Hello everyone,
Here's the report on the Media Summit held on 25th October in Edinburgh. The report is followed by the stateless codec discussion two days earlier.
Note: this is bcc'd to the meeting attendees plus a few others. I didn't use cc as the list servers tend to reject messages with too many recipients in cc / to headers.
Most presenters used slides some of which are already available here (expect more in the near future):
URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/
The original announcement for the meeting is here:
URL:https://www.spinics.net/lists/linux-media/msg141095.html
The raw notes can be found here:
URL:http://www.retiisi.org.uk/~sailus/v4l2/notes/osseu18-media.html
Attendees ---------
Brad Love Ezequiel Garcia Gustavo Padovan Hans Verkuil Helen Koike Hidenori Yamaji Ivan Kalinin Jacopo Mondi Kieran Bingham Laurent Pinchart Mauro Chebab Maxime Ripard Michael Grzeschik Michael Ira Krufky Niklas Söderlund Patrick Lai Paul Elder Peter Griffin Ralph Clark Ricardo Ribalda Sakari Ailus Sean Young Seung-Woo Kim Stefan Klug Vinod Koul
CEC status - Hans Verkuil -------------------------
Hans prensented an update on CEC status. Besides the slides, noteworthy information is maintained here:
URL:https://hverkuil.home.xs4all.nl/cec-status.txt
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-cec-status.pdf
rc-core status report - Sean Young ----------------------------------
(Contributed by Sean Young)
In the last year all staging lirc drivers have been either removed or ported to rc-core. Decoding of the more obscure IR protocols and protocol variants can now be done with BPF, with support in both the kernel and ir-keytable (which is in v4l-utils). Generally we're in a good situation wrt IR support.
There is some more ancient hardware (serial or usb-serial) that does not have support but not sure if anyone cares. kernel-doc is a little sparse and does not cover BPF IR decoding, so that needs improving. There was a discussion on enabling builds with CONFIG_RC_CORE=n. Sean suggested we could have rc_allocate_driver() return NULL and have the drivers deal with this gracefully, i.e. their probe functions should continue without IR. Mauro said there should be a per-driver config option (as is done for saa7134 for example).
No conclusion was reached on this.
Persistent storage of controls - Ricardo Ribalda ------------------------------------------------
Ricardo gave a presentation on a proposed solution for using the V4L2 control framework as an interface for updating control value defaults on sensor EEPROM.
Sensors commonly come with device specific tuning information that's embedded in the device EEPROM. Whereas this is also very common for raw cameras on mobile devices, the discussion this time was concentrated on industrial cameras.
The EEPROM contents may be written by the sensor vendor but occasionally may need to be updated by customers. Setting the control default value was suggested as the exact mechanism to do this.
The proposal was to use controls as the interface to update sensor tuning information in the EEPROM.
There were arguments for and against the approach:
+ Drivers usually get these things right: relying on an user space program to do this is an additional dependency. + Re-use of an existing interface (root priviledge check may be added).
- Partial solution only: EEPROM contents may need to be updated for other reasons as well, and a "spotty" implementation for updating certain EEPROM locations seems very use case specific. - Changes required to the control framework for this --- defaults are not settable at the moment. - The need is very use case specific, and adding support for that in a generic framework does not seem to fit very well.
The general consensus appears to be not to change the control framework this way, but to continue to update the EEPROM using a specific user space program.
Tooling for sub-system tree maintenance - Laurent Pinchart ----------------------------------------------------------
Laurent talked about the DRM tree maintenance model.
The DRM tree has switched to co-maintainer model. This has made it possible to share the burden of tree maintenance, removing bottlenecks they've had.
The larger number of people having (and using) their commit rights has created the need for a more strict rules for the tree maintenance, and subsequently a tool to implement it. It's called "DIM", the DRM Inglorious Maintenance tool. This is a command line tool that works as a front-end to execute the workflow.
URL:https://01.org/linuxgraphics/gfx-docs/maintainer-tools/dim.html
In particular what's worth noting:
- The conflicts are resolved by the committer, not by the tree maintainer.
- DIM stores conflict resolutions (as resolved by developers) to a shared cache.
- DIM makes doing common mistakes harder by using sanity checks.
There are about 50 people who currently have commit rights to the DRM tree. There are no reports of commit rights having been forcibly removed as of yet. This strongly suggests that the model is workable.
The use of the tool puts additional responsibilities as well as some burden to the committers. Before the patches may be pushed, they are first compiled on developer's machine. That requires time, and without special arrangements such as having a second local workspace, and that time is away from productive work.
The discussion that followed was concentrated on the possibility of using a similar model for the media tree. While the suggestion was initially met by mostly favourable reception, there were concerns as well.
V4L2 *was* maintained generally according to the suggested model --- albeit without the proposed tools or process that needed to be strictly followed. There was once an incident which involved merging around 9000 lines of unreviewed code in a lot of places. What followed was not pretty, and this eventually lead to loss of multiple developers.
Could this happen again? The DRM tree has not suffered such incidents, and generally it understood such incident could be addressed by simply reverting such a patch and removing commit rights if necessary. (Editor note: we have reverted the media tree master state to an earlier commit many times for various reasons. Could it be one of the reasons the 9000 line patch was not reverted was that the version control wasn't based on git??)
Some opined that we do not have a bottleneck in reviewing patches and getting them merged whilst others thought this was not the case. It is certainly true that a very large number of patches (around 500 in the last kernel release) went in through the media tree. It still appears that there would be more patches and more drivers to get in if the throughput was higher.
Current status of testing on the media tree - Sakari ----------------------------------------------------
The common practice in media subsystem development is that developers do test their patches before submitting them. This is an unwritten rule: sometimes patches end up not being tested after making slight changes to them, or they have been tested on a different kernel version. The developer may also simply forget to test the patch.
Besides this, it is not uncommon that changing the kernel configuration or switching to a different architecture will cause a compilation warning or an error.
The 0-day bot will catch some of these errors before the patches are merged, but that testing does not fully cover all the possible cases. There are some common pain points in V4L2-related Kconfig options (plain V4L2, MC or MC + subdev uAPI); newly submitted drivers may in fact require one of these, but the developer may not have realised that and so this ends up not being taken into account in Kconfig.
Once the review is done, and after being applied to the sub-maintainer tree, a patch is applied to Mauro's local tree and Mauro performs additional tests on it. These tests currently prevent a fair number of problems reaching a wider audience than the media developers.
On the other hand, whenever an issue is found, the patch will have to be fixed by the sub-maintainer or the developer. This is hardly ideal, as the problem has existed usually for a month or two before being spotted --- by a program. These checks should be instead performed on the patch when it's submitted.
Automated testing - Ezequiel Garcia -----------------------------------
Ideal Continuous Integration process consists of the following steps:
1. patch submission 2. review and approval 3. merge
The core question is "what level of quality standards do we want to enforce". The maintenance process should be modelled around this question, and not the other way around. Automated testing can be a part of enforcing the quality standards.
There are three steps:
1. Define the quality standard 2. Define how to quantify quality in respect to the standard 3. Define how to enforce the standards
On the tooling side, an uAPI test tool exists. It's called v4l2-compliance, and new drivers are required to pass the v4l2-compliance test. It has quite a few favourable properties:
- Complete in terms of the uAPI coverage - Quick and easy to run - Nice output format for humans & scripts
There are some issues as well:
- No codec support (stateful or stateless) - No SDR or touch support - Frequently updated (distribution shipped v4l2-compliance useless) - Only one contributor
Ezequiel noted that some people think that v4l2-compliance is changing too often but Hans responded that this is a necessity. The API gets amended occasionally and the existing API gets new tests. Mauro proposed moving v4l2-compliance to the kernel source tree but Hans preferred keeping it separate. That way it's easier to develop it.
To address the problem of only a single contributor, it was suggested that people implementing new APIs would need to provide the tests for v4l2-compliance as well. To achieve this, the v4l2-compliance codebase needs some cleanup to make it easier to contribute. The codebase is larger and there is no documentation.
V4l2-compliance also covers MC, V4L2 and V4L2 sub-device uAPIs.
DVB will require its own test tooling; it is not covered by v4l2-compliance. In order to facilitate automated testing, a virtual DVB driver would be useful as well. The task was added to the list of projects needing volunteers:
URL:https://linuxtv.org/wiki/index.php/Media_Open_Source_Projects:_Looking_for_Volunteers
There are some other test tools that could cover V4L2 but at the moment it seems somewhat far-fetched any of them would be used to test V4L2 in the near future:
- kselftest - kunit - gst-validate - ktf (https://github.com/oracle/ktf, http://heim.ifi.uio.no/~knuto/ktf/)
KernelCI is a test automation system that supports automated compile and boot testing. As a newly added feature, additional tests may be implemented. This is what Collabora has implemented, effectively the current demo system runs v4l2-compliance on virtual drivers in a virtual machines (LAVA slaves).
A sample of the current test report is here:
URL:https://www.mail-archive.com/linux-media@vger.kernel.org/msg135787.html
The established way to run KernelCI tests is off the head of the branches of the stable and development kernel trees, including linux-next. This is not useful as such to support automated testing of patches for the media tree: the patches need to be tested before they are merged, not after merging.
In the discusion that followed among a slightly smaller group of people, it was suggested that tests could be run from select developer kernel trees, from any branch. If a developer needs long-term storage, (s)he could have another tree which would not be subject automated test builds. Alternatively, the branch name could be used as a basis for triggering an automated build, but this could end up being too restrictive.
Merging the next rc1 by the maintainer would be no special case: the branch would be tested in similar way than the developer branches containing patches, and tests should need to pass before pushing the content to the media tree master branch.
Ezequiel wished that people would reply to his e-mail to express their wishes on the testing needs (see sample report above).
Stateless codecs - Hans Verkuil -------------------------------
Support for stateless codecs will be merged for v4.20 with an Allwinner staging codec driver.
The earlier stateless codec discussion ended up concluding that the bitstream parsing is application specific, so there will be no need for a generic implementation that was previously foreseen. The question that remains is: should there be a simple parser for compliance testing?
All main applications support libva which was developed as the codec API to be used with Intel GPUs. A libVA frontend was written to support the Cedrus stateless V4L2 decoder driver. It remains to be seen whether the same implementation could be used as such for the other stateless codec drivers or whether changes, or in the worst case a parallel implementation, would be needed.
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-codec-userspace.pdf
New versions of the old IOCTLs - Hans Verkuil ---------------------------------------------
V4L2 is an old API with shifting focus in terms of functionality and hardware supported. While there has been lots of changes to the two during the existence of V4L2, some of the API is unchanged since the old times. While the API is usable for the purpose, it is needlessly clunky: it is often not obvious how an IOCTL is related to the task at hand (such as using S_PARM to set the frame interval) or the API does not use year 2038-safe timestamps (struct v4l2_buffer). These APIs deserve to be updated.
* VIDIOC_*_PARM
In the case of VIDIOC_G_PARM and VIDIOC_S_PARM, the IOCTLs are only used to set and get the frame interval. In this case, what can be done, is to add a new IOCTL definition, with the same IOCTL number and with binary-equivalent IOCTL argument struct that only contains the field for the frame rate itself. This is binary-compatible with the existing code and no compatibility code will be needed. The new IOCTLs will be called VIDIOC_G_FRAME_INTERVAL and VIDIOC_S_FRAME_INTERVAL.
* VIDIOC_ENUM_FRAME_INTERVALS
Besides discrete set of supported frame intervals, VIDIOC_ENUM_FRAME_INTERVALS has stepwise frame interval as well. Stepwise could be removed as the Qualcomm venus codec and uvc (100 ns units) are the only users. Additionally, the buffer type should be added to struct v4l2_frmivalenum.
There was also a discussion related to enumerating frame intervals in units of ns vs. fractional seconds. The reasoning using a fraction is that this way the frame interval on many standards can be conveyed precisely. Somebody recalled "flick", that is is the common denominator of the frame rates on all TV standards. Drivers could simply move to use the flick as the denominator, to make frame interval reporting uniform across the drivers.
* struct v4l2_buffer
struct v4l2_buffer is an age-old struct. There are a few issues in it:
- The timestamp is not 2038-safe. - The multi-plane implementation is a mess. - Differing implementation for the end single-plane and multi-plane APIs is confusing for both applications and drivers.
The proposal is to create a new v4l2_buffer struct. The differences to the old one would be:
- __u64 timestamps. These are 2038-safe. The timestamp source is maintained, i.e. the type remains CLOCK_MONOTONIC apart from certain drivers (e.g. UVC) that lets the user choose the timestamp. - Put the planes right to struct v4l2_buffer. The plane struct would also be changed; the new plane struct would be called v4l2_ext_plane. - While at it, the plane description can be improved: - The start of data from the beginning of the plane memory. - Add width and height to the buffer? This would make image size changes easier for the codec. (Ed. note: pixel format as well. But this approach could only partially support what the request API is for.) - Unify single- and multi-planar APIs.
The new struct could be called v4l2_ext_buffer.
As the new IOCTL argument struct will have has different syntax as well as semantics, it deserves to be named differently. Compatibility code will be needed to convert the users of the old IOCTLs to the new struct used internally by the kernel and drivers, and then back to the user.
* struct v4l2_create_buffers
Of the format, only the pix.fmt.sizeimage field is effectively used by the drivers supporting VIDIOC_CREATE_BUFS. This could be simplified, by just providing the desired buffer size instead of the entire v4l2_format struct. The user would be instructed to use TRY_FMT to obtain that buffer size.
The need to delete buffers seems to have eventually surfaced. That was expected, but it wasn't known when this would happen. As the buffer index range would become non-contiguous, it should be possible to create buffers one by one only, as otherwise the indices of the additional buffers would no longer be communicated to the user unambiguously.
So there would be new IOCTLs:
- VIDIOC_CREATE_BUF - Create a single buffer of given size (plus other non-format related aspects) - VIDIOC_DELETE_BUF - Delete a single buffer - VIDIOC_DELETE_ALL_BUFS - Delete all buffers
The naming still requires some work. The opposite of create is "destroy", not "delete".
* struct v4l2_pix_format vs. struct v4l2_pix_format_mplane
Working with the two structs depending on whether the format is multi-planar or not is painful. While we're doing changes in the area, the two could be unified as well. (Editor note: this could be still orthogonal to the buffers, so it could be done separately as well. We'll see.)
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-new-ioctls.pdf
Fault tolerant V4L2 - Kieran Bingham ------------------------------------
Kieran presented a system where the media hardware complex consisted of eight more or less independent camera sensors that naturally end up being within a single media device.
The current implementation, as well as the API, necessitates that all devices in a media device probe successfully before the entire media device is exposed to the user. Otherwise the user would see with a partial view of the device, without the knowledge it is such.
To address the problem, additional information need to be provided to the user space. In particular:
- Events on the media device to tell the graph has changed.
- Graph version number is incremented at graph change (already implemented).
- The property API could be applicable --- placeholders for entities that have not yet appeared?
- Alternative: known entities that have failed to probe created in the media graph and marked "disable" or "failed".
- Query the state of media graph completeness.
That way, even when the devices in a media controller device appear one by one, the user space will be able to have all the necessary information on the registration state of the device.
Complex cameras - Mauro Chehab ------------------------------
Some new laptops integrate a raw Bayer camera + ISP instead of a USB webcam. This is expected to increase, as the solution is generally cheaper and results in better quality images --- as long as all the pieces of the puzzle are in place, including the proprietary 3A library.
Still, such devices need to be supported.
(Ed. note: there were two talks related to this topic given in the ELc-E.)
URL:https://www.youtube.com/watch?v=KpaNNJr92CY&index=31&list=PLbzoR-pLrL6qThA7SAbhVfuMbjZsJX1CY URL:https://www.youtube.com/watch?v=GIhV7tiUji0&index=60&list=PLbzoR-pLrL6qThA7SAbhVfuMbjZsJX1CY
Development process - All -------------------------
Topic-wise this is continuation of the "Tooling for sub-system tree maintenance", "Current status of testing on the media tree" and "Automated testing" topics above.
The question here is whether there's something that could be improved in the media development process and if so, how could that be done.
What came up was a suggestion to have multi-committer tree in a similar manner as the DRM developers do. This was seen to be more interesting for developers than simply being asked to review patches.
It certainly does raise the need for more precise rules for what may be committed to the multi-committer tree, when etc.
It was also requested that experienced driver maintainers would send pull requests on patches to their drivers instead of going through a sub-maintainer (pre-agreed with the relevant (sub)maintainer). This would take some work away from sub-maintainers, but not the maintainer.
No firm decisions were reached in this topic. Perhaps this could be tried out?
There was also a request to document the sub-maintainer names in the wiki so that it'd be easier for people to figure out who to ping if their patches do not get merged.
linuxtv.org hosting - All -------------------------
Mauro noted that linuxtv.org is currently hosted in a virtual machine somewhere in a German university. The administrator of the virtual machine has not been involved with Video4Linux for some time but has been kind to provide us the hosting over the years.
It has been recognised that there is a need to find a new hosting location for the virtual machine. There is also a question of the domain name linuxtv.org. Discussion followed.
What could be agreed on rather immediately was that the domain name should be owned by "us". "Us" is not a legal entity at the moment, and a practical arrangement to achieve that could be to find a new association to own the domain name.
The hosting of the virtual machine could possibly be handled by the same association. In practice this would likely mean a virtual machine on a hosting provider. Ideally this would be paid for by a company or a group of companies.
No decisions were reached on the topic.
Tuesday's stateless codec discussion ------------------------------------
Hans presented a summary of this in his stateless codec status presentation, here are a bit more details.
We had a discussion (first in the Microsoft sponsor suite, then at the bar) on how to support user space for the stateless codecs better. The expected outcome of that would be a rough understanding how a stateless codec user space library would look like.
The raw notes are available here:
URL:http://www.retiisi.org.uk/~sailus/v4l2/notes/osseu18-codecs.html
* Attendees
Alexandre Courbot Chris Healy Ezequiel Garcia Hans Verkuil Kieran Bingham Laurent Pinchart Maxime Ripard Mauro Carvalho Chehab Nicolas Dufresne Niklas Söderlund Philip Zabell Sakari Ailus Tomasz Figa Victor Jáquez
* Buffer management
Nicolas reported an issue in V4L2 buffer management. The V4L2 decouples the buffers from the format, and assumes all queued buffers (at a given point of time) have the same format. (Ed. note: the request API could be used to address this, but that particular features is not yet supported.)
* User space library
The existing projects generally integrate their own bitstream parsers for codecs. There are subtle reasons why that tends to be the case, instead of using more generic parsers. There are differences in error handling, for instance, or other matters of policy, the variation which could be difficult to fully offer using a generic API.
Maxime noted that VLC recently released a new parser meant to be used as a library, and that could be useful. Nicolas believes that we'd need a parser library independent of any other code base to avoid pulling in extra libraries and this parser would need to be maintained. It could be difficult to find the volunteers to do that.
Does ChromeOS have its own parser? Alexandre believes it does, but little was known beyond that.
There's also the language problem: ffmpeg and gstreamer are written in C, the ChomeOS parser in C++, VLC is moving to Rust. What do we pick, how do we ensure interoperability?
* libVA re-use
As a short-term solution, implementing a generic wrapper using the V4L2 stateless codec API to offer libVA API would enable generic applications to use the V4L2 stateless codec drivers as most applications already support libVA.
70 % of the applications use FFMPEG, which has a software codec API that is nearly identical to the V4L2 statless codec API. It would be trivial for applications to switch to V4L2 natively.
Mauro would like us to explain our plans to Intel to avoid surprises later on.
* Source code hosting
libva is hosted on freedesktop. Should we host the libva-v4l2-codec backend there, or host it on linuxtv.org? Hans would prefer linuxtv.org as it's "closer to our kernel implementation".
* Backend support in libva
libva loads backends in order, and picks the first one that reports it can support the platform. There is also an environment variable that can specify a backend. Ezequiel enquired how to support platforms that would have multiple hardware codecs. libva doesn't seem to support this at the moment. Nicolas reported that there's an Intel SoC that have both an Intel graphics core and a Vega64 graphics core that both have a codec.
Hans said that a platform that expose multiple codecs will likely be used for specialized applications, and requiring those to implement codec support directly is acceptable. Our main focus should be to support the common case.
* Vendor support
NVidia is following our progress and is interested in using the V4L2 stateless API. On the userspace side, vdpau is pretty much dead, they have moved to nvdec. OMX is being phasing out, in particular that is taking place for RaspberryPi now.
* Tooling
bootlin has developed a debugging tool called v4l2-request-test (https://github.com/bootlin/v4l2-request-test) that has been very useful to debug the codec driver without going through the full userspace stack. This is worth mentioning and integrating.
* API discussions
Using buffer indices as handles to reference frames
This has been proposed by Tomasz, and Hans has serious concerns, he believes that having userspace predict what buffer indices will be used in the future is very fragile and would prefer using a separate 64-bit cookie associated with v4l2_buffers.
Using capture buffer indices as reference frame handles requires predicting the buffer index on the capture queue which the output queue frames will be decoded into. We could use the output queue buffer index instead, but that wouldn't work with multi-slice decoding (multiple output buffers for a single capture buffer). Using a cookie set by userspace on the output side, then copied to the capture queue by the driver, solves that problem. All slices queued on the output queue for the same decoded picture will have the same cookie value (userspace will have to ensure that).
Tomasz would prefer a buffer index-based solution, to avoid keeping a cookie-index map in userspace. Due to how V4L2 works, enqueuing a new dmabuf handle on the capture side for a V4L2 buffer with a given index will effectively delete the corresponding cookie, so userspace would need to ensure it doesn't overwrite buffers; (Tomasz: To clarify, I don't see the significant benefit of using cookies over indices. It makes it easier for user space, because it doesn't have to predict the CAPTURE buffers, but still is error prone because of the buffer requeuing problem. For now it would be good to see how it translates into real code, though. In the meantime I can try to find a better idea.)
On Sun, Nov 18, 2018 at 7:45 AM Sakari Ailus sakari.ailus@iki.fi wrote:
Hello everyone,
Here's the report on the Media Summit held on 25th October in Edinburgh. The report is followed by the stateless codec discussion two days earlier.
Note: this is bcc'd to the meeting attendees plus a few others. I didn't use cc as the list servers tend to reject messages with too many recipients in cc / to headers.
Most presenters used slides some of which are already available here (expect more in the near future):
URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/
The original announcement for the meeting is here:
URL:https://www.spinics.net/lists/linux-media/msg141095.html
The raw notes can be found here:
URL:http://www.retiisi.org.uk/~sailus/v4l2/notes/osseu18-media.html
Thanks Sakari for editing the notes. Let me share my thoughts inline.
[snip]
Automated testing - Ezequiel Garcia
Ideal Continuous Integration process consists of the following steps:
1. patch submission 2. review and approval 3. mergeThe core question is "what level of quality standards do we want to enforce". The maintenance process should be modelled around this question, and not the other way around. Automated testing can be a part of enforcing the quality standards.
There are three steps:
1. Define the quality standard 2. Define how to quantify quality in respect to the standard 3. Define how to enforce the standardsOn the tooling side, an uAPI test tool exists. It's called v4l2-compliance, and new drivers are required to pass the v4l2-compliance test. It has quite a few favourable properties:
- Complete in terms of the uAPI coverage
- Quick and easy to run
- Nice output format for humans & scripts
There are some issues as well:
- No codec support (stateful or stateless)
- No SDR or touch support
- Frequently updated (distribution shipped v4l2-compliance useless)
- Only one contributor
Ezequiel noted that some people think that v4l2-compliance is changing too often but Hans responded that this is a necessity. The API gets amended occasionally and the existing API gets new tests. Mauro proposed moving v4l2-compliance to the kernel source tree but Hans preferred keeping it separate. That way it's easier to develop it.
To address the problem of only a single contributor, it was suggested that people implementing new APIs would need to provide the tests for v4l2-compliance as well. To achieve this, the v4l2-compliance codebase needs some cleanup to make it easier to contribute. The codebase is larger and there is no documentation.
V4l2-compliance also covers MC, V4L2 and V4L2 sub-device uAPIs.
DVB will require its own test tooling; it is not covered by v4l2-compliance. In order to facilitate automated testing, a virtual DVB driver would be useful as well. The task was added to the list of projects needing volunteers:
URL:https://linuxtv.org/wiki/index.php/Media_Open_Source_Projects:_Looking_for_Volunteers
There are some other test tools that could cover V4L2 but at the moment it seems somewhat far-fetched any of them would be used to test V4L2 in the near future:
- kselftest - kunit - gst-validate - ktf (https://github.com/oracle/ktf, http://heim.ifi.uio.no/~knuto/ktf/)KernelCI is a test automation system that supports automated compile and boot testing. As a newly added feature, additional tests may be implemented. This is what Collabora has implemented, effectively the current demo system runs v4l2-compliance on virtual drivers in a virtual machines (LAVA slaves).
A sample of the current test report is here:
URL:https://www.mail-archive.com/linux-media@vger.kernel.org/msg135787.html
The established way to run KernelCI tests is off the head of the branches of the stable and development kernel trees, including linux-next. This is not useful as such to support automated testing of patches for the media tree: the patches need to be tested before they are merged, not after merging.
In the discusion that followed among a slightly smaller group of people, it was suggested that tests could be run from select developer kernel trees, from any branch. If a developer needs long-term storage, (s)he could have another tree which would not be subject automated test builds. Alternatively, the branch name could be used as a basis for triggering an automated build, but this could end up being too restrictive.
Merging the next rc1 by the maintainer would be no special case: the branch would be tested in similar way than the developer branches containing patches, and tests should need to pass before pushing the content to the media tree master branch.
Ezequiel wished that people would reply to his e-mail to express their wishes on the testing needs (see sample report above).
I'd really love having codec tests there, but we're still working to finalize the API, so not much to say about testing. :( The ideal mode would test all the sequences defined by the API, including erroneous operations from the userspace to verify error handling.
Stateless codecs - Hans Verkuil
Support for stateless codecs will be merged for v4.20 with an Allwinner staging codec driver.
The earlier stateless codec discussion ended up concluding that the bitstream parsing is application specific, so there will be no need for a generic implementation that was previously foreseen. The question that remains is: should there be a simple parser for compliance testing?
All main applications support libva which was developed as the codec API to be used with Intel GPUs. A libVA frontend was written to support the Cedrus stateless V4L2 decoder driver. It remains to be seen whether the same implementation could be used as such for the other stateless codec drivers or whether changes, or in the worst case a parallel implementation, would be needed.
I believe it should work for Rockchip VPU as well.
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-codec-userspace.pdf
New versions of the old IOCTLs - Hans Verkuil
V4L2 is an old API with shifting focus in terms of functionality and hardware supported. While there has been lots of changes to the two during the existence of V4L2, some of the API is unchanged since the old times. While the API is usable for the purpose, it is needlessly clunky: it is often not obvious how an IOCTL is related to the task at hand (such as using S_PARM to set the frame interval) or the API does not use year 2038-safe timestamps (struct v4l2_buffer). These APIs deserve to be updated.
[snip]
- struct v4l2_buffer
struct v4l2_buffer is an age-old struct. There are a few issues in it:
- The timestamp is not 2038-safe.
- The multi-plane implementation is a mess.
- Differing implementation for the end single-plane and multi-plane APIs is confusing for both applications and drivers.
The proposal is to create a new v4l2_buffer struct. The differences to the old one would be:
- __u64 timestamps. These are 2038-safe. The timestamp source is maintained, i.e. the type remains CLOCK_MONOTONIC apart from certain drivers (e.g. UVC) that lets the user choose the timestamp.
- Put the planes right to struct v4l2_buffer. The plane struct would also be changed; the new plane struct would be called v4l2_ext_plane.
- While at it, the plane description can be improved: - The start of data from the beginning of the plane memory.
This is a must, since otherwise there is no way to have one DMA-buf contain multiple planes at offsets, which actually is a common case in the graphics world.
- Add width and height to the buffer? This would make image size changes easier for the codec. (Ed. note: pixel format as well. But this approach could only partially support what the request API is for.)
- Unify single- and multi-planar APIs.
The new struct could be called v4l2_ext_buffer.
As the new IOCTL argument struct will have has different syntax as well as semantics, it deserves to be named differently. Compatibility code will be needed to convert the users of the old IOCTLs to the new struct used internally by the kernel and drivers, and then back to the user.
- struct v4l2_create_buffers
Of the format, only the pix.fmt.sizeimage field is effectively used by the drivers supporting VIDIOC_CREATE_BUFS. This could be simplified, by just providing the desired buffer size instead of the entire v4l2_format struct. The user would be instructed to use TRY_FMT to obtain that buffer size.
There is a problem with using TRY_FMT for stateful codecs, because once the CAPTURE format is established from the stream metadata, any format operations would only accept formats that are compatible with current stream. Currently TRY_FMT is defined to behave exactly as S_FMT, except that it doesn't update the active format.
Perhaps it would actually make sense to keep the full v4l2_format struct in VIDIOC_CREATE_BUFS and actually make the implementation compute the size based on the other fields (if sizeimage is left 0 for example or smaller than needed for the format)?
The need to delete buffers seems to have eventually surfaced. That was expected, but it wasn't known when this would happen. As the buffer index range would become non-contiguous, it should be possible to create buffers one by one only, as otherwise the indices of the additional buffers would no longer be communicated to the user unambiguously.
allocated = 0; while (allocated < to_allocate) { // ... create_bufs.count = to_allocate - allocated; ret = ioctl(fd, VIDIOC_CREATE_BUFS, &create_bufs); allocated += create_bufs.count; // ... add [create_bufs.index, create_bufs.index + create_bufs.count - 1] to the list of buffer indices }
So there would be new IOCTLs:
- VIDIOC_CREATE_BUF - Create a single buffer of given size (plus other non-format related aspects)
What would this ioctl give us over VIDIOC_CREATE_BUF with count=1?
- VIDIOC_DELETE_BUF - Delete a single buffer
- VIDIOC_DELETE_ALL_BUFS - Delete all buffers
The naming still requires some work. The opposite of create is "destroy", not "delete".
- struct v4l2_pix_format vs. struct v4l2_pix_format_mplane
Working with the two structs depending on whether the format is multi-planar or not is painful. While we're doing changes in the area, the two could be unified as well. (Editor note: this could be still orthogonal to the buffers, so it could be done separately as well. We'll see.)
This is a huge pain on both userspace and driver sides.
On a related note, the split into M and non-M formats also poses a userspace compatibility problem, as many drivers that expose M formats fail to expose support for corresponding non-M formats, only because the implementation becomes messy if you have to deal with both.
[snip]
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-new-ioctls.pdf
[snip]
linuxtv.org hosting - All
Mauro noted that linuxtv.org is currently hosted in a virtual machine somewhere in a German university. The administrator of the virtual machine has not been involved with Video4Linux for some time but has been kind to provide us the hosting over the years.
It has been recognised that there is a need to find a new hosting location for the virtual machine. There is also a question of the domain name linuxtv.org. Discussion followed.
What could be agreed on rather immediately was that the domain name should be owned by "us". "Us" is not a legal entity at the moment, and a practical arrangement to achieve that could be to find a new association to own the domain name.
The hosting of the virtual machine could possibly be handled by the same association. In practice this would likely mean a virtual machine on a hosting provider. Ideally this would be paid for by a company or a group of companies.
No decisions were reached on the topic.
Any chance that we could have the toolkit on linuxtv.org improved? Examples: - The new patchwork (as on lore.kernel.org or patchwork.freedesktop.org) has a lot of useful features that our linuxtv one misses (for example tracking of patches within a series or tracking versions of patches), - The color scheme of diff views in the web git is not very readable (the dark blue items in particular): https://git.linuxtv.org/media_tree.git/diff/?id2=d148b85e8b0779b910f3120a1b7... Making it consistent with the patchwork scheme would solve the problem.
Best regards, Tomasz
Hello everyone,
And thanks for the detailed report!
On Mon, 2018-11-19 at 13:09 +0900, Tomasz Figa wrote:
On Sun, Nov 18, 2018 at 7:45 AM Sakari Ailus sakari.ailus@iki.fi wrote:
Hello everyone,
Here's the report on the Media Summit held on 25th October in Edinburgh. The report is followed by the stateless codec discussion two days earlier.
Note: this is bcc'd to the meeting attendees plus a few others. I didn't use cc as the list servers tend to reject messages with too many recipients in cc / to headers.
Most presenters used slides some of which are already available here (expect more in the near future):
URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/
The original announcement for the meeting is here:
URL:https://www.spinics.net/lists/linux-media/msg141095.html
The raw notes can be found here:
URL:http://www.retiisi.org.uk/~sailus/v4l2/notes/osseu18-media.html
Thanks Sakari for editing the notes. Let me share my thoughts inline.
[snip]
Automated testing - Ezequiel Garcia
Ideal Continuous Integration process consists of the following steps:
1. patch submission 2. review and approval 3. mergeThe core question is "what level of quality standards do we want to enforce". The maintenance process should be modelled around this question, and not the other way around. Automated testing can be a part of enforcing the quality standards.
There are three steps:
1. Define the quality standard 2. Define how to quantify quality in respect to the standard 3. Define how to enforce the standardsOn the tooling side, an uAPI test tool exists. It's called v4l2-compliance, and new drivers are required to pass the v4l2-compliance test. It has quite a few favourable properties:
- Complete in terms of the uAPI coverage
- Quick and easy to run
- Nice output format for humans & scripts
There are some issues as well:
- No codec support (stateful or stateless)
- No SDR or touch support
- Frequently updated (distribution shipped v4l2-compliance useless)
- Only one contributor
Ezequiel noted that some people think that v4l2-compliance is changing too often but Hans responded that this is a necessity. The API gets amended occasionally and the existing API gets new tests. Mauro proposed moving v4l2-compliance to the kernel source tree but Hans preferred keeping it separate. That way it's easier to develop it.
To address the problem of only a single contributor, it was suggested that people implementing new APIs would need to provide the tests for v4l2-compliance as well. To achieve this, the v4l2-compliance codebase needs some cleanup to make it easier to contribute. The codebase is larger and there is no documentation.
V4l2-compliance also covers MC, V4L2 and V4L2 sub-device uAPIs.
DVB will require its own test tooling; it is not covered by v4l2-compliance. In order to facilitate automated testing, a virtual DVB driver would be useful as well. The task was added to the list of projects needing volunteers:
URL:https://linuxtv.org/wiki/index.php/Media_Open_Source_Projects:_Looking_for_Volunteers
There are some other test tools that could cover V4L2 but at the moment it seems somewhat far-fetched any of them would be used to test V4L2 in the near future:
- kselftest - kunit - gst-validate - ktf (https://github.com/oracle/ktf, http://heim.ifi.uio.no/~knuto/ktf/)KernelCI is a test automation system that supports automated compile and boot testing. As a newly added feature, additional tests may be implemented. This is what Collabora has implemented, effectively the current demo system runs v4l2-compliance on virtual drivers in a virtual machines (LAVA slaves).
A sample of the current test report is here:
URL:https://www.mail-archive.com/linux-media@vger.kernel.org/msg135787.html
The established way to run KernelCI tests is off the head of the branches of the stable and development kernel trees, including linux-next. This is not useful as such to support automated testing of patches for the media tree: the patches need to be tested before they are merged, not after merging.
In the discusion that followed among a slightly smaller group of people, it was suggested that tests could be run from select developer kernel trees, from any branch. If a developer needs long-term storage, (s)he could have another tree which would not be subject automated test builds. Alternatively, the branch name could be used as a basis for triggering an automated build, but this could end up being too restrictive.
Merging the next rc1 by the maintainer would be no special case: the branch would be tested in similar way than the developer branches containing patches, and tests should need to pass before pushing the content to the media tree master branch.
Ezequiel wished that people would reply to his e-mail to express their wishes on the testing needs (see sample report above).
I'd really love having codec tests there, but we're still working to finalize the API, so not much to say about testing. :( The ideal mode would test all the sequences defined by the API, including erroneous operations from the userspace to verify error handling.
Stateless codecs - Hans Verkuil
Support for stateless codecs will be merged for v4.20 with an Allwinner staging codec driver.
The earlier stateless codec discussion ended up concluding that the bitstream parsing is application specific, so there will be no need for a generic implementation that was previously foreseen. The question that remains is: should there be a simple parser for compliance testing?
All main applications support libva which was developed as the codec API to be used with Intel GPUs. A libVA frontend was written to support the Cedrus stateless V4L2 decoder driver. It remains to be seen whether the same implementation could be used as such for the other stateless codec drivers or whether changes, or in the worst case a parallel implementation, would be needed.
I believe it should work for Rockchip VPU as well.
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-codec-userspace.pdf
New versions of the old IOCTLs - Hans Verkuil
V4L2 is an old API with shifting focus in terms of functionality and hardware supported. While there has been lots of changes to the two during the existence of V4L2, some of the API is unchanged since the old times. While the API is usable for the purpose, it is needlessly clunky: it is often not obvious how an IOCTL is related to the task at hand (such as using S_PARM to set the frame interval) or the API does not use year 2038-safe timestamps (struct v4l2_buffer). These APIs deserve to be updated.
[snip]
- struct v4l2_buffer
struct v4l2_buffer is an age-old struct. There are a few issues in it:
- The timestamp is not 2038-safe.
- The multi-plane implementation is a mess.
- Differing implementation for the end single-plane and multi-plane APIs is confusing for both applications and drivers.
The proposal is to create a new v4l2_buffer struct. The differences to the old one would be:
- __u64 timestamps. These are 2038-safe. The timestamp source is maintained, i.e. the type remains CLOCK_MONOTONIC apart from certain drivers (e.g. UVC) that lets the user choose the timestamp.
- Put the planes right to struct v4l2_buffer. The plane struct would also be changed; the new plane struct would be called v4l2_ext_plane.
- While at it, the plane description can be improved: - The start of data from the beginning of the plane memory.
This is a must, since otherwise there is no way to have one DMA-buf contain multiple planes at offsets, which actually is a common case in the graphics world.
- Add width and height to the buffer? This would make image size changes easier for the codec. (Ed. note: pixel format as well. But this approach could only partially support what the request API is for.)
- Unify single- and multi-planar APIs.
The new struct could be called v4l2_ext_buffer.
As the new IOCTL argument struct will have has different syntax as well as semantics, it deserves to be named differently. Compatibility code will be needed to convert the users of the old IOCTLs to the new struct used internally by the kernel and drivers, and then back to the user.
- struct v4l2_create_buffers
Of the format, only the pix.fmt.sizeimage field is effectively used by the drivers supporting VIDIOC_CREATE_BUFS. This could be simplified, by just providing the desired buffer size instead of the entire v4l2_format struct. The user would be instructed to use TRY_FMT to obtain that buffer size.
There is a problem with using TRY_FMT for stateful codecs, because once the CAPTURE format is established from the stream metadata, any format operations would only accept formats that are compatible with current stream. Currently TRY_FMT is defined to behave exactly as S_FMT, except that it doesn't update the active format.
Perhaps it would actually make sense to keep the full v4l2_format struct in VIDIOC_CREATE_BUFS and actually make the implementation compute the size based on the other fields (if sizeimage is left 0 for example or smaller than needed for the format)?
The need to delete buffers seems to have eventually surfaced. That was expected, but it wasn't known when this would happen. As the buffer index range would become non-contiguous, it should be possible to create buffers one by one only, as otherwise the indices of the additional buffers would no longer be communicated to the user unambiguously.
allocated = 0; while (allocated < to_allocate) { // ... create_bufs.count = to_allocate - allocated; ret = ioctl(fd, VIDIOC_CREATE_BUFS, &create_bufs); allocated += create_bufs.count; // ... add [create_bufs.index, create_bufs.index + create_bufs.count - 1] to the list of buffer indices }
So there would be new IOCTLs:
- VIDIOC_CREATE_BUF - Create a single buffer of given size (plus other non-format related aspects)
What would this ioctl give us over VIDIOC_CREATE_BUF with count=1?
- VIDIOC_DELETE_BUF - Delete a single buffer
- VIDIOC_DELETE_ALL_BUFS - Delete all buffers
The naming still requires some work. The opposite of create is "destroy", not "delete".
- struct v4l2_pix_format vs. struct v4l2_pix_format_mplane
Working with the two structs depending on whether the format is multi-planar or not is painful. While we're doing changes in the area, the two could be unified as well. (Editor note: this could be still orthogonal to the buffers, so it could be done separately as well. We'll see.)
This is a huge pain on both userspace and driver sides.
On a related note, the split into M and non-M formats also poses a userspace compatibility problem, as many drivers that expose M formats fail to expose support for corresponding non-M formats, only because the implementation becomes messy if you have to deal with both.
[snip]
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-new-ioctls.pdf
[snip]
linuxtv.org hosting - All
Mauro noted that linuxtv.org is currently hosted in a virtual machine somewhere in a German university. The administrator of the virtual machine has not been involved with Video4Linux for some time but has been kind to provide us the hosting over the years.
It has been recognised that there is a need to find a new hosting location for the virtual machine. There is also a question of the domain name linuxtv.org. Discussion followed.
What could be agreed on rather immediately was that the domain name should be owned by "us". "Us" is not a legal entity at the moment, and a practical arrangement to achieve that could be to find a new association to own the domain name.
The hosting of the virtual machine could possibly be handled by the same association. In practice this would likely mean a virtual machine on a hosting provider. Ideally this would be paid for by a company or a group of companies.
No decisions were reached on the topic.
Any chance that we could have the toolkit on linuxtv.org improved? Examples:
- The new patchwork (as on lore.kernel.org or
patchwork.freedesktop.org) has a lot of useful features that our linuxtv one misses (for example tracking of patches within a series or tracking versions of patches),
I agree with this wholeheartedly. In particular, the feature for tracking series of patches is very valuable.
Thanks, Ezequiel
Sakari,
Em Sun, 18 Nov 2018 00:45:02 +0200 Sakari Ailus sakari.ailus@iki.fi escreveu:
Hello everyone,
Sorry for taking so long to review this. Was very busy those days.
It follows my comments.
Here's the report on the Media Summit held on 25th October in Edinburgh. The report is followed by the stateless codec discussion two days earlier.
Note: this is bcc'd to the meeting attendees plus a few others. I didn't use cc as the list servers tend to reject messages with too many recipients in cc / to headers.
Most presenters used slides some of which are already available here (expect more in the near future):
URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/
The original announcement for the meeting is here:
URL:https://www.spinics.net/lists/linux-media/msg141095.html
The raw notes can be found here:
URL:http://www.retiisi.org.uk/~sailus/v4l2/notes/osseu18-media.html
Attendees
Brad Love Ezequiel Garcia Gustavo Padovan Hans Verkuil Helen Koike Hidenori Yamaji Ivan Kalinin Jacopo Mondi Kieran Bingham Laurent Pinchart Mauro Chebab Maxime Ripard Michael Grzeschik Michael Ira Krufky Niklas Söderlund Patrick Lai Paul Elder Peter Griffin Ralph Clark Ricardo Ribalda Sakari Ailus Sean Young Seung-Woo Kim Stefan Klug Vinod Koul
CEC status - Hans Verkuil
Hans prensented an update on CEC status. Besides the slides, noteworthy information is maintained here:
URL:https://hverkuil.home.xs4all.nl/cec-status.txt
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-cec-status.pdf
It makes sense to add a quick summary of the main points to the meeting report that was there at the slide deck, in order to make the report more complete.
rc-core status report - Sean Young
(Contributed by Sean Young)
Sorry, I didn't understand what you're meaning here. The status report was made by Sean. No need to repeat it.
In the last year all staging lirc drivers have been either removed or ported to rc-core. Decoding of the more obscure IR protocols and protocol variants can now be done with BPF, with support in both the kernel and ir-keytable (which is in v4l-utils). Generally we're in a good situation wrt IR support.
There is some more ancient hardware (serial or usb-serial) that does not have support but not sure if anyone cares. kernel-doc is a little sparse and does not cover BPF IR decoding, so that needs improving. There was a discussion on enabling builds with CONFIG_RC_CORE=n. Sean suggested we could have rc_allocate_driver() return NULL and have the drivers deal with this gracefully, i.e. their probe functions should continue without IR. Mauro said there should be a per-driver config option (as is done for saa7134 for example).
Please break each topic on different paragraphs, as it makes easier to read and comment.
No conclusion was reached on this.
No conclusion was reached on what?
Persistent storage of controls - Ricardo Ribalda
Ricardo gave a presentation on a proposed solution for using the V4L2 control framework as an interface for updating control value defaults on sensor EEPROM.
Sensors commonly come with device specific tuning information that's embedded in the device EEPROM. Whereas this is also very common for raw cameras on mobile devices, the discussion this time was concentrated on industrial cameras.
The EEPROM contents may be written by the sensor vendor but occasionally may need to be updated by customers. Setting the control default value was suggested as the exact mechanism to do this.
The proposal was to use controls as the interface to update sensor tuning information in the EEPROM.
There were arguments for and against the approach:
- Drivers usually get these things right: relying on an user space program to do this is an additional dependency.
- Re-use of an existing interface (root priviledge check may be added).
- Partial solution only: EEPROM contents may need to be updated for other reasons as well, and a "spotty" implementation for updating certain EEPROM locations seems very use case specific.
- Changes required to the control framework for this --- defaults are not settable at the moment.
- The need is very use case specific, and adding support for that in a generic framework does not seem to fit very well.
I remember I mentioned, as an alternative, to use the firmware API, if one wants to update the eeprom contents. If I'm not mistaken, Ricardo opted not using it.
Ricardo?
The general consensus appears to be not to change the control framework this way, but to continue to update the EEPROM using a specific user space program.
Tooling for sub-system tree maintenance - Laurent Pinchart
Laurent talked about the DRM tree maintenance model.
The DRM tree has switched to co-maintainer model. This has made it possible to share the burden of tree maintenance, removing bottlenecks they've had.
The larger number of people having (and using) their commit rights has created the need for a more strict rules for the tree maintenance, and subsequently a tool to implement it. It's called "DIM", the DRM Inglorious Maintenance tool. This is a command line tool that works as a front-end to execute the workflow.
URL:https://01.org/linuxgraphics/gfx-docs/maintainer-tools/dim.html
In particular what's worth noting:
The conflicts are resolved by the committer, not by the tree maintainer.
DIM stores conflict resolutions (as resolved by developers) to a shared cache.
DIM makes doing common mistakes harder by using sanity checks.
There are about 50 people who currently have commit rights to the DRM tree. There are no reports of commit rights having been forcibly removed as of yet. This strongly suggests that the model is workable.
The use of the tool puts additional responsibilities as well as some burden to the committers. Before the patches may be pushed, they are first compiled on developer's machine. That requires time, and without special arrangements such as having a second local workspace, and that time is away from productive work.
The discussion that followed was concentrated on the possibility of using a similar model for the media tree. While the suggestion was initially met by mostly favourable reception, there were concerns as well.
V4L2 *was* maintained generally according to the suggested model --- albeit without the proposed tools or process that needed to be strictly followed. There was once an incident which involved merging around 9000 lines of unreviewed code in a lot of places. What followed was not pretty, and this eventually lead to loss of multiple developers.
Could this happen again? The DRM tree has not suffered such incidents, and generally it understood such incident could be addressed by simply reverting such a patch and removing commit rights if necessary.
(Editor note: we have reverted the media tree master state to an earlier commit many times for various reasons. Could it be one of the reasons the 9000 line patch was not reverted was that the version control wasn't based on git??)
We actually reverted it, but it caused a huge confusion and produced lots of discussions. We lost several active developers: people that were not happy by the 9000 lines patchset stepping on everyone's feet and people that were not happy by reverting it.
Some opined that we do not have a bottleneck in reviewing patches and getting them merged whilst others thought this was not the case. It is certainly true that a very large number of patches (around 500 in the last kernel release) went in through the media tree.
It still appears that there would be more patches and more drivers to get in if the throughput was higher.
I'm not so sure about that (if we expect good quality patches), specially while we don't have any automatic testing tool to double check some stuff.
As a result of those discussions, One of the things that we've agreed there is to give trees at LinuxTV for more active developers that we trust enough to skip a sub-maintainer's review.
We also agreed to try to improve the tooling at linuxtv.org, in order to try to improve our processes (although this discussion was actually split on other topics, like KernelCI and linuxtv infra).
Current status of testing on the media tree - Sakari
The common practice in media subsystem development is that developers do test their patches before submitting them. This is an unwritten rule: sometimes patches end up not being tested after making slight changes to them, or they have been tested on a different kernel version. The developer may also simply forget to test the patch.
Besides this, it is not uncommon that changing the kernel configuration or switching to a different architecture will cause a compilation warning or an error.
The 0-day bot will catch some of these errors before the patches are merged, but that testing does not fully cover all the possible cases. There are some common pain points in V4L2-related Kconfig options (plain V4L2, MC or MC + subdev uAPI); newly submitted drivers may in fact require one of these, but the developer may not have realised that and so this ends up not being taken into account in Kconfig.
Once the review is done, and after being applied to the sub-maintainer tree, a patch is applied to Mauro's local tree and Mauro performs additional tests on it. These tests currently prevent a fair number of problems reaching a wider audience than the media developers.
On the other hand, whenever an issue is found, the patch will have to be fixed by the sub-maintainer or the developer. This is hardly ideal, as the problem has existed usually for a month or two before being spotted --- by a program. These checks should be instead performed on the patch when it's submitted.
Automated testing - Ezequiel Garcia
Ideal Continuous Integration process consists of the following steps:
- patch submission
- review and approval
- merge
The core question is "what level of quality standards do we want to enforce". The maintenance process should be modelled around this question, and not the other way around. Automated testing can be a part of enforcing the quality standards.
There are three steps:
- Define the quality standard
- Define how to quantify quality in respect to the standard
- Define how to enforce the standards
On the tooling side, an uAPI test tool exists. It's called v4l2-compliance, and new drivers are required to pass the v4l2-compliance test. It has quite a few favourable properties:
- Complete in terms of the uAPI coverage
- Quick and easy to run
- Nice output format for humans & scripts
There are some issues as well:
- No codec support (stateful or stateless)
- No SDR or touch support
- Frequently updated (distribution shipped v4l2-compliance useless)
- Only one contributor
Ezequiel noted that some people think that v4l2-compliance is changing too often but Hans responded that this is a necessity. The API gets amended occasionally and the existing API gets new tests. Mauro proposed moving v4l2-compliance to the kernel source tree but Hans preferred keeping it separate. That way it's easier to develop it.
To address the problem of only a single contributor, it was suggested that people implementing new APIs would need to provide the tests for v4l2-compliance as well. To achieve this, the v4l2-compliance codebase needs some cleanup to make it easier to contribute. The codebase is larger and there is no documentation.
V4l2-compliance also covers MC, V4L2 and V4L2 sub-device uAPIs.
DVB will require its own test tooling; it is not covered by v4l2-compliance. In order to facilitate automated testing, a virtual DVB driver would be useful as well. The task was added to the list of projects needing volunteers:
URL:https://linuxtv.org/wiki/index.php/Media_Open_Source_Projects:_Looking_for_Volunteers
There are some other test tools that could cover V4L2 but at the moment it seems somewhat far-fetched any of them would be used to test V4L2 in the near future:
- kselftest
- kunit
- gst-validate
- ktf (https://github.com/oracle/ktf, http://heim.ifi.uio.no/~knuto/ktf/)
KernelCI is a test automation system that supports automated compile and boot testing. As a newly added feature, additional tests may be implemented. This is what Collabora has implemented, effectively the current demo system runs v4l2-compliance on virtual drivers in a virtual machines (LAVA slaves).
A sample of the current test report is here:
URL:https://www.mail-archive.com/linux-media@vger.kernel.org/msg135787.html
The established way to run KernelCI tests is off the head of the branches of the stable and development kernel trees, including linux-next. This is not useful as such to support automated testing of patches for the media tree: the patches need to be tested before they are merged, not after merging.
In the discusion that followed among a slightly smaller group of people, it was suggested that tests could be run from select developer kernel trees, from any branch. If a developer needs long-term storage, (s)he could have another tree which would not be subject automated test builds. Alternatively, the branch name could be used as a basis for triggering an automated build, but this could end up being too restrictive.
Merging the next rc1 by the maintainer would be no special case: the branch would be tested in similar way than the developer branches containing patches, and tests should need to pass before pushing the content to the media tree master branch.
Ezequiel wished that people would reply to his e-mail to express their wishes on the testing needs (see sample report above).
Stateless codecs - Hans Verkuil
Support for stateless codecs will be merged for v4.20 with an Allwinner staging codec driver.
The earlier stateless codec discussion ended up concluding that the bitstream parsing is application specific, so there will be no need for a generic implementation that was previously foreseen. The question that remains is: should there be a simple parser for compliance testing?
All main applications support libva which was developed as the codec API to be used with Intel GPUs. A libVA frontend was written to support the Cedrus stateless V4L2 decoder driver. It remains to be seen whether the same implementation could be used as such for the other stateless codec drivers or whether changes, or in the worst case a parallel implementation, would be needed.
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-codec-userspace.pdf
New versions of the old IOCTLs - Hans Verkuil
V4L2 is an old API with shifting focus in terms of functionality and hardware supported. While there has been lots of changes to the two during the existence of V4L2, some of the API is unchanged since the old times. While the API is usable for the purpose, it is needlessly clunky: it is often not obvious how an IOCTL is related to the task at hand (such as using S_PARM to set the frame interval) or the API does not use year 2038-safe timestamps (struct v4l2_buffer). These APIs deserve to be updated.
- VIDIOC_*_PARM
In the case of VIDIOC_G_PARM and VIDIOC_S_PARM, the IOCTLs are only used to set and get the frame interval.
In this case, what can be done, is to add a new IOCTL definition, with the same IOCTL number and with binary-equivalent IOCTL argument struct that only contains the field for the frame rate itself. This is binary-compatible with the existing code and no compatibility code will be needed. The new IOCTLs will be called VIDIOC_G_FRAME_INTERVAL and VIDIOC_S_FRAME_INTERVAL.
- VIDIOC_ENUM_FRAME_INTERVALS
Besides discrete set of supported frame intervals, VIDIOC_ENUM_FRAME_INTERVALS has stepwise frame interval as well. Stepwise could be removed as the Qualcomm venus codec and uvc (100 ns units) are the only users. Additionally, the buffer type should be added to struct v4l2_frmivalenum.
There was also a discussion related to enumerating frame intervals in units of ns vs. fractional seconds. The reasoning using a fraction is that this way the frame interval on many standards can be conveyed precisely. Somebody recalled "flick", that is is the common denominator of the frame rates on all TV standards. Drivers could simply move to use the flick as the denominator, to make frame interval reporting uniform across the drivers.
- struct v4l2_buffer
struct v4l2_buffer is an age-old struct. There are a few issues in it:
- The timestamp is not 2038-safe.
- The multi-plane implementation is a mess.
- Differing implementation for the end single-plane and multi-plane APIs is confusing for both applications and drivers.
The proposal is to create a new v4l2_buffer struct. The differences to the old one would be:
- __u64 timestamps. These are 2038-safe. The timestamp source is maintained, i.e. the type remains CLOCK_MONOTONIC apart from certain drivers (e.g. UVC) that lets the user choose the timestamp.
- Put the planes right to struct v4l2_buffer. The plane struct would also be changed; the new plane struct would be called v4l2_ext_plane.
- While at it, the plane description can be improved:
- The start of data from the beginning of the plane memory.
- Add width and height to the buffer? This would make image size changes easier for the codec. (Ed. note: pixel format as well. But this approach could only partially support what the request API is for.)
- Unify single- and multi-planar APIs.
The new struct could be called v4l2_ext_buffer.
As the new IOCTL argument struct will have has different syntax as well as
s/have has/have/
semantics, it deserves to be named differently. Compatibility code will be needed to convert the users of the old IOCTLs to the new struct used internally by the kernel and drivers, and then back to the user.
- struct v4l2_create_buffers
Of the format, only the pix.fmt.sizeimage field is effectively used by the drivers supporting VIDIOC_CREATE_BUFS. This could be simplified, by just providing the desired buffer size instead of the entire v4l2_format struct. The user would be instructed to use TRY_FMT to obtain that buffer size.
The need to delete buffers seems to have eventually surfaced. That was expected, but it wasn't known when this would happen. As the buffer index range would become non-contiguous, it should be possible to create buffers one by one only, as otherwise the indices of the additional buffers would no longer be communicated to the user unambiguously.
So there would be new IOCTLs:
- VIDIOC_CREATE_BUF - Create a single buffer of given size (plus other non-format related aspects)
- VIDIOC_DELETE_BUF - Delete a single buffer
- VIDIOC_DELETE_ALL_BUFS - Delete all buffers
The naming still requires some work. The opposite of create is "destroy", not "delete".
- struct v4l2_pix_format vs. struct v4l2_pix_format_mplane
Working with the two structs depending on whether the format is multi-planar or not is painful. While we're doing changes in the area, the two could be unified as well.
(Editor note: this could be still orthogonal to the buffers, so it could be done separately as well. We'll see.)
I suspect that those "editor note" (as any post-meeting notes) don't belong to the final report.
But yeah, perhaps this could be done seprarately. Let's discuss it when actual patches gets posted.
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-new-ioctls.pdf
I guess there was an action plan for that, based on the discussions (maybe some of them ended by being merged with the presentation on the above?).
Hans,
Did you take any notes about the actions to be taken? I found very helpful to have an action plan item below the topics where we made such plan.
Fault tolerant V4L2 - Kieran Bingham
Kieran presented a system where the media hardware complex consisted of eight more or less independent camera sensors that naturally end up being within a single media device.
The current implementation, as well as the API, necessitates that all devices in a media device probe successfully before the entire media device is exposed to the user. Otherwise the user would see with a partial view of the device, without the knowledge it is such.
To address the problem, additional information need to be provided to the user space. In particular:
Events on the media device to tell the graph has changed.
Graph version number is incremented at graph change (already implemented).
The property API could be applicable --- placeholders for entities that have not yet appeared?
- Alternative: known entities that have failed to probe created in the media graph and marked "disable" or "failed".
Query the state of media graph completeness.
That way, even when the devices in a media controller device appear one by one, the user space will be able to have all the necessary information on the registration state of the device.
Complex cameras - Mauro Chehab
Some new laptops integrate a raw Bayer camera + ISP instead of a USB webcam. This is expected to increase, as the solution is generally cheaper and results in better quality images --- as long as all the pieces of the puzzle are in place, including the proprietary 3A library.
Still, such devices need to be supported.
(Ed. note: there were two talks related to this topic given in the ELc-E.)
URL:https://www.youtube.com/watch?v=KpaNNJr92CY&index=31&list=PLbzoR-pLrL6qThA7SAbhVfuMbjZsJX1CY URL:https://www.youtube.com/watch?v=GIhV7tiUji0&index=60&list=PLbzoR-pLrL6qThA7SAbhVfuMbjZsJX1CY
In this specific case, it is worth to keep the note, as those presentations happened at ELC-Eu and were explicitly mentioned there during the discussions.
Development process - All
Topic-wise this is continuation of the "Tooling for sub-system tree maintenance", "Current status of testing on the media tree" and "Automated testing" topics above.
The question here is whether there's something that could be improved in the media development process and if so, how could that be done.
What came up was a suggestion to have multi-committer tree in a similar manner as the DRM developers do. This was seen to be more interesting for developers than simply being asked to review patches.
It certainly does raise the need for more precise rules for what may be committed to the multi-committer tree, when etc.
It was also requested that experienced driver maintainers would send pull requests on patches to their drivers instead of going through a sub-maintainer (pre-agreed with the relevant (sub)maintainer). This would take some work away from sub-maintainers, but not the maintainer.
No firm decisions were reached in this topic. Perhaps this could be tried out?
We did decide to experment the "experienced driver" maintainership model.
Btw, I already added an account for one such developer :-)
There was also a request to document the sub-maintainer names in the wiki so that it'd be easier for people to figure out who to ping if their patches do not get merged.
I'm ok with that, but, after the LPC, I suspect that the best is to document it in sync with the per-subsystem profile. I'm waiting for Don to submit an updated patchset, in order to rebase our subsystem's profile.
linuxtv.org hosting - All
Mauro noted that linuxtv.org is currently hosted in a virtual machine somewhere in a German university. The administrator of the virtual machine has not been involved with Video4Linux for some time but has been kind to provide us the hosting over the years.
It has been recognised that there is a need to find a new hosting location for the virtual machine. There is also a question of the domain name linuxtv.org. Discussion followed.
What could be agreed on rather immediately was that the domain name should be owned by "us". "Us" is not a legal entity at the moment, and a practical arrangement to achieve that could be to find a new association to own the domain name.
The hosting of the virtual machine could possibly be handled by the same association. In practice this would likely mean a virtual machine on a hosting provider. Ideally this would be paid for by a company or a group of companies.
No decisions were reached on the topic.
There was actually one decision: to talk with Linux Foundation about that. Laurent was against, but the majority was ok with the idea.
Tuesday's stateless codec discussion
Hans presented a summary of this in his stateless codec status presentation, here are a bit more details.
We had a discussion (first in the Microsoft sponsor suite, then at the bar)
I don't think that the room location is relevant for the report :-)
Also, we should split this on a separate report, as this was another meeting and not all people listed above participated on it.
on how to support user space for the stateless codecs better. The expected outcome of that would be a rough understanding how a stateless codec user space library would look like.
The raw notes are available here:
URL:http://www.retiisi.org.uk/~sailus/v4l2/notes/osseu18-codecs.html
Attendees
Alexandre Courbot Chris Healy Ezequiel Garcia Hans Verkuil Kieran Bingham Laurent Pinchart Maxime Ripard Mauro Carvalho Chehab Nicolas Dufresne Niklas Söderlund Philip Zabell Sakari Ailus Tomasz Figa Victor Jáquez
Buffer management
Nicolas reported an issue in V4L2 buffer management. The V4L2 decouples the buffers from the format, and assumes all queued buffers (at a given point of time) have the same format. (Ed. note: the request API could be used to address this, but that particular features is not yet supported.)
- User space library
The existing projects generally integrate their own bitstream parsers for codecs. There are subtle reasons why that tends to be the case, instead of using more generic parsers. There are differences in error handling, for instance, or other matters of policy, the variation which could be difficult to fully offer using a generic API.
Maxime noted that VLC recently released a new parser meant to be used as a library, and that could be useful. Nicolas believes that we'd need a parser library independent of any other code base to avoid pulling in extra libraries and this parser would need to be maintained. It could be difficult to find the volunteers to do that.
Does ChromeOS have its own parser? Alexandre believes it does, but little was known beyond that.
There's also the language problem: ffmpeg and gstreamer are written in C, the ChomeOS parser in C++, VLC is moving to Rust. What do we pick, how do we ensure interoperability?
- libVA re-use
As a short-term solution, implementing a generic wrapper using the V4L2 stateless codec API to offer libVA API would enable generic applications to use the V4L2 stateless codec drivers as most applications already support libVA.
70 % of the applications use FFMPEG, which has a software codec API that is nearly identical to the V4L2 statless codec API. It would be trivial for applications to switch to V4L2 natively.
Mauro would like us to explain our plans to Intel to avoid surprises later on.
To be clear: it was said at the meeting that libVA is sponsored an maintained by Intel. If we're willing to use it, we should sync with them, in order to avoid unexpected surprises if they change it in a way that would cause problems for the V4L2 stateless coded implementation.
- Source code hosting
libva is hosted on freedesktop. Should we host the libva-v4l2-codec backend there, or host it on linuxtv.org? Hans would prefer linuxtv.org as it's "closer to our kernel implementation".
- Backend support in libva
libva loads backends in order, and picks the first one that reports it can support the platform. There is also an environment variable that can specify a backend. Ezequiel enquired how to support platforms that would have multiple hardware codecs. libva doesn't seem to support this at the moment. Nicolas reported that there's an Intel SoC that have both an Intel graphics core and a Vega64 graphics core that both have a codec.
Hans said that a platform that expose multiple codecs will likely be used for specialized applications, and requiring those to implement codec support directly is acceptable. Our main focus should be to support the common case.
- Vendor support
NVidia is following our progress and is interested in using the V4L2 stateless API. On the userspace side, vdpau is pretty much dead, they have moved to nvdec. OMX is being phasing out, in particular that is taking place for RaspberryPi now.
- Tooling
bootlin has developed a debugging tool called v4l2-request-test (https://github.com/bootlin/v4l2-request-test) that has been very useful to debug the codec driver without going through the full userspace stack. This is worth mentioning and integrating.
- API discussions
Using buffer indices as handles to reference frames
This has been proposed by Tomasz, and Hans has serious concerns, he believes that having userspace predict what buffer indices will be used in the future is very fragile and would prefer using a separate 64-bit cookie associated with v4l2_buffers.
Using capture buffer indices as reference frame handles requires predicting the buffer index on the capture queue which the output queue frames will be decoded into. We could use the output queue buffer index instead, but that wouldn't work with multi-slice decoding (multiple output buffers for a single capture buffer). Using a cookie set by userspace on the output side, then copied to the capture queue by the driver, solves that problem. All slices queued on the output queue for the same decoded picture will have the same cookie value (userspace will have to ensure that).
Tomasz would prefer a buffer index-based solution, to avoid keeping a cookie-index map in userspace. Due to how V4L2 works, enqueuing a new dmabuf handle on the capture side for a V4L2 buffer with a given index will effectively delete the corresponding cookie, so userspace would need to ensure it doesn't overwrite buffers; (Tomasz: To clarify, I don't see the significant benefit of using cookies over indices. It makes it easier for user space, because it doesn't have to predict the CAPTURE buffers, but still is error prone because of the buffer requeuing problem. For now it would be good to see how it translates into real code, though. In the meantime I can try to find a better idea.)
Once we have a final version for both Tuesday meeting and the Linux Media Summit, I'll post it at the "news" section of linuxtv, add the group photo and the links for the presentation and for our nightly dinner.
Thanks, Mauro
Hi Mauro,
On Wed, Dec 12, 2018 at 05:30:02AM -0200, Mauro Carvalho Chehab wrote:
Sakari,
Em Sun, 18 Nov 2018 00:45:02 +0200 Sakari Ailus sakari.ailus@iki.fi escreveu:
Hello everyone,
Sorry for taking so long to review this. Was very busy those days.
Likewise in my reply. Please see my comments below. Let me know if you're fine with the proposed changes.
It follows my comments.
Here's the report on the Media Summit held on 25th October in Edinburgh. The report is followed by the stateless codec discussion two days earlier.
Note: this is bcc'd to the meeting attendees plus a few others. I didn't use cc as the list servers tend to reject messages with too many recipients in cc / to headers.
Most presenters used slides some of which are already available here (expect more in the near future):
URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/
The original announcement for the meeting is here:
URL:https://www.spinics.net/lists/linux-media/msg141095.html
The raw notes can be found here:
URL:http://www.retiisi.org.uk/~sailus/v4l2/notes/osseu18-media.html
Attendees
Brad Love Ezequiel Garcia Gustavo Padovan Hans Verkuil Helen Koike Hidenori Yamaji Ivan Kalinin Jacopo Mondi Kieran Bingham Laurent Pinchart Mauro Chebab Maxime Ripard Michael Grzeschik Michael Ira Krufky Niklas Söderlund Patrick Lai Paul Elder Peter Griffin Ralph Clark Ricardo Ribalda Sakari Ailus Sean Young Seung-Woo Kim Stefan Klug Vinod Koul
CEC status - Hans Verkuil
Hans prensented an update on CEC status. Besides the slides, noteworthy information is maintained here:
URL:https://hverkuil.home.xs4all.nl/cec-status.txt
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-cec-status.pdf
It makes sense to add a quick summary of the main points to the meeting report that was there at the slide deck, in order to make the report more complete.
Bullet points from the slide:
- cec-gpio error injection support
- tda998x (including BeagleBoard Bone support after gpiolib changes)
- ChromeOS EC CEC
- In progress: omap5/dra7xx/am57xx TI (waiting for DSS redesign to land)
- In progress: SECO cec driver (for UDOO x86 boards, expected for 4.21)
- DisplayPort CEC-Tunneling-over-AUX for i915, nouveau, amdgpu
- MegaChips 2900 chipset based adapters seems to support this protocol very well
- Continuing work on CEC utilities, esp. the compliance test: it is in continuous use at Cisco.
rc-core status report - Sean Young
(Contributed by Sean Young)
Sorry, I didn't understand what you're meaning here. The status report was made by Sean. No need to repeat it.
Sean wrote the summary as well, the rest is written by me.
In the last year all staging lirc drivers have been either removed or ported to rc-core. Decoding of the more obscure IR protocols and protocol variants can now be done with BPF, with support in both the kernel and ir-keytable (which is in v4l-utils). Generally we're in a good situation wrt IR support.
There is some more ancient hardware (serial or usb-serial) that does not have support but not sure if anyone cares. kernel-doc is a little sparse and does not cover BPF IR decoding, so that needs improving. There was a discussion on enabling builds with CONFIG_RC_CORE=n. Sean suggested we could have rc_allocate_driver() return NULL and have the drivers deal with this gracefully, i.e. their probe functions should continue without IR. Mauro said there should be a per-driver config option (as is done for saa7134 for example).
Please break each topic on different paragraphs, as it makes easier to read and comment.
No conclusion was reached on this.
No conclusion was reached on what?
That probably raises more questions than it answers. I'll drop the line.
Persistent storage of controls - Ricardo Ribalda
Ricardo gave a presentation on a proposed solution for using the V4L2 control framework as an interface for updating control value defaults on sensor EEPROM.
Sensors commonly come with device specific tuning information that's embedded in the device EEPROM. Whereas this is also very common for raw cameras on mobile devices, the discussion this time was concentrated on industrial cameras.
The EEPROM contents may be written by the sensor vendor but occasionally may need to be updated by customers. Setting the control default value was suggested as the exact mechanism to do this.
The proposal was to use controls as the interface to update sensor tuning information in the EEPROM.
There were arguments for and against the approach:
- Drivers usually get these things right: relying on an user space program to do this is an additional dependency.
- Re-use of an existing interface (root priviledge check may be added).
- Partial solution only: EEPROM contents may need to be updated for other reasons as well, and a "spotty" implementation for updating certain EEPROM locations seems very use case specific.
- Changes required to the control framework for this --- defaults are not settable at the moment.
- The need is very use case specific, and adding support for that in a generic framework does not seem to fit very well.
I remember I mentioned, as an alternative, to use the firmware API, if one wants to update the eeprom contents. If I'm not mistaken, Ricardo opted not using it.
Ricardo?
I let Ricardo to comment that.
The general consensus appears to be not to change the control framework this way, but to continue to update the EEPROM using a specific user space program.
Tooling for sub-system tree maintenance - Laurent Pinchart
Laurent talked about the DRM tree maintenance model.
The DRM tree has switched to co-maintainer model. This has made it possible to share the burden of tree maintenance, removing bottlenecks they've had.
The larger number of people having (and using) their commit rights has created the need for a more strict rules for the tree maintenance, and subsequently a tool to implement it. It's called "DIM", the DRM Inglorious Maintenance tool. This is a command line tool that works as a front-end to execute the workflow.
URL:https://01.org/linuxgraphics/gfx-docs/maintainer-tools/dim.html
In particular what's worth noting:
The conflicts are resolved by the committer, not by the tree maintainer.
DIM stores conflict resolutions (as resolved by developers) to a shared cache.
DIM makes doing common mistakes harder by using sanity checks.
There are about 50 people who currently have commit rights to the DRM tree. There are no reports of commit rights having been forcibly removed as of yet. This strongly suggests that the model is workable.
The use of the tool puts additional responsibilities as well as some burden to the committers. Before the patches may be pushed, they are first compiled on developer's machine. That requires time, and without special arrangements such as having a second local workspace, and that time is away from productive work.
The discussion that followed was concentrated on the possibility of using a similar model for the media tree. While the suggestion was initially met by mostly favourable reception, there were concerns as well.
V4L2 *was* maintained generally according to the suggested model --- albeit without the proposed tools or process that needed to be strictly followed. There was once an incident which involved merging around 9000 lines of unreviewed code in a lot of places. What followed was not pretty, and this eventually lead to loss of multiple developers.
Could this happen again? The DRM tree has not suffered such incidents, and generally it understood such incident could be addressed by simply reverting such a patch and removing commit rights if necessary.
(Editor note: we have reverted the media tree master state to an earlier commit many times for various reasons. Could it be one of the reasons the 9000 line patch was not reverted was that the version control wasn't based on git??)
We actually reverted it, but it caused a huge confusion and produced lots of discussions. We lost several active developers: people that were not happy by the 9000 lines patchset stepping on everyone's feet and people that were not happy by reverting it.
Do you happen to remember any details? Did that "reverting" for instance involve rolling back to the state before the offending patch after more comments had been done?
That said, I feel this is not overly important. The DRM folks have proved this model works. Still I agree this is good to remember and document, but I don't see us getting into such situation _even if_ we'd switch to a similar way of working.
Some opined that we do not have a bottleneck in reviewing patches and getting them merged whilst others thought this was not the case. It is certainly true that a very large number of patches (around 500 in the last kernel release) went in through the media tree.
It still appears that there would be more patches and more drivers to get in if the throughput was higher.
I'm not so sure about that (if we expect good quality patches), specially while we don't have any automatic testing tool to double check some stuff.
I agree. To help improving the process from here, we do need automated testing. I don't think anyone has really even argued against adding automated testing.
Considering the amount of coverage in the meeting as well as the interest in general, it's just a question of time until we have something quite usable.
As a result of those discussions, One of the things that we've agreed there is to give trees at LinuxTV for more active developers that we trust enough to skip a sub-maintainer's review.
We used to have more active developer involvement. Anyway, I'll add a note on this.
We also agreed to try to improve the tooling at linuxtv.org, in order to try to improve our processes (although this discussion was actually split on other topics, like KernelCI and linuxtv infra).
How about:
It was also agreed to try to improve tooling at linuxtv.org to streamline the workflow. (Ed. note: also see testing related topics below.)
Current status of testing on the media tree - Sakari
The common practice in media subsystem development is that developers do test their patches before submitting them. This is an unwritten rule: sometimes patches end up not being tested after making slight changes to them, or they have been tested on a different kernel version. The developer may also simply forget to test the patch.
Besides this, it is not uncommon that changing the kernel configuration or switching to a different architecture will cause a compilation warning or an error.
The 0-day bot will catch some of these errors before the patches are merged, but that testing does not fully cover all the possible cases. There are some common pain points in V4L2-related Kconfig options (plain V4L2, MC or MC + subdev uAPI); newly submitted drivers may in fact require one of these, but the developer may not have realised that and so this ends up not being taken into account in Kconfig.
Once the review is done, and after being applied to the sub-maintainer tree, a patch is applied to Mauro's local tree and Mauro performs additional tests on it. These tests currently prevent a fair number of problems reaching a wider audience than the media developers.
On the other hand, whenever an issue is found, the patch will have to be fixed by the sub-maintainer or the developer. This is hardly ideal, as the problem has existed usually for a month or two before being spotted --- by a program. These checks should be instead performed on the patch when it's submitted.
Automated testing - Ezequiel Garcia
Ideal Continuous Integration process consists of the following steps:
- patch submission
- review and approval
- merge
The core question is "what level of quality standards do we want to enforce". The maintenance process should be modelled around this question, and not the other way around. Automated testing can be a part of enforcing the quality standards.
There are three steps:
- Define the quality standard
- Define how to quantify quality in respect to the standard
- Define how to enforce the standards
On the tooling side, an uAPI test tool exists. It's called v4l2-compliance, and new drivers are required to pass the v4l2-compliance test. It has quite a few favourable properties:
- Complete in terms of the uAPI coverage
- Quick and easy to run
- Nice output format for humans & scripts
There are some issues as well:
- No codec support (stateful or stateless)
- No SDR or touch support
- Frequently updated (distribution shipped v4l2-compliance useless)
- Only one contributor
Ezequiel noted that some people think that v4l2-compliance is changing too often but Hans responded that this is a necessity. The API gets amended occasionally and the existing API gets new tests. Mauro proposed moving v4l2-compliance to the kernel source tree but Hans preferred keeping it separate. That way it's easier to develop it.
To address the problem of only a single contributor, it was suggested that people implementing new APIs would need to provide the tests for v4l2-compliance as well. To achieve this, the v4l2-compliance codebase needs some cleanup to make it easier to contribute. The codebase is larger and there is no documentation.
V4l2-compliance also covers MC, V4L2 and V4L2 sub-device uAPIs.
DVB will require its own test tooling; it is not covered by v4l2-compliance. In order to facilitate automated testing, a virtual DVB driver would be useful as well. The task was added to the list of projects needing volunteers:
URL:https://linuxtv.org/wiki/index.php/Media_Open_Source_Projects:_Looking_for_Volunteers
There are some other test tools that could cover V4L2 but at the moment it seems somewhat far-fetched any of them would be used to test V4L2 in the near future:
- kselftest
- kunit
- gst-validate
- ktf (https://github.com/oracle/ktf, http://heim.ifi.uio.no/~knuto/ktf/)
KernelCI is a test automation system that supports automated compile and boot testing. As a newly added feature, additional tests may be implemented. This is what Collabora has implemented, effectively the current demo system runs v4l2-compliance on virtual drivers in a virtual machines (LAVA slaves).
A sample of the current test report is here:
URL:https://www.mail-archive.com/linux-media@vger.kernel.org/msg135787.html
The established way to run KernelCI tests is off the head of the branches of the stable and development kernel trees, including linux-next. This is not useful as such to support automated testing of patches for the media tree: the patches need to be tested before they are merged, not after merging.
In the discusion that followed among a slightly smaller group of people, it was suggested that tests could be run from select developer kernel trees, from any branch. If a developer needs long-term storage, (s)he could have another tree which would not be subject automated test builds. Alternatively, the branch name could be used as a basis for triggering an automated build, but this could end up being too restrictive.
Merging the next rc1 by the maintainer would be no special case: the branch would be tested in similar way than the developer branches containing patches, and tests should need to pass before pushing the content to the media tree master branch.
Ezequiel wished that people would reply to his e-mail to express their wishes on the testing needs (see sample report above).
Stateless codecs - Hans Verkuil
Support for stateless codecs will be merged for v4.20 with an Allwinner staging codec driver.
The earlier stateless codec discussion ended up concluding that the bitstream parsing is application specific, so there will be no need for a generic implementation that was previously foreseen. The question that remains is: should there be a simple parser for compliance testing?
All main applications support libva which was developed as the codec API to be used with Intel GPUs. A libVA frontend was written to support the Cedrus stateless V4L2 decoder driver. It remains to be seen whether the same implementation could be used as such for the other stateless codec drivers or whether changes, or in the worst case a parallel implementation, would be needed.
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-codec-userspace.pdf
New versions of the old IOCTLs - Hans Verkuil
V4L2 is an old API with shifting focus in terms of functionality and hardware supported. While there has been lots of changes to the two during the existence of V4L2, some of the API is unchanged since the old times. While the API is usable for the purpose, it is needlessly clunky: it is often not obvious how an IOCTL is related to the task at hand (such as using S_PARM to set the frame interval) or the API does not use year 2038-safe timestamps (struct v4l2_buffer). These APIs deserve to be updated.
- VIDIOC_*_PARM
In the case of VIDIOC_G_PARM and VIDIOC_S_PARM, the IOCTLs are only used to set and get the frame interval.
In this case, what can be done, is to add a new IOCTL definition, with the same IOCTL number and with binary-equivalent IOCTL argument struct that only contains the field for the frame rate itself. This is binary-compatible with the existing code and no compatibility code will be needed. The new IOCTLs will be called VIDIOC_G_FRAME_INTERVAL and VIDIOC_S_FRAME_INTERVAL.
- VIDIOC_ENUM_FRAME_INTERVALS
Besides discrete set of supported frame intervals, VIDIOC_ENUM_FRAME_INTERVALS has stepwise frame interval as well. Stepwise could be removed as the Qualcomm venus codec and uvc (100 ns units) are the only users. Additionally, the buffer type should be added to struct v4l2_frmivalenum.
There was also a discussion related to enumerating frame intervals in units of ns vs. fractional seconds. The reasoning using a fraction is that this way the frame interval on many standards can be conveyed precisely. Somebody recalled "flick", that is is the common denominator of the frame rates on all TV standards. Drivers could simply move to use the flick as the denominator, to make frame interval reporting uniform across the drivers.
- struct v4l2_buffer
struct v4l2_buffer is an age-old struct. There are a few issues in it:
- The timestamp is not 2038-safe.
- The multi-plane implementation is a mess.
- Differing implementation for the end single-plane and multi-plane APIs is confusing for both applications and drivers.
The proposal is to create a new v4l2_buffer struct. The differences to the old one would be:
- __u64 timestamps. These are 2038-safe. The timestamp source is maintained, i.e. the type remains CLOCK_MONOTONIC apart from certain drivers (e.g. UVC) that lets the user choose the timestamp.
- Put the planes right to struct v4l2_buffer. The plane struct would also be changed; the new plane struct would be called v4l2_ext_plane.
- While at it, the plane description can be improved:
- The start of data from the beginning of the plane memory.
- Add width and height to the buffer? This would make image size changes easier for the codec. (Ed. note: pixel format as well. But this approach could only partially support what the request API is for.)
- Unify single- and multi-planar APIs.
The new struct could be called v4l2_ext_buffer.
As the new IOCTL argument struct will have has different syntax as well as
s/have has/have/
Will fix.
semantics, it deserves to be named differently. Compatibility code will be needed to convert the users of the old IOCTLs to the new struct used internally by the kernel and drivers, and then back to the user.
- struct v4l2_create_buffers
Of the format, only the pix.fmt.sizeimage field is effectively used by the drivers supporting VIDIOC_CREATE_BUFS. This could be simplified, by just providing the desired buffer size instead of the entire v4l2_format struct. The user would be instructed to use TRY_FMT to obtain that buffer size.
The need to delete buffers seems to have eventually surfaced. That was expected, but it wasn't known when this would happen. As the buffer index range would become non-contiguous, it should be possible to create buffers one by one only, as otherwise the indices of the additional buffers would no longer be communicated to the user unambiguously.
So there would be new IOCTLs:
- VIDIOC_CREATE_BUF - Create a single buffer of given size (plus other non-format related aspects)
- VIDIOC_DELETE_BUF - Delete a single buffer
- VIDIOC_DELETE_ALL_BUFS - Delete all buffers
The naming still requires some work. The opposite of create is "destroy", not "delete".
- struct v4l2_pix_format vs. struct v4l2_pix_format_mplane
Working with the two structs depending on whether the format is multi-planar or not is painful. While we're doing changes in the area, the two could be unified as well.
(Editor note: this could be still orthogonal to the buffers, so it could be done separately as well. We'll see.)
I suspect that those "editor note" (as any post-meeting notes) don't belong to the final report.
I wanted to make the report more readable for people who are not actively working on V4L2 development and are thus likely not able to make such connections.
But yeah, perhaps this could be done seprarately. Let's discuss it when actual patches gets posted.
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-new-ioctls.pdf
I guess there was an action plan for that, based on the discussions (maybe some of them ended by being merged with the presentation on the above?).
Hans,
Did you take any notes about the actions to be taken? I found very helpful to have an action plan item below the topics where we made such plan.
Fault tolerant V4L2 - Kieran Bingham
Kieran presented a system where the media hardware complex consisted of eight more or less independent camera sensors that naturally end up being within a single media device.
The current implementation, as well as the API, necessitates that all devices in a media device probe successfully before the entire media device is exposed to the user. Otherwise the user would see with a partial view of the device, without the knowledge it is such.
To address the problem, additional information need to be provided to the user space. In particular:
Events on the media device to tell the graph has changed.
Graph version number is incremented at graph change (already implemented).
The property API could be applicable --- placeholders for entities that have not yet appeared?
- Alternative: known entities that have failed to probe created in the media graph and marked "disable" or "failed".
Query the state of media graph completeness.
That way, even when the devices in a media controller device appear one by one, the user space will be able to have all the necessary information on the registration state of the device.
Complex cameras - Mauro Chehab
Some new laptops integrate a raw Bayer camera + ISP instead of a USB webcam. This is expected to increase, as the solution is generally cheaper and results in better quality images --- as long as all the pieces of the puzzle are in place, including the proprietary 3A library.
Still, such devices need to be supported.
(Ed. note: there were two talks related to this topic given in the ELc-E.)
URL:https://www.youtube.com/watch?v=KpaNNJr92CY&index=31&list=PLbzoR-pLrL6qThA7SAbhVfuMbjZsJX1CY URL:https://www.youtube.com/watch?v=GIhV7tiUji0&index=60&list=PLbzoR-pLrL6qThA7SAbhVfuMbjZsJX1CY
In this specific case, it is worth to keep the note, as those presentations happened at ELC-Eu and were explicitly mentioned there during the discussions.
Development process - All
Topic-wise this is continuation of the "Tooling for sub-system tree maintenance", "Current status of testing on the media tree" and "Automated testing" topics above.
The question here is whether there's something that could be improved in the media development process and if so, how could that be done.
What came up was a suggestion to have multi-committer tree in a similar manner as the DRM developers do. This was seen to be more interesting for developers than simply being asked to review patches.
It certainly does raise the need for more precise rules for what may be committed to the multi-committer tree, when etc.
It was also requested that experienced driver maintainers would send pull requests on patches to their drivers instead of going through a sub-maintainer (pre-agreed with the relevant (sub)maintainer). This would take some work away from sub-maintainers, but not the maintainer.
No firm decisions were reached in this topic. Perhaps this could be tried out?
We did decide to experment the "experienced driver" maintainership model.
Btw, I already added an account for one such developer :-)
Ok, so we're actually trying this out. Great! :-)
There was also a request to document the sub-maintainer names in the wiki so that it'd be easier for people to figure out who to ping if their patches do not get merged.
I'm ok with that, but, after the LPC, I suspect that the best is to document it in sync with the per-subsystem profile. I'm waiting for Don to submit an updated patchset, in order to rebase our subsystem's profile.
Ok. There's a list in the wiki but I think few people end up finding it when they needed it. :-I
linuxtv.org hosting - All
Mauro noted that linuxtv.org is currently hosted in a virtual machine somewhere in a German university. The administrator of the virtual machine has not been involved with Video4Linux for some time but has been kind to provide us the hosting over the years.
It has been recognised that there is a need to find a new hosting location for the virtual machine. There is also a question of the domain name linuxtv.org. Discussion followed.
What could be agreed on rather immediately was that the domain name should be owned by "us". "Us" is not a legal entity at the moment, and a practical arrangement to achieve that could be to find a new association to own the domain name.
The hosting of the virtual machine could possibly be handled by the same association. In practice this would likely mean a virtual machine on a hosting provider. Ideally this would be paid for by a company or a group of companies.
No decisions were reached on the topic.
There was actually one decision: to talk with Linux Foundation about that. Laurent was against, but the majority was ok with the idea.
I remember Laurent was not the only one expressing concerns related to using such hosting providers. I rather remember hearing this from quite a few people in the discussion. Either way, the decisions related to e.g. hosting will be taken later when more information is available, including what LF has to offer.
How about replacing the original last paragraph with:
Mauro will discuss with LF to find out what they can offer. Concerns were expressed over other organisations providing us with hosting we are not in charge of ourselves. Other options related to domain ownership and hosting will be researched as well.
Tuesday's stateless codec discussion
Hans presented a summary of this in his stateless codec status presentation, here are a bit more details.
We had a discussion (first in the Microsoft sponsor suite, then at the bar)
I don't think that the room location is relevant for the report :-)
:-D
Also, we should split this on a separate report, as this was another meeting and not all people listed above participated on it.
Works for me.
on how to support user space for the stateless codecs better. The expected outcome of that would be a rough understanding how a stateless codec user space library would look like.
The raw notes are available here:
URL:http://www.retiisi.org.uk/~sailus/v4l2/notes/osseu18-codecs.html
Attendees
Alexandre Courbot Chris Healy Ezequiel Garcia Hans Verkuil Kieran Bingham Laurent Pinchart Maxime Ripard Mauro Carvalho Chehab Nicolas Dufresne Niklas Söderlund Philip Zabell Sakari Ailus Tomasz Figa Victor Jáquez
Buffer management
Nicolas reported an issue in V4L2 buffer management. The V4L2 decouples the buffers from the format, and assumes all queued buffers (at a given point of time) have the same format. (Ed. note: the request API could be used to address this, but that particular features is not yet supported.)
- User space library
The existing projects generally integrate their own bitstream parsers for codecs. There are subtle reasons why that tends to be the case, instead of using more generic parsers. There are differences in error handling, for instance, or other matters of policy, the variation which could be difficult to fully offer using a generic API.
Maxime noted that VLC recently released a new parser meant to be used as a library, and that could be useful. Nicolas believes that we'd need a parser library independent of any other code base to avoid pulling in extra libraries and this parser would need to be maintained. It could be difficult to find the volunteers to do that.
Does ChromeOS have its own parser? Alexandre believes it does, but little was known beyond that.
There's also the language problem: ffmpeg and gstreamer are written in C, the ChomeOS parser in C++, VLC is moving to Rust. What do we pick, how do we ensure interoperability?
- libVA re-use
As a short-term solution, implementing a generic wrapper using the V4L2 stateless codec API to offer libVA API would enable generic applications to use the V4L2 stateless codec drivers as most applications already support libVA.
70 % of the applications use FFMPEG, which has a software codec API that is nearly identical to the V4L2 statless codec API. It would be trivial for applications to switch to V4L2 natively.
Mauro would like us to explain our plans to Intel to avoid surprises later on.
To be clear: it was said at the meeting that libVA is sponsored an maintained by Intel. If we're willing to use it, we should sync with them, in order to avoid unexpected surprises if they change it in a way that would cause problems for the V4L2 stateless coded implementation.
How about, instead of the original:
We need to explain our plans to libVA maintainers to better coordinate libVA API development in a way the V4L2 stateless codecs are taken into account.
- Source code hosting
libva is hosted on freedesktop. Should we host the libva-v4l2-codec backend there, or host it on linuxtv.org? Hans would prefer linuxtv.org as it's "closer to our kernel implementation".
- Backend support in libva
libva loads backends in order, and picks the first one that reports it can support the platform. There is also an environment variable that can specify a backend. Ezequiel enquired how to support platforms that would have multiple hardware codecs. libva doesn't seem to support this at the moment. Nicolas reported that there's an Intel SoC that have both an Intel graphics core and a Vega64 graphics core that both have a codec.
Hans said that a platform that expose multiple codecs will likely be used for specialized applications, and requiring those to implement codec support directly is acceptable. Our main focus should be to support the common case.
- Vendor support
NVidia is following our progress and is interested in using the V4L2 stateless API. On the userspace side, vdpau is pretty much dead, they have moved to nvdec. OMX is being phasing out, in particular that is taking place for RaspberryPi now.
- Tooling
bootlin has developed a debugging tool called v4l2-request-test (https://github.com/bootlin/v4l2-request-test) that has been very useful to debug the codec driver without going through the full userspace stack. This is worth mentioning and integrating.
- API discussions
Using buffer indices as handles to reference frames
This has been proposed by Tomasz, and Hans has serious concerns, he believes that having userspace predict what buffer indices will be used in the future is very fragile and would prefer using a separate 64-bit cookie associated with v4l2_buffers.
Using capture buffer indices as reference frame handles requires predicting the buffer index on the capture queue which the output queue frames will be decoded into. We could use the output queue buffer index instead, but that wouldn't work with multi-slice decoding (multiple output buffers for a single capture buffer). Using a cookie set by userspace on the output side, then copied to the capture queue by the driver, solves that problem. All slices queued on the output queue for the same decoded picture will have the same cookie value (userspace will have to ensure that).
Tomasz would prefer a buffer index-based solution, to avoid keeping a cookie-index map in userspace. Due to how V4L2 works, enqueuing a new dmabuf handle on the capture side for a V4L2 buffer with a given index will effectively delete the corresponding cookie, so userspace would need to ensure it doesn't overwrite buffers; (Tomasz: To clarify, I don't see the significant benefit of using cookies over indices. It makes it easier for user space, because it doesn't have to predict the CAPTURE buffers, but still is error prone because of the buffer requeuing problem. For now it would be good to see how it translates into real code, though. In the meantime I can try to find a better idea.)
Once we have a final version for both Tuesday meeting and the Linux Media Summit, I'll post it at the "news" section of linuxtv, add the group photo and the links for the presentation and for our nightly dinner.
Sounds good.
Hello,
On Mon, Mar 11, 2019 at 01:23:58PM +0200, Sakari Ailus wrote:
On Wed, Dec 12, 2018 at 05:30:02AM -0200, Mauro Carvalho Chehab wrote:
Em Sun, 18 Nov 2018 00:45:02 +0200 Sakari Ailus escreveu:
Hello everyone,
Sorry for taking so long to review this. Was very busy those days.
Likewise in my reply. Please see my comments below. Let me know if you're fine with the proposed changes.
Same here, this is my long overdue reply.
It follows my comments.
Here's the report on the Media Summit held on 25th October in Edinburgh. The report is followed by the stateless codec discussion two days earlier.
Note: this is bcc'd to the meeting attendees plus a few others. I didn't use cc as the list servers tend to reject messages with too many recipients in cc / to headers.
Most presenters used slides some of which are already available here (expect more in the near future):
URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/
The original announcement for the meeting is here:
URL:https://www.spinics.net/lists/linux-media/msg141095.html
The raw notes can be found here:
URL:http://www.retiisi.org.uk/~sailus/v4l2/notes/osseu18-media.html
Attendees
Brad Love Ezequiel Garcia Gustavo Padovan Hans Verkuil Helen Koike Hidenori Yamaji Ivan Kalinin Jacopo Mondi Kieran Bingham Laurent Pinchart Mauro Chebab Maxime Ripard Michael Grzeschik Michael Ira Krufky Niklas Söderlund Patrick Lai Paul Elder Peter Griffin Ralph Clark Ricardo Ribalda Sakari Ailus Sean Young Seung-Woo Kim Stefan Klug Vinod Koul
CEC status - Hans Verkuil
Hans prensented an update on CEC status. Besides the slides, noteworthy information is maintained here:
URL:https://hverkuil.home.xs4all.nl/cec-status.txt
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-cec-status.pdf
It makes sense to add a quick summary of the main points to the meeting report that was there at the slide deck, in order to make the report more complete.
Bullet points from the slide:
cec-gpio error injection support
tda998x (including BeagleBoard Bone support after gpiolib changes)
ChromeOS EC CEC
In progress: omap5/dra7xx/am57xx TI (waiting for DSS redesign to land)
In progress: SECO cec driver (for UDOO x86 boards, expected for 4.21)
DisplayPort CEC-Tunneling-over-AUX for i915, nouveau, amdgpu
MegaChips 2900 chipset based adapters seems to support this protocol very well
Continuing work on CEC utilities, esp. the compliance test: it is in continuous use at Cisco.
rc-core status report - Sean Young
(Contributed by Sean Young)
Sorry, I didn't understand what you're meaning here. The status report was made by Sean. No need to repeat it.
Sean wrote the summary as well, the rest is written by me.
In the last year all staging lirc drivers have been either removed or ported to rc-core. Decoding of the more obscure IR protocols and protocol variants can now be done with BPF, with support in both the kernel and ir-keytable (which is in v4l-utils). Generally we're in a good situation wrt IR support.
There is some more ancient hardware (serial or usb-serial) that does not have support but not sure if anyone cares. kernel-doc is a little sparse and does not cover BPF IR decoding, so that needs improving. There was a discussion on enabling builds with CONFIG_RC_CORE=n. Sean suggested we could have rc_allocate_driver() return NULL and have the drivers deal with this gracefully, i.e. their probe functions should continue without IR. Mauro said there should be a per-driver config option (as is done for saa7134 for example).
Please break each topic on different paragraphs, as it makes easier to read and comment.
No conclusion was reached on this.
No conclusion was reached on what?
That probably raises more questions than it answers. I'll drop the line.
Persistent storage of controls - Ricardo Ribalda
Ricardo gave a presentation on a proposed solution for using the V4L2 control framework as an interface for updating control value defaults on sensor EEPROM.
Sensors commonly come with device specific tuning information that's embedded in the device EEPROM. Whereas this is also very common for raw cameras on mobile devices, the discussion this time was concentrated on industrial cameras.
The EEPROM contents may be written by the sensor vendor but occasionally may need to be updated by customers. Setting the control default value was suggested as the exact mechanism to do this.
The proposal was to use controls as the interface to update sensor tuning information in the EEPROM.
There were arguments for and against the approach:
- Drivers usually get these things right: relying on an user space program to do this is an additional dependency.
- Re-use of an existing interface (root priviledge check may be added).
- Partial solution only: EEPROM contents may need to be updated for other reasons as well, and a "spotty" implementation for updating certain EEPROM locations seems very use case specific.
- Changes required to the control framework for this --- defaults are not settable at the moment.
- The need is very use case specific, and adding support for that in a generic framework does not seem to fit very well.
I remember I mentioned, as an alternative, to use the firmware API, if one wants to update the eeprom contents. If I'm not mistaken, Ricardo opted not using it.
Ricardo?
I let Ricardo to comment that.
The general consensus appears to be not to change the control framework this way, but to continue to update the EEPROM using a specific user space program.
Tooling for sub-system tree maintenance - Laurent Pinchart
Laurent talked about the DRM tree maintenance model.
The DRM tree has switched to co-maintainer model. This has made it possible to share the burden of tree maintenance, removing bottlenecks they've had.
The larger number of people having (and using) their commit rights has created the need for a more strict rules for the tree maintenance, and subsequently a tool to implement it. It's called "DIM", the DRM Inglorious Maintenance tool. This is a command line tool that works as a front-end to execute the workflow.
URL:https://01.org/linuxgraphics/gfx-docs/maintainer-tools/dim.html
In particular what's worth noting:
The conflicts are resolved by the committer, not by the tree maintainer.
DIM stores conflict resolutions (as resolved by developers) to a shared cache.
DIM makes doing common mistakes harder by using sanity checks.
There are about 50 people who currently have commit rights to the DRM tree. There are no reports of commit rights having been forcibly removed as of yet. This strongly suggests that the model is workable.
The use of the tool puts additional responsibilities as well as some burden to the committers. Before the patches may be pushed, they are first compiled on developer's machine. That requires time, and without special arrangements such as having a second local workspace, and that time is away from productive work.
The discussion that followed was concentrated on the possibility of using a similar model for the media tree. While the suggestion was initially met by mostly favourable reception, there were concerns as well.
V4L2 *was* maintained generally according to the suggested model --- albeit without the proposed tools or process that needed to be strictly followed. There was once an incident which involved merging around 9000 lines of unreviewed code in a lot of places. What followed was not pretty, and this eventually lead to loss of multiple developers.
Could this happen again? The DRM tree has not suffered such incidents, and generally it understood such incident could be addressed by simply reverting such a patch and removing commit rights if necessary.
(Editor note: we have reverted the media tree master state to an earlier commit many times for various reasons. Could it be one of the reasons the 9000 line patch was not reverted was that the version control wasn't based on git??)
We actually reverted it, but it caused a huge confusion and produced lots of discussions. We lost several active developers: people that were not happy by the 9000 lines patchset stepping on everyone's feet and people that were not happy by reverting it.
Do you happen to remember any details? Did that "reverting" for instance involve rolling back to the state before the offending patch after more comments had been done?
I'd like a more detailed version of that story too. It's hard to comment on this particular example without knowing what happened.
That said, I feel this is not overly important. The DRM folks have proved this model works. Still I agree this is good to remember and document, but I don't see us getting into such situation _even if_ we'd switch to a similar way of working.
Agreed. I don't think the "9000 lines revert" is relevant anymore as such. What matters is how to keep existing and attract new developers, by making the linux-media subsystem attractive and easy and pleasant to work with.
Some opined that we do not have a bottleneck in reviewing patches and getting them merged whilst others thought this was not the case. It is certainly true that a very large number of patches (around 500 in the last kernel release) went in through the media tree.
It still appears that there would be more patches and more drivers to get in if the throughput was higher.
I'm not so sure about that (if we expect good quality patches), specially while we don't have any automatic testing tool to double check some stuff.
I agree. To help improving the process from here, we do need automated testing. I don't think anyone has really even argued against adding automated testing.
It won't be enough though. Testing is crucial to scale, but isn't a substitute for review, it won't solve the review bottleneck by itself.
Considering the amount of coverage in the meeting as well as the interest in general, it's just a question of time until we have something quite usable.
As a result of those discussions, One of the things that we've agreed there is to give trees at LinuxTV for more active developers that we trust enough to skip a sub-maintainer's review.
No, please. *Nobody* *ever* should have review bypass rights, there is no single exception to that rule. I fully agree we should get more people on-board and give them trees on linuxtv.org, but that's not about skipping review.
We used to have more active developer involvement. Anyway, I'll add a note on this.
We also agreed to try to improve the tooling at linuxtv.org, in order to try to improve our processes (although this discussion was actually split on other topics, like KernelCI and linuxtv infra).
How about:
It was also agreed to try to improve tooling at linuxtv.org to streamline the workflow. (Ed. note: also see testing related topics below.)
Current status of testing on the media tree - Sakari
The common practice in media subsystem development is that developers do test their patches before submitting them. This is an unwritten rule: sometimes patches end up not being tested after making slight changes to them, or they have been tested on a different kernel version. The developer may also simply forget to test the patch.
Besides this, it is not uncommon that changing the kernel configuration or switching to a different architecture will cause a compilation warning or an error.
The 0-day bot will catch some of these errors before the patches are merged, but that testing does not fully cover all the possible cases. There are some common pain points in V4L2-related Kconfig options (plain V4L2, MC or MC + subdev uAPI); newly submitted drivers may in fact require one of these, but the developer may not have realised that and so this ends up not being taken into account in Kconfig.
Once the review is done, and after being applied to the sub-maintainer tree, a patch is applied to Mauro's local tree and Mauro performs additional tests on it. These tests currently prevent a fair number of problems reaching a wider audience than the media developers.
On the other hand, whenever an issue is found, the patch will have to be fixed by the sub-maintainer or the developer. This is hardly ideal, as the problem has existed usually for a month or two before being spotted --- by a program. These checks should be instead performed on the patch when it's submitted.
Automated testing - Ezequiel Garcia
Ideal Continuous Integration process consists of the following steps:
- patch submission
- review and approval
- merge
The core question is "what level of quality standards do we want to enforce". The maintenance process should be modelled around this question, and not the other way around. Automated testing can be a part of enforcing the quality standards.
There are three steps:
- Define the quality standard
- Define how to quantify quality in respect to the standard
- Define how to enforce the standards
On the tooling side, an uAPI test tool exists. It's called v4l2-compliance, and new drivers are required to pass the v4l2-compliance test. It has quite a few favourable properties:
- Complete in terms of the uAPI coverage
- Quick and easy to run
- Nice output format for humans & scripts
There are some issues as well:
- No codec support (stateful or stateless)
- No SDR or touch support
- Frequently updated (distribution shipped v4l2-compliance useless)
- Only one contributor
Ezequiel noted that some people think that v4l2-compliance is changing too often but Hans responded that this is a necessity. The API gets amended occasionally and the existing API gets new tests. Mauro proposed moving v4l2-compliance to the kernel source tree but Hans preferred keeping it separate. That way it's easier to develop it.
To address the problem of only a single contributor, it was suggested that people implementing new APIs would need to provide the tests for v4l2-compliance as well. To achieve this, the v4l2-compliance codebase needs some cleanup to make it easier to contribute. The codebase is larger and there is no documentation.
V4l2-compliance also covers MC, V4L2 and V4L2 sub-device uAPIs.
DVB will require its own test tooling; it is not covered by v4l2-compliance. In order to facilitate automated testing, a virtual DVB driver would be useful as well. The task was added to the list of projects needing volunteers:
URL:https://linuxtv.org/wiki/index.php/Media_Open_Source_Projects:_Looking_for_Volunteers
There are some other test tools that could cover V4L2 but at the moment it seems somewhat far-fetched any of them would be used to test V4L2 in the near future:
- kselftest
- kunit
- gst-validate
- ktf (https://github.com/oracle/ktf, http://heim.ifi.uio.no/~knuto/ktf/)
KernelCI is a test automation system that supports automated compile and boot testing. As a newly added feature, additional tests may be implemented. This is what Collabora has implemented, effectively the current demo system runs v4l2-compliance on virtual drivers in a virtual machines (LAVA slaves).
A sample of the current test report is here:
URL:https://www.mail-archive.com/linux-media@vger.kernel.org/msg135787.html
The established way to run KernelCI tests is off the head of the branches of the stable and development kernel trees, including linux-next. This is not useful as such to support automated testing of patches for the media tree: the patches need to be tested before they are merged, not after merging.
In the discusion that followed among a slightly smaller group of people, it was suggested that tests could be run from select developer kernel trees, from any branch. If a developer needs long-term storage, (s)he could have another tree which would not be subject automated test builds. Alternatively, the branch name could be used as a basis for triggering an automated build, but this could end up being too restrictive.
Merging the next rc1 by the maintainer would be no special case: the branch would be tested in similar way than the developer branches containing patches, and tests should need to pass before pushing the content to the media tree master branch.
Ezequiel wished that people would reply to his e-mail to express their wishes on the testing needs (see sample report above).
Stateless codecs - Hans Verkuil
Support for stateless codecs will be merged for v4.20 with an Allwinner staging codec driver.
The earlier stateless codec discussion ended up concluding that the bitstream parsing is application specific, so there will be no need for a generic implementation that was previously foreseen. The question that remains is: should there be a simple parser for compliance testing?
All main applications support libva which was developed as the codec API to be used with Intel GPUs. A libVA frontend was written to support the Cedrus stateless V4L2 decoder driver. It remains to be seen whether the same implementation could be used as such for the other stateless codec drivers or whether changes, or in the worst case a parallel implementation, would be needed.
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-codec-userspace.pdf
New versions of the old IOCTLs - Hans Verkuil
V4L2 is an old API with shifting focus in terms of functionality and hardware supported. While there has been lots of changes to the two during the existence of V4L2, some of the API is unchanged since the old times. While the API is usable for the purpose, it is needlessly clunky: it is often not obvious how an IOCTL is related to the task at hand (such as using S_PARM to set the frame interval) or the API does not use year 2038-safe timestamps (struct v4l2_buffer). These APIs deserve to be updated.
- VIDIOC_*_PARM
In the case of VIDIOC_G_PARM and VIDIOC_S_PARM, the IOCTLs are only used to set and get the frame interval.
In this case, what can be done, is to add a new IOCTL definition, with the same IOCTL number and with binary-equivalent IOCTL argument struct that only contains the field for the frame rate itself. This is binary-compatible with the existing code and no compatibility code will be needed. The new IOCTLs will be called VIDIOC_G_FRAME_INTERVAL and VIDIOC_S_FRAME_INTERVAL.
- VIDIOC_ENUM_FRAME_INTERVALS
Besides discrete set of supported frame intervals, VIDIOC_ENUM_FRAME_INTERVALS has stepwise frame interval as well. Stepwise could be removed as the Qualcomm venus codec and uvc (100 ns units) are the only users. Additionally, the buffer type should be added to struct v4l2_frmivalenum.
There was also a discussion related to enumerating frame intervals in units of ns vs. fractional seconds. The reasoning using a fraction is that this way the frame interval on many standards can be conveyed precisely. Somebody recalled "flick", that is is the common denominator of the frame rates on all TV standards. Drivers could simply move to use the flick as the denominator, to make frame interval reporting uniform across the drivers.
- struct v4l2_buffer
struct v4l2_buffer is an age-old struct. There are a few issues in it:
- The timestamp is not 2038-safe.
- The multi-plane implementation is a mess.
- Differing implementation for the end single-plane and multi-plane APIs is confusing for both applications and drivers.
The proposal is to create a new v4l2_buffer struct. The differences to the old one would be:
- __u64 timestamps. These are 2038-safe. The timestamp source is maintained, i.e. the type remains CLOCK_MONOTONIC apart from certain drivers (e.g. UVC) that lets the user choose the timestamp.
- Put the planes right to struct v4l2_buffer. The plane struct would also be changed; the new plane struct would be called v4l2_ext_plane.
- While at it, the plane description can be improved:
- The start of data from the beginning of the plane memory.
- Add width and height to the buffer? This would make image size changes easier for the codec. (Ed. note: pixel format as well. But this approach could only partially support what the request API is for.)
- Unify single- and multi-planar APIs.
The new struct could be called v4l2_ext_buffer.
As the new IOCTL argument struct will have has different syntax as well as
s/have has/have/
Will fix.
semantics, it deserves to be named differently. Compatibility code will be needed to convert the users of the old IOCTLs to the new struct used internally by the kernel and drivers, and then back to the user.
- struct v4l2_create_buffers
Of the format, only the pix.fmt.sizeimage field is effectively used by the drivers supporting VIDIOC_CREATE_BUFS. This could be simplified, by just providing the desired buffer size instead of the entire v4l2_format struct. The user would be instructed to use TRY_FMT to obtain that buffer size.
The need to delete buffers seems to have eventually surfaced. That was expected, but it wasn't known when this would happen. As the buffer index range would become non-contiguous, it should be possible to create buffers one by one only, as otherwise the indices of the additional buffers would no longer be communicated to the user unambiguously.
So there would be new IOCTLs:
- VIDIOC_CREATE_BUF - Create a single buffer of given size (plus other non-format related aspects)
- VIDIOC_DELETE_BUF - Delete a single buffer
- VIDIOC_DELETE_ALL_BUFS - Delete all buffers
The naming still requires some work. The opposite of create is "destroy", not "delete".
- struct v4l2_pix_format vs. struct v4l2_pix_format_mplane
Working with the two structs depending on whether the format is multi-planar or not is painful. While we're doing changes in the area, the two could be unified as well.
(Editor note: this could be still orthogonal to the buffers, so it could be done separately as well. We'll see.)
I suspect that those "editor note" (as any post-meeting notes) don't belong to the final report.
I wanted to make the report more readable for people who are not actively working on V4L2 development and are thus likely not able to make such connections.
But yeah, perhaps this could be done seprarately. Let's discuss it when actual patches gets posted.
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-new-ioctls.pdf
I guess there was an action plan for that, based on the discussions (maybe some of them ended by being merged with the presentation on the above?).
Hans,
Did you take any notes about the actions to be taken? I found very helpful to have an action plan item below the topics where we made such plan.
Fault tolerant V4L2 - Kieran Bingham
Kieran presented a system where the media hardware complex consisted of eight more or less independent camera sensors that naturally end up being within a single media device.
The current implementation, as well as the API, necessitates that all devices in a media device probe successfully before the entire media device is exposed to the user. Otherwise the user would see with a partial view of the device, without the knowledge it is such.
To address the problem, additional information need to be provided to the user space. In particular:
Events on the media device to tell the graph has changed.
Graph version number is incremented at graph change (already implemented).
The property API could be applicable --- placeholders for entities that have not yet appeared?
- Alternative: known entities that have failed to probe created in the media graph and marked "disable" or "failed".
Query the state of media graph completeness.
That way, even when the devices in a media controller device appear one by one, the user space will be able to have all the necessary information on the registration state of the device.
Complex cameras - Mauro Chehab
Some new laptops integrate a raw Bayer camera + ISP instead of a USB webcam. This is expected to increase, as the solution is generally cheaper and results in better quality images --- as long as all the pieces of the puzzle are in place, including the proprietary 3A library.
Still, such devices need to be supported.
(Ed. note: there were two talks related to this topic given in the ELc-E.)
URL:https://www.youtube.com/watch?v=KpaNNJr92CY&index=31&list=PLbzoR-pLrL6qThA7SAbhVfuMbjZsJX1CY URL:https://www.youtube.com/watch?v=GIhV7tiUji0&index=60&list=PLbzoR-pLrL6qThA7SAbhVfuMbjZsJX1CY
In this specific case, it is worth to keep the note, as those presentations happened at ELC-Eu and were explicitly mentioned there during the discussions.
Development process - All
Topic-wise this is continuation of the "Tooling for sub-system tree maintenance", "Current status of testing on the media tree" and "Automated testing" topics above.
The question here is whether there's something that could be improved in the media development process and if so, how could that be done.
What came up was a suggestion to have multi-committer tree in a similar manner as the DRM developers do. This was seen to be more interesting for developers than simply being asked to review patches.
It certainly does raise the need for more precise rules for what may be committed to the multi-committer tree, when etc.
It was also requested that experienced driver maintainers would send pull requests on patches to their drivers instead of going through a sub-maintainer (pre-agreed with the relevant (sub)maintainer). This would take some work away from sub-maintainers, but not the maintainer.
No firm decisions were reached in this topic. Perhaps this could be tried out?
We did decide to experment the "experienced driver" maintainership model.
Btw, I already added an account for one such developer :-)
Ok, so we're actually trying this out. Great! :-)
There was also a request to document the sub-maintainer names in the wiki so that it'd be easier for people to figure out who to ping if their patches do not get merged.
I'm ok with that, but, after the LPC, I suspect that the best is to document it in sync with the per-subsystem profile. I'm waiting for Don to submit an updated patchset, in order to rebase our subsystem's profile.
Ok. There's a list in the wiki but I think few people end up finding it when they needed it. :-I
linuxtv.org hosting - All
Mauro noted that linuxtv.org is currently hosted in a virtual machine somewhere in a German university. The administrator of the virtual machine has not been involved with Video4Linux for some time but has been kind to provide us the hosting over the years.
It has been recognised that there is a need to find a new hosting location for the virtual machine. There is also a question of the domain name linuxtv.org. Discussion followed.
What could be agreed on rather immediately was that the domain name should be owned by "us". "Us" is not a legal entity at the moment, and a practical arrangement to achieve that could be to find a new association to own the domain name.
The hosting of the virtual machine could possibly be handled by the same association. In practice this would likely mean a virtual machine on a hosting provider. Ideally this would be paid for by a company or a group of companies.
No decisions were reached on the topic.
There was actually one decision: to talk with Linux Foundation about that. Laurent was against, but the majority was ok with the idea.
I remember Laurent was not the only one expressing concerns related to using such hosting providers. I rather remember hearing this from quite a few people in the discussion. Either way, the decisions related to e.g. hosting will be taken later when more information is available, including what LF has to offer.
I agree with Sakari, there was clearly no majority, that's not true.
How about replacing the original last paragraph with:
Mauro will discuss with LF to find out what they can offer. Concerns were expressed over other organisations providing us with hosting we are not in charge of ourselves. Other options related to domain ownership and hosting will be researched as well.
Works for me.
Tuesday's stateless codec discussion
Hans presented a summary of this in his stateless codec status presentation, here are a bit more details.
We had a discussion (first in the Microsoft sponsor suite, then at the bar)
I don't think that the room location is relevant for the report :-)
It adds colours to the report though :-)
:-D
Also, we should split this on a separate report, as this was another meeting and not all people listed above participated on it.
Works for me.
on how to support user space for the stateless codecs better. The expected outcome of that would be a rough understanding how a stateless codec user space library would look like.
The raw notes are available here:
URL:http://www.retiisi.org.uk/~sailus/v4l2/notes/osseu18-codecs.html
Attendees
Alexandre Courbot Chris Healy Ezequiel Garcia Hans Verkuil Kieran Bingham Laurent Pinchart Maxime Ripard Mauro Carvalho Chehab Nicolas Dufresne Niklas Söderlund Philip Zabell Sakari Ailus Tomasz Figa Victor Jáquez
Buffer management
Nicolas reported an issue in V4L2 buffer management. The V4L2 decouples the buffers from the format, and assumes all queued buffers (at a given point of time) have the same format. (Ed. note: the request API could be used to address this, but that particular features is not yet supported.)
- User space library
The existing projects generally integrate their own bitstream parsers for codecs. There are subtle reasons why that tends to be the case, instead of using more generic parsers. There are differences in error handling, for instance, or other matters of policy, the variation which could be difficult to fully offer using a generic API.
Maxime noted that VLC recently released a new parser meant to be used as a library, and that could be useful. Nicolas believes that we'd need a parser library independent of any other code base to avoid pulling in extra libraries and this parser would need to be maintained. It could be difficult to find the volunteers to do that.
Does ChromeOS have its own parser? Alexandre believes it does, but little was known beyond that.
There's also the language problem: ffmpeg and gstreamer are written in C, the ChomeOS parser in C++, VLC is moving to Rust. What do we pick, how do we ensure interoperability?
- libVA re-use
As a short-term solution, implementing a generic wrapper using the V4L2 stateless codec API to offer libVA API would enable generic applications to use the V4L2 stateless codec drivers as most applications already support libVA.
70 % of the applications use FFMPEG, which has a software codec API that is nearly identical to the V4L2 statless codec API. It would be trivial for applications to switch to V4L2 natively.
Mauro would like us to explain our plans to Intel to avoid surprises later on.
To be clear: it was said at the meeting that libVA is sponsored an maintained by Intel. If we're willing to use it, we should sync with them, in order to avoid unexpected surprises if they change it in a way that would cause problems for the V4L2 stateless coded implementation.
How about, instead of the original:
We need to explain our plans to libVA maintainers to better coordinate libVA API development in a way the V4L2 stateless codecs are taken into account.
- Source code hosting
libva is hosted on freedesktop. Should we host the libva-v4l2-codec backend there, or host it on linuxtv.org? Hans would prefer linuxtv.org as it's "closer to our kernel implementation".
- Backend support in libva
libva loads backends in order, and picks the first one that reports it can support the platform. There is also an environment variable that can specify a backend. Ezequiel enquired how to support platforms that would have multiple hardware codecs. libva doesn't seem to support this at the moment. Nicolas reported that there's an Intel SoC that have both an Intel graphics core and a Vega64 graphics core that both have a codec.
Hans said that a platform that expose multiple codecs will likely be used for specialized applications, and requiring those to implement codec support directly is acceptable. Our main focus should be to support the common case.
- Vendor support
NVidia is following our progress and is interested in using the V4L2 stateless API. On the userspace side, vdpau is pretty much dead, they have moved to nvdec. OMX is being phasing out, in particular that is taking place for RaspberryPi now.
- Tooling
bootlin has developed a debugging tool called v4l2-request-test (https://github.com/bootlin/v4l2-request-test) that has been very useful to debug the codec driver without going through the full userspace stack. This is worth mentioning and integrating.
- API discussions
Using buffer indices as handles to reference frames
This has been proposed by Tomasz, and Hans has serious concerns, he believes that having userspace predict what buffer indices will be used in the future is very fragile and would prefer using a separate 64-bit cookie associated with v4l2_buffers.
Using capture buffer indices as reference frame handles requires predicting the buffer index on the capture queue which the output queue frames will be decoded into. We could use the output queue buffer index instead, but that wouldn't work with multi-slice decoding (multiple output buffers for a single capture buffer). Using a cookie set by userspace on the output side, then copied to the capture queue by the driver, solves that problem. All slices queued on the output queue for the same decoded picture will have the same cookie value (userspace will have to ensure that).
Tomasz would prefer a buffer index-based solution, to avoid keeping a cookie-index map in userspace. Due to how V4L2 works, enqueuing a new dmabuf handle on the capture side for a V4L2 buffer with a given index will effectively delete the corresponding cookie, so userspace would need to ensure it doesn't overwrite buffers; (Tomasz: To clarify, I don't see the significant benefit of using cookies over indices. It makes it easier for user space, because it doesn't have to predict the CAPTURE buffers, but still is error prone because of the buffer requeuing problem. For now it would be good to see how it translates into real code, though. In the meantime I can try to find a better idea.)
Once we have a final version for both Tuesday meeting and the Linux Media Summit, I'll post it at the "news" section of linuxtv, add the group photo and the links for the presentation and for our nightly dinner.
Sounds good.
Hi
On Mon, Mar 11, 2019 at 1:29 PM Laurent Pinchart laurent.pinchart@ideasonboard.com wrote:
Hello,
On Mon, Mar 11, 2019 at 01:23:58PM +0200, Sakari Ailus wrote:
On Wed, Dec 12, 2018 at 05:30:02AM -0200, Mauro Carvalho Chehab wrote:
Em Sun, 18 Nov 2018 00:45:02 +0200 Sakari Ailus escreveu:
Hello everyone,
Sorry for taking so long to review this. Was very busy those days.
Likewise in my reply. Please see my comments below. Let me know if you're fine with the proposed changes.
Same here, this is my long overdue reply.
It follows my comments.
Here's the report on the Media Summit held on 25th October in Edinburgh. The report is followed by the stateless codec discussion two days earlier.
Note: this is bcc'd to the meeting attendees plus a few others. I didn't use cc as the list servers tend to reject messages with too many recipients in cc / to headers.
Most presenters used slides some of which are already available here (expect more in the near future):
URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/
The original announcement for the meeting is here:
URL:https://www.spinics.net/lists/linux-media/msg141095.html
The raw notes can be found here:
URL:http://www.retiisi.org.uk/~sailus/v4l2/notes/osseu18-media.html
Attendees
Brad Love Ezequiel Garcia Gustavo Padovan Hans Verkuil Helen Koike Hidenori Yamaji Ivan Kalinin Jacopo Mondi Kieran Bingham Laurent Pinchart Mauro Chebab Maxime Ripard Michael Grzeschik Michael Ira Krufky Niklas Söderlund Patrick Lai Paul Elder Peter Griffin Ralph Clark Ricardo Ribalda Sakari Ailus Sean Young Seung-Woo Kim Stefan Klug Vinod Koul
CEC status - Hans Verkuil
Hans prensented an update on CEC status. Besides the slides, noteworthy information is maintained here:
URL:https://hverkuil.home.xs4all.nl/cec-status.txt
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-cec-status.pdf
It makes sense to add a quick summary of the main points to the meeting report that was there at the slide deck, in order to make the report more complete.
Bullet points from the slide:
cec-gpio error injection support
tda998x (including BeagleBoard Bone support after gpiolib changes)
ChromeOS EC CEC
In progress: omap5/dra7xx/am57xx TI (waiting for DSS redesign to land)
In progress: SECO cec driver (for UDOO x86 boards, expected for 4.21)
DisplayPort CEC-Tunneling-over-AUX for i915, nouveau, amdgpu
MegaChips 2900 chipset based adapters seems to support this protocol very well
Continuing work on CEC utilities, esp. the compliance test: it is in continuous use at Cisco.
rc-core status report - Sean Young
(Contributed by Sean Young)
Sorry, I didn't understand what you're meaning here. The status report was made by Sean. No need to repeat it.
Sean wrote the summary as well, the rest is written by me.
In the last year all staging lirc drivers have been either removed or ported to rc-core. Decoding of the more obscure IR protocols and protocol variants can now be done with BPF, with support in both the kernel and ir-keytable (which is in v4l-utils). Generally we're in a good situation wrt IR support.
There is some more ancient hardware (serial or usb-serial) that does not have support but not sure if anyone cares. kernel-doc is a little sparse and does not cover BPF IR decoding, so that needs improving. There was a discussion on enabling builds with CONFIG_RC_CORE=n. Sean suggested we could have rc_allocate_driver() return NULL and have the drivers deal with this gracefully, i.e. their probe functions should continue without IR. Mauro said there should be a per-driver config option (as is done for saa7134 for example).
Please break each topic on different paragraphs, as it makes easier to read and comment.
No conclusion was reached on this.
No conclusion was reached on what?
That probably raises more questions than it answers. I'll drop the line.
Persistent storage of controls - Ricardo Ribalda
Ricardo gave a presentation on a proposed solution for using the V4L2 control framework as an interface for updating control value defaults on sensor EEPROM.
Sensors commonly come with device specific tuning information that's embedded in the device EEPROM. Whereas this is also very common for raw cameras on mobile devices, the discussion this time was concentrated on industrial cameras.
The EEPROM contents may be written by the sensor vendor but occasionally may need to be updated by customers. Setting the control default value was suggested as the exact mechanism to do this.
The proposal was to use controls as the interface to update sensor tuning information in the EEPROM.
There were arguments for and against the approach:
- Drivers usually get these things right: relying on an user space program to do this is an additional dependency.
- Re-use of an existing interface (root priviledge check may be added).
- Partial solution only: EEPROM contents may need to be updated for other reasons as well, and a "spotty" implementation for updating certain EEPROM locations seems very use case specific.
- Changes required to the control framework for this --- defaults are not settable at the moment.
- The need is very use case specific, and adding support for that in a generic framework does not seem to fit very well.
I remember I mentioned, as an alternative, to use the firmware API, if one wants to update the eeprom contents. If I'm not mistaken, Ricardo opted not using it.
Ricardo?
I let Ricardo to comment that.
Sorry, I completely missed the conversation.
The problem with the firmware API is that it is reading a file from userspace: like /lib/firmware/camera_values.raw
Provisioning that file, might be an issue. You want to be able to sell a camera to a client, and that client shall be able to use any linux distro at his disposal without having to install any extra file. Consider that camera_values contains values such as the list of dead pixels, that is completely sensor dependent.
I think that it would be a better approach to use pmem-regions instead of mtd ( Documentation/devicetree/bindings/pmem/pmem-region.txt ) as Jacopo suggested.
Once things settle down on my side (have been a bit crazy since october) I want to send a small patchset to demo what we are doing, which is better than discussing about abstract ideas.
Thanks!
The general consensus appears to be not to change the control framework this way, but to continue to update the EEPROM using a specific user space program.
Tooling for sub-system tree maintenance - Laurent Pinchart
Laurent talked about the DRM tree maintenance model.
The DRM tree has switched to co-maintainer model. This has made it possible to share the burden of tree maintenance, removing bottlenecks they've had.
The larger number of people having (and using) their commit rights has created the need for a more strict rules for the tree maintenance, and subsequently a tool to implement it. It's called "DIM", the DRM Inglorious Maintenance tool. This is a command line tool that works as a front-end to execute the workflow.
URL:https://01.org/linuxgraphics/gfx-docs/maintainer-tools/dim.html
In particular what's worth noting:
The conflicts are resolved by the committer, not by the tree maintainer.
DIM stores conflict resolutions (as resolved by developers) to a shared cache.
DIM makes doing common mistakes harder by using sanity checks.
There are about 50 people who currently have commit rights to the DRM tree. There are no reports of commit rights having been forcibly removed as of yet. This strongly suggests that the model is workable.
The use of the tool puts additional responsibilities as well as some burden to the committers. Before the patches may be pushed, they are first compiled on developer's machine. That requires time, and without special arrangements such as having a second local workspace, and that time is away from productive work.
The discussion that followed was concentrated on the possibility of using a similar model for the media tree. While the suggestion was initially met by mostly favourable reception, there were concerns as well.
V4L2 *was* maintained generally according to the suggested model --- albeit without the proposed tools or process that needed to be strictly followed. There was once an incident which involved merging around 9000 lines of unreviewed code in a lot of places. What followed was not pretty, and this eventually lead to loss of multiple developers.
Could this happen again? The DRM tree has not suffered such incidents, and generally it understood such incident could be addressed by simply reverting such a patch and removing commit rights if necessary.
(Editor note: we have reverted the media tree master state to an earlier commit many times for various reasons. Could it be one of the reasons the 9000 line patch was not reverted was that the version control wasn't based on git??)
We actually reverted it, but it caused a huge confusion and produced lots of discussions. We lost several active developers: people that were not happy by the 9000 lines patchset stepping on everyone's feet and people that were not happy by reverting it.
Do you happen to remember any details? Did that "reverting" for instance involve rolling back to the state before the offending patch after more comments had been done?
I'd like a more detailed version of that story too. It's hard to comment on this particular example without knowing what happened.
That said, I feel this is not overly important. The DRM folks have proved this model works. Still I agree this is good to remember and document, but I don't see us getting into such situation _even if_ we'd switch to a similar way of working.
Agreed. I don't think the "9000 lines revert" is relevant anymore as such. What matters is how to keep existing and attract new developers, by making the linux-media subsystem attractive and easy and pleasant to work with.
Some opined that we do not have a bottleneck in reviewing patches and getting them merged whilst others thought this was not the case. It is certainly true that a very large number of patches (around 500 in the last kernel release) went in through the media tree.
It still appears that there would be more patches and more drivers to get in if the throughput was higher.
I'm not so sure about that (if we expect good quality patches), specially while we don't have any automatic testing tool to double check some stuff.
I agree. To help improving the process from here, we do need automated testing. I don't think anyone has really even argued against adding automated testing.
It won't be enough though. Testing is crucial to scale, but isn't a substitute for review, it won't solve the review bottleneck by itself.
Considering the amount of coverage in the meeting as well as the interest in general, it's just a question of time until we have something quite usable.
As a result of those discussions, One of the things that we've agreed there is to give trees at LinuxTV for more active developers that we trust enough to skip a sub-maintainer's review.
No, please. *Nobody* *ever* should have review bypass rights, there is no single exception to that rule. I fully agree we should get more people on-board and give them trees on linuxtv.org, but that's not about skipping review.
We used to have more active developer involvement. Anyway, I'll add a note on this.
We also agreed to try to improve the tooling at linuxtv.org, in order to try to improve our processes (although this discussion was actually split on other topics, like KernelCI and linuxtv infra).
How about:
It was also agreed to try to improve tooling at linuxtv.org to streamline the workflow. (Ed. note: also see testing related topics below.)
Current status of testing on the media tree - Sakari
The common practice in media subsystem development is that developers do test their patches before submitting them. This is an unwritten rule: sometimes patches end up not being tested after making slight changes to them, or they have been tested on a different kernel version. The developer may also simply forget to test the patch.
Besides this, it is not uncommon that changing the kernel configuration or switching to a different architecture will cause a compilation warning or an error.
The 0-day bot will catch some of these errors before the patches are merged, but that testing does not fully cover all the possible cases. There are some common pain points in V4L2-related Kconfig options (plain V4L2, MC or MC + subdev uAPI); newly submitted drivers may in fact require one of these, but the developer may not have realised that and so this ends up not being taken into account in Kconfig.
Once the review is done, and after being applied to the sub-maintainer tree, a patch is applied to Mauro's local tree and Mauro performs additional tests on it. These tests currently prevent a fair number of problems reaching a wider audience than the media developers.
On the other hand, whenever an issue is found, the patch will have to be fixed by the sub-maintainer or the developer. This is hardly ideal, as the problem has existed usually for a month or two before being spotted --- by a program. These checks should be instead performed on the patch when it's submitted.
Automated testing - Ezequiel Garcia
Ideal Continuous Integration process consists of the following steps:
- patch submission
- review and approval
- merge
The core question is "what level of quality standards do we want to enforce". The maintenance process should be modelled around this question, and not the other way around. Automated testing can be a part of enforcing the quality standards.
There are three steps:
- Define the quality standard
- Define how to quantify quality in respect to the standard
- Define how to enforce the standards
On the tooling side, an uAPI test tool exists. It's called v4l2-compliance, and new drivers are required to pass the v4l2-compliance test. It has quite a few favourable properties:
- Complete in terms of the uAPI coverage
- Quick and easy to run
- Nice output format for humans & scripts
There are some issues as well:
- No codec support (stateful or stateless)
- No SDR or touch support
- Frequently updated (distribution shipped v4l2-compliance useless)
- Only one contributor
Ezequiel noted that some people think that v4l2-compliance is changing too often but Hans responded that this is a necessity. The API gets amended occasionally and the existing API gets new tests. Mauro proposed moving v4l2-compliance to the kernel source tree but Hans preferred keeping it separate. That way it's easier to develop it.
To address the problem of only a single contributor, it was suggested that people implementing new APIs would need to provide the tests for v4l2-compliance as well. To achieve this, the v4l2-compliance codebase needs some cleanup to make it easier to contribute. The codebase is larger and there is no documentation.
V4l2-compliance also covers MC, V4L2 and V4L2 sub-device uAPIs.
DVB will require its own test tooling; it is not covered by v4l2-compliance. In order to facilitate automated testing, a virtual DVB driver would be useful as well. The task was added to the list of projects needing volunteers:
URL:https://linuxtv.org/wiki/index.php/Media_Open_Source_Projects:_Looking_for_Volunteers
There are some other test tools that could cover V4L2 but at the moment it seems somewhat far-fetched any of them would be used to test V4L2 in the near future:
- kselftest
- kunit
- gst-validate
- ktf (https://github.com/oracle/ktf, http://heim.ifi.uio.no/~knuto/ktf/)
KernelCI is a test automation system that supports automated compile and boot testing. As a newly added feature, additional tests may be implemented. This is what Collabora has implemented, effectively the current demo system runs v4l2-compliance on virtual drivers in a virtual machines (LAVA slaves).
A sample of the current test report is here:
URL:https://www.mail-archive.com/linux-media@vger.kernel.org/msg135787.html
The established way to run KernelCI tests is off the head of the branches of the stable and development kernel trees, including linux-next. This is not useful as such to support automated testing of patches for the media tree: the patches need to be tested before they are merged, not after merging.
In the discusion that followed among a slightly smaller group of people, it was suggested that tests could be run from select developer kernel trees, from any branch. If a developer needs long-term storage, (s)he could have another tree which would not be subject automated test builds. Alternatively, the branch name could be used as a basis for triggering an automated build, but this could end up being too restrictive.
Merging the next rc1 by the maintainer would be no special case: the branch would be tested in similar way than the developer branches containing patches, and tests should need to pass before pushing the content to the media tree master branch.
Ezequiel wished that people would reply to his e-mail to express their wishes on the testing needs (see sample report above).
Stateless codecs - Hans Verkuil
Support for stateless codecs will be merged for v4.20 with an Allwinner staging codec driver.
The earlier stateless codec discussion ended up concluding that the bitstream parsing is application specific, so there will be no need for a generic implementation that was previously foreseen. The question that remains is: should there be a simple parser for compliance testing?
All main applications support libva which was developed as the codec API to be used with Intel GPUs. A libVA frontend was written to support the Cedrus stateless V4L2 decoder driver. It remains to be seen whether the same implementation could be used as such for the other stateless codec drivers or whether changes, or in the worst case a parallel implementation, would be needed.
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-codec-userspace.pdf
New versions of the old IOCTLs - Hans Verkuil
V4L2 is an old API with shifting focus in terms of functionality and hardware supported. While there has been lots of changes to the two during the existence of V4L2, some of the API is unchanged since the old times. While the API is usable for the purpose, it is needlessly clunky: it is often not obvious how an IOCTL is related to the task at hand (such as using S_PARM to set the frame interval) or the API does not use year 2038-safe timestamps (struct v4l2_buffer). These APIs deserve to be updated.
- VIDIOC_*_PARM
In the case of VIDIOC_G_PARM and VIDIOC_S_PARM, the IOCTLs are only used to set and get the frame interval.
In this case, what can be done, is to add a new IOCTL definition, with the same IOCTL number and with binary-equivalent IOCTL argument struct that only contains the field for the frame rate itself. This is binary-compatible with the existing code and no compatibility code will be needed. The new IOCTLs will be called VIDIOC_G_FRAME_INTERVAL and VIDIOC_S_FRAME_INTERVAL.
- VIDIOC_ENUM_FRAME_INTERVALS
Besides discrete set of supported frame intervals, VIDIOC_ENUM_FRAME_INTERVALS has stepwise frame interval as well. Stepwise could be removed as the Qualcomm venus codec and uvc (100 ns units) are the only users. Additionally, the buffer type should be added to struct v4l2_frmivalenum.
There was also a discussion related to enumerating frame intervals in units of ns vs. fractional seconds. The reasoning using a fraction is that this way the frame interval on many standards can be conveyed precisely. Somebody recalled "flick", that is is the common denominator of the frame rates on all TV standards. Drivers could simply move to use the flick as the denominator, to make frame interval reporting uniform across the drivers.
- struct v4l2_buffer
struct v4l2_buffer is an age-old struct. There are a few issues in it:
- The timestamp is not 2038-safe.
- The multi-plane implementation is a mess.
- Differing implementation for the end single-plane and multi-plane APIs is confusing for both applications and drivers.
The proposal is to create a new v4l2_buffer struct. The differences to the old one would be:
- __u64 timestamps. These are 2038-safe. The timestamp source is maintained, i.e. the type remains CLOCK_MONOTONIC apart from certain drivers (e.g. UVC) that lets the user choose the timestamp.
- Put the planes right to struct v4l2_buffer. The plane struct would also be changed; the new plane struct would be called v4l2_ext_plane.
- While at it, the plane description can be improved:
- The start of data from the beginning of the plane memory.
- Add width and height to the buffer? This would make image size changes easier for the codec. (Ed. note: pixel format as well. But this approach could only partially support what the request API is for.)
- Unify single- and multi-planar APIs.
The new struct could be called v4l2_ext_buffer.
As the new IOCTL argument struct will have has different syntax as well as
s/have has/have/Will fix.
semantics, it deserves to be named differently. Compatibility code will be needed to convert the users of the old IOCTLs to the new struct used internally by the kernel and drivers, and then back to the user.
- struct v4l2_create_buffers
Of the format, only the pix.fmt.sizeimage field is effectively used by the drivers supporting VIDIOC_CREATE_BUFS. This could be simplified, by just providing the desired buffer size instead of the entire v4l2_format struct. The user would be instructed to use TRY_FMT to obtain that buffer size.
The need to delete buffers seems to have eventually surfaced. That was expected, but it wasn't known when this would happen. As the buffer index range would become non-contiguous, it should be possible to create buffers one by one only, as otherwise the indices of the additional buffers would no longer be communicated to the user unambiguously.
So there would be new IOCTLs:
- VIDIOC_CREATE_BUF - Create a single buffer of given size (plus other non-format related aspects)
- VIDIOC_DELETE_BUF - Delete a single buffer
- VIDIOC_DELETE_ALL_BUFS - Delete all buffers
The naming still requires some work. The opposite of create is "destroy", not "delete".
- struct v4l2_pix_format vs. struct v4l2_pix_format_mplane
Working with the two structs depending on whether the format is multi-planar or not is painful. While we're doing changes in the area, the two could be unified as well.
(Editor note: this could be still orthogonal to the buffers, so it could be done separately as well. We'll see.)
I suspect that those "editor note" (as any post-meeting notes) don't belong to the final report.
I wanted to make the report more readable for people who are not actively working on V4L2 development and are thus likely not able to make such connections.
But yeah, perhaps this could be done seprarately. Let's discuss it when actual patches gets posted.
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-new-ioctls.pdf
I guess there was an action plan for that, based on the discussions (maybe some of them ended by being merged with the presentation on the above?).
Hans,
Did you take any notes about the actions to be taken? I found very helpful to have an action plan item below the topics where we made such plan.
Fault tolerant V4L2 - Kieran Bingham
Kieran presented a system where the media hardware complex consisted of eight more or less independent camera sensors that naturally end up being within a single media device.
The current implementation, as well as the API, necessitates that all devices in a media device probe successfully before the entire media device is exposed to the user. Otherwise the user would see with a partial view of the device, without the knowledge it is such.
To address the problem, additional information need to be provided to the user space. In particular:
Events on the media device to tell the graph has changed.
Graph version number is incremented at graph change (already implemented).
The property API could be applicable --- placeholders for entities that have not yet appeared?
- Alternative: known entities that have failed to probe created in the media graph and marked "disable" or "failed".
Query the state of media graph completeness.
That way, even when the devices in a media controller device appear one by one, the user space will be able to have all the necessary information on the registration state of the device.
Complex cameras - Mauro Chehab
Some new laptops integrate a raw Bayer camera + ISP instead of a USB webcam. This is expected to increase, as the solution is generally cheaper and results in better quality images --- as long as all the pieces of the puzzle are in place, including the proprietary 3A library.
Still, such devices need to be supported.
(Ed. note: there were two talks related to this topic given in the ELc-E.)
URL:https://www.youtube.com/watch?v=KpaNNJr92CY&index=31&list=PLbzoR-pLrL6qThA7SAbhVfuMbjZsJX1CY URL:https://www.youtube.com/watch?v=GIhV7tiUji0&index=60&list=PLbzoR-pLrL6qThA7SAbhVfuMbjZsJX1CY
In this specific case, it is worth to keep the note, as those presentations happened at ELC-Eu and were explicitly mentioned there during the discussions.
Development process - All
Topic-wise this is continuation of the "Tooling for sub-system tree maintenance", "Current status of testing on the media tree" and "Automated testing" topics above.
The question here is whether there's something that could be improved in the media development process and if so, how could that be done.
What came up was a suggestion to have multi-committer tree in a similar manner as the DRM developers do. This was seen to be more interesting for developers than simply being asked to review patches.
It certainly does raise the need for more precise rules for what may be committed to the multi-committer tree, when etc.
It was also requested that experienced driver maintainers would send pull requests on patches to their drivers instead of going through a sub-maintainer (pre-agreed with the relevant (sub)maintainer). This would take some work away from sub-maintainers, but not the maintainer.
No firm decisions were reached in this topic. Perhaps this could be tried out?
We did decide to experment the "experienced driver" maintainership model.
Btw, I already added an account for one such developer :-)
Ok, so we're actually trying this out. Great! :-)
There was also a request to document the sub-maintainer names in the wiki so that it'd be easier for people to figure out who to ping if their patches do not get merged.
I'm ok with that, but, after the LPC, I suspect that the best is to document it in sync with the per-subsystem profile. I'm waiting for Don to submit an updated patchset, in order to rebase our subsystem's profile.
Ok. There's a list in the wiki but I think few people end up finding it when they needed it. :-I
linuxtv.org hosting - All
Mauro noted that linuxtv.org is currently hosted in a virtual machine somewhere in a German university. The administrator of the virtual machine has not been involved with Video4Linux for some time but has been kind to provide us the hosting over the years.
It has been recognised that there is a need to find a new hosting location for the virtual machine. There is also a question of the domain name linuxtv.org. Discussion followed.
What could be agreed on rather immediately was that the domain name should be owned by "us". "Us" is not a legal entity at the moment, and a practical arrangement to achieve that could be to find a new association to own the domain name.
The hosting of the virtual machine could possibly be handled by the same association. In practice this would likely mean a virtual machine on a hosting provider. Ideally this would be paid for by a company or a group of companies.
No decisions were reached on the topic.
There was actually one decision: to talk with Linux Foundation about that. Laurent was against, but the majority was ok with the idea.
I remember Laurent was not the only one expressing concerns related to using such hosting providers. I rather remember hearing this from quite a few people in the discussion. Either way, the decisions related to e.g. hosting will be taken later when more information is available, including what LF has to offer.
I agree with Sakari, there was clearly no majority, that's not true.
How about replacing the original last paragraph with:
Mauro will discuss with LF to find out what they can offer. Concerns were expressed over other organisations providing us with hosting we are not in charge of ourselves. Other options related to domain ownership and hosting will be researched as well.
Works for me.
Tuesday's stateless codec discussion
Hans presented a summary of this in his stateless codec status presentation, here are a bit more details.
We had a discussion (first in the Microsoft sponsor suite, then at the bar)
I don't think that the room location is relevant for the report :-)
It adds colours to the report though :-)
:-D
Also, we should split this on a separate report, as this was another meeting and not all people listed above participated on it.
Works for me.
on how to support user space for the stateless codecs better. The expected outcome of that would be a rough understanding how a stateless codec user space library would look like.
The raw notes are available here:
URL:http://www.retiisi.org.uk/~sailus/v4l2/notes/osseu18-codecs.html
Attendees
Alexandre Courbot Chris Healy Ezequiel Garcia Hans Verkuil Kieran Bingham Laurent Pinchart Maxime Ripard Mauro Carvalho Chehab Nicolas Dufresne Niklas Söderlund Philip Zabell Sakari Ailus Tomasz Figa Victor Jáquez
Buffer management
Nicolas reported an issue in V4L2 buffer management. The V4L2 decouples the buffers from the format, and assumes all queued buffers (at a given point of time) have the same format. (Ed. note: the request API could be used to address this, but that particular features is not yet supported.)
- User space library
The existing projects generally integrate their own bitstream parsers for codecs. There are subtle reasons why that tends to be the case, instead of using more generic parsers. There are differences in error handling, for instance, or other matters of policy, the variation which could be difficult to fully offer using a generic API.
Maxime noted that VLC recently released a new parser meant to be used as a library, and that could be useful. Nicolas believes that we'd need a parser library independent of any other code base to avoid pulling in extra libraries and this parser would need to be maintained. It could be difficult to find the volunteers to do that.
Does ChromeOS have its own parser? Alexandre believes it does, but little was known beyond that.
There's also the language problem: ffmpeg and gstreamer are written in C, the ChomeOS parser in C++, VLC is moving to Rust. What do we pick, how do we ensure interoperability?
- libVA re-use
As a short-term solution, implementing a generic wrapper using the V4L2 stateless codec API to offer libVA API would enable generic applications to use the V4L2 stateless codec drivers as most applications already support libVA.
70 % of the applications use FFMPEG, which has a software codec API that is nearly identical to the V4L2 statless codec API. It would be trivial for applications to switch to V4L2 natively.
Mauro would like us to explain our plans to Intel to avoid surprises later on.
To be clear: it was said at the meeting that libVA is sponsored an maintained by Intel. If we're willing to use it, we should sync with them, in order to avoid unexpected surprises if they change it in a way that would cause problems for the V4L2 stateless coded implementation.
How about, instead of the original:
We need to explain our plans to libVA maintainers to better coordinate libVA API development in a way the V4L2 stateless codecs are taken into account.
- Source code hosting
libva is hosted on freedesktop. Should we host the libva-v4l2-codec backend there, or host it on linuxtv.org? Hans would prefer linuxtv.org as it's "closer to our kernel implementation".
- Backend support in libva
libva loads backends in order, and picks the first one that reports it can support the platform. There is also an environment variable that can specify a backend. Ezequiel enquired how to support platforms that would have multiple hardware codecs. libva doesn't seem to support this at the moment. Nicolas reported that there's an Intel SoC that have both an Intel graphics core and a Vega64 graphics core that both have a codec.
Hans said that a platform that expose multiple codecs will likely be used for specialized applications, and requiring those to implement codec support directly is acceptable. Our main focus should be to support the common case.
- Vendor support
NVidia is following our progress and is interested in using the V4L2 stateless API. On the userspace side, vdpau is pretty much dead, they have moved to nvdec. OMX is being phasing out, in particular that is taking place for RaspberryPi now.
- Tooling
bootlin has developed a debugging tool called v4l2-request-test (https://github.com/bootlin/v4l2-request-test) that has been very useful to debug the codec driver without going through the full userspace stack. This is worth mentioning and integrating.
- API discussions
Using buffer indices as handles to reference frames
This has been proposed by Tomasz, and Hans has serious concerns, he believes that having userspace predict what buffer indices will be used in the future is very fragile and would prefer using a separate 64-bit cookie associated with v4l2_buffers.
Using capture buffer indices as reference frame handles requires predicting the buffer index on the capture queue which the output queue frames will be decoded into. We could use the output queue buffer index instead, but that wouldn't work with multi-slice decoding (multiple output buffers for a single capture buffer). Using a cookie set by userspace on the output side, then copied to the capture queue by the driver, solves that problem. All slices queued on the output queue for the same decoded picture will have the same cookie value (userspace will have to ensure that).
Tomasz would prefer a buffer index-based solution, to avoid keeping a cookie-index map in userspace. Due to how V4L2 works, enqueuing a new dmabuf handle on the capture side for a V4L2 buffer with a given index will effectively delete the corresponding cookie, so userspace would need to ensure it doesn't overwrite buffers; (Tomasz: To clarify, I don't see the significant benefit of using cookies over indices. It makes it easier for user space, because it doesn't have to predict the CAPTURE buffers, but still is error prone because of the buffer requeuing problem. For now it would be good to see how it translates into real code, though. In the meantime I can try to find a better idea.)
Once we have a final version for both Tuesday meeting and the Linux Media Summit, I'll post it at the "news" section of linuxtv, add the group photo and the links for the presentation and for our nightly dinner.
Sounds good.
-- Regards,
Laurent Pinchart
media-workshop mailing list media-workshop@linuxtv.org https://www.linuxtv.org/cgi-bin/mailman/listinfo/media-workshop
Em Mon, 11 Mar 2019 14:29:14 +0200 Laurent Pinchart laurent.pinchart@ideasonboard.com escreveu:
We actually reverted it, but it caused a huge confusion and produced lots of discussions. We lost several active developers: people that were not happy by the 9000 lines patchset stepping on everyone's feet and people that were not happy by reverting it.
Do you happen to remember any details? Did that "reverting" for instance involve rolling back to the state before the offending patch after more comments had been done?
I'd like a more detailed version of that story too. It's hard to comment on this particular example without knowing what happened.
You can take a look at the lore archives in 2008. There are lots of threads from that time related to it. There were even some discussions at LKML. I won't myself review the past, as those were very painful days. Unfortunately, on that time, we weren't logging the IRC discussions. A lot of the heat happened there.
That said, I feel this is not overly important. The DRM folks have proved this model works. Still I agree this is good to remember and document, but I don't see us getting into such situation _even if_ we'd switch to a similar way of working.
Agreed. I don't think the "9000 lines revert" is relevant anymore as such. What matters is how to keep existing and attract new developers, by making the linux-media subsystem attractive and easy and pleasant to work with.
No. What's relevant is that we tried a multiple committers model in the past worked fine for 3 years, but it didn't survive to the first crisis.
I'm OK to make the subsystem more attractive, provided that we won't do the same mistakes. So, any change should happen on baby steps, and on a way that it won't repeat the problem where a single developer would be able to make everybody else unhappy.
Some opined that we do not have a bottleneck in reviewing patches and getting them merged whilst others thought this was not the case. It is certainly true that a very large number of patches (around 500 in the last kernel release) went in through the media tree.
It still appears that there would be more patches and more drivers to get in if the throughput was higher.
I'm not so sure about that (if we expect good quality patches), specially while we don't have any automatic testing tool to double check some stuff.
I agree. To help improving the process from here, we do need automated testing. I don't think anyone has really even argued against adding automated testing.
It won't be enough though. Testing is crucial to scale, but isn't a substitute for review, it won't solve the review bottleneck by itself.
I agree.
Considering the amount of coverage in the meeting as well as the interest in general, it's just a question of time until we have something quite usable.
As a result of those discussions, One of the things that we've agreed there is to give trees at LinuxTV for more active developers that we trust enough to skip a sub-maintainer's review.
No, please. *Nobody* *ever* should have review bypass rights, there is no single exception to that rule. I fully agree we should get more people on-board and give them trees on linuxtv.org, but that's not about skipping review.
This is what it was agreed there, and it applies only to drivers, not to the subsystem's core.
We can review such policy on a next media summit.
In any case, I take a look on all patches I receive. If I notice something that I believe it would require sub-maintainers review, I'll forward the pull request to a sub-maintainer.
Thanks, Mauro
Em Mon, 11 Mar 2019 13:23:58 +0200 Sakari Ailus sakari.ailus@iki.fi escreveu:
Hi Mauro,
On Wed, Dec 12, 2018 at 05:30:02AM -0200, Mauro Carvalho Chehab wrote:
Sakari,
Em Sun, 18 Nov 2018 00:45:02 +0200 Sakari Ailus sakari.ailus@iki.fi escreveu:
Hello everyone,
Sorry for taking so long to review this. Was very busy those days.
Likewise in my reply. Please see my comments below. Let me know if you're fine with the proposed changes.
It follows my comments.
Here's the report on the Media Summit held on 25th October in Edinburgh. The report is followed by the stateless codec discussion two days earlier.
Note: this is bcc'd to the meeting attendees plus a few others. I didn't use cc as the list servers tend to reject messages with too many recipients in cc / to headers.
Most presenters used slides some of which are already available here (expect more in the near future):
URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/
The original announcement for the meeting is here:
URL:https://www.spinics.net/lists/linux-media/msg141095.html
The raw notes can be found here:
URL:http://www.retiisi.org.uk/~sailus/v4l2/notes/osseu18-media.html
Attendees
Brad Love Ezequiel Garcia Gustavo Padovan Hans Verkuil Helen Koike Hidenori Yamaji Ivan Kalinin Jacopo Mondi Kieran Bingham Laurent Pinchart Mauro Chebab Maxime Ripard Michael Grzeschik Michael Ira Krufky Niklas Söderlund Patrick Lai Paul Elder Peter Griffin Ralph Clark Ricardo Ribalda Sakari Ailus Sean Young Seung-Woo Kim Stefan Klug Vinod Koul
CEC status - Hans Verkuil
Hans prensented an update on CEC status. Besides the slides, noteworthy information is maintained here:
URL:https://hverkuil.home.xs4all.nl/cec-status.txt
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-cec-status.pdf
It makes sense to add a quick summary of the main points to the meeting report that was there at the slide deck, in order to make the report more complete.
Bullet points from the slide:
cec-gpio error injection support
tda998x (including BeagleBoard Bone support after gpiolib changes)
ChromeOS EC CEC
In progress: omap5/dra7xx/am57xx TI (waiting for DSS redesign to land)
In progress: SECO cec driver (for UDOO x86 boards, expected for 4.21)
DisplayPort CEC-Tunneling-over-AUX for i915, nouveau, amdgpu
MegaChips 2900 chipset based adapters seems to support this protocol very well
Continuing work on CEC utilities, esp. the compliance test: it is in continuous use at Cisco.
OK!
rc-core status report - Sean Young
(Contributed by Sean Young)
Sorry, I didn't understand what you're meaning here. The status report was made by Sean. No need to repeat it.
Sean wrote the summary as well, the rest is written by me.
I see. Well, I don't think we need to track this, as the Etherpad notes itself (with was used as the basis for the report) was authored by several participants of the Media Summit.
In the last year all staging lirc drivers have been either removed or ported to rc-core. Decoding of the more obscure IR protocols and protocol variants can now be done with BPF, with support in both the kernel and ir-keytable (which is in v4l-utils). Generally we're in a good situation wrt IR support.
There is some more ancient hardware (serial or usb-serial) that does not have support but not sure if anyone cares. kernel-doc is a little sparse and does not cover BPF IR decoding, so that needs improving. There was a discussion on enabling builds with CONFIG_RC_CORE=n. Sean suggested we could have rc_allocate_driver() return NULL and have the drivers deal with this gracefully, i.e. their probe functions should continue without IR. Mauro said there should be a per-driver config option (as is done for saa7134 for example).
Please break each topic on different paragraphs, as it makes easier to read and comment.
No conclusion was reached on this.
No conclusion was reached on what?
That probably raises more questions than it answers. I'll drop the line.
Ok.
Persistent storage of controls - Ricardo Ribalda
Ricardo gave a presentation on a proposed solution for using the V4L2 control framework as an interface for updating control value defaults on sensor EEPROM.
Sensors commonly come with device specific tuning information that's embedded in the device EEPROM. Whereas this is also very common for raw cameras on mobile devices, the discussion this time was concentrated on industrial cameras.
The EEPROM contents may be written by the sensor vendor but occasionally may need to be updated by customers. Setting the control default value was suggested as the exact mechanism to do this.
The proposal was to use controls as the interface to update sensor tuning information in the EEPROM.
There were arguments for and against the approach:
- Drivers usually get these things right: relying on an user space program to do this is an additional dependency.
- Re-use of an existing interface (root priviledge check may be added).
- Partial solution only: EEPROM contents may need to be updated for other reasons as well, and a "spotty" implementation for updating certain EEPROM locations seems very use case specific.
- Changes required to the control framework for this --- defaults are not settable at the moment.
- The need is very use case specific, and adding support for that in a generic framework does not seem to fit very well.
I remember I mentioned, as an alternative, to use the firmware API, if one wants to update the eeprom contents. If I'm not mistaken, Ricardo opted not using it.
Ricardo?
I let Ricardo to comment that.
The general consensus appears to be not to change the control framework this way, but to continue to update the EEPROM using a specific user space program.
Tooling for sub-system tree maintenance - Laurent Pinchart
Laurent talked about the DRM tree maintenance model.
The DRM tree has switched to co-maintainer model. This has made it possible to share the burden of tree maintenance, removing bottlenecks they've had.
The larger number of people having (and using) their commit rights has created the need for a more strict rules for the tree maintenance, and subsequently a tool to implement it. It's called "DIM", the DRM Inglorious Maintenance tool. This is a command line tool that works as a front-end to execute the workflow.
URL:https://01.org/linuxgraphics/gfx-docs/maintainer-tools/dim.html
In particular what's worth noting:
The conflicts are resolved by the committer, not by the tree maintainer.
DIM stores conflict resolutions (as resolved by developers) to a shared cache.
DIM makes doing common mistakes harder by using sanity checks.
There are about 50 people who currently have commit rights to the DRM tree. There are no reports of commit rights having been forcibly removed as of yet. This strongly suggests that the model is workable.
The use of the tool puts additional responsibilities as well as some burden to the committers. Before the patches may be pushed, they are first compiled on developer's machine. That requires time, and without special arrangements such as having a second local workspace, and that time is away from productive work.
The discussion that followed was concentrated on the possibility of using a similar model for the media tree. While the suggestion was initially met by mostly favourable reception, there were concerns as well.
V4L2 *was* maintained generally according to the suggested model --- albeit without the proposed tools or process that needed to be strictly followed. There was once an incident which involved merging around 9000 lines of unreviewed code in a lot of places. What followed was not pretty, and this eventually lead to loss of multiple developers.
Could this happen again? The DRM tree has not suffered such incidents, and generally it understood such incident could be addressed by simply reverting such a patch and removing commit rights if necessary.
(Editor note: we have reverted the media tree master state to an earlier commit many times for various reasons. Could it be one of the reasons the 9000 line patch was not reverted was that the version control wasn't based on git??)
We actually reverted it, but it caused a huge confusion and produced lots of discussions. We lost several active developers: people that were not happy by the 9000 lines patchset stepping on everyone's feet and people that were not happy by reverting it.
Do you happen to remember any details? Did that "reverting" for instance involve rolling back to the state before the offending patch after more comments had been done?
Don't remember the exact details anymore. This was at the time we used Mercurial to manage the tree. It was either a patch reverting the previous stuff, or more likely, we just dropped the patch from it.
That said, I feel this is not overly important. The DRM folks have proved this model works.
So far, it is working for them, but, last time I asked, it seems that DRM didn't have yet an issue where a developer with commit rights that had started to deliberately violate their policies.
For media, the multi-commit model worked fine for us by the time we used it (from 2005 to ~2008), until we had a major issue and had to change it.
Still I agree this is good to remember and document, but I don't see us getting into such situation _even if_ we'd switch to a similar way of working.
As we had this situation in the past, I would prefer to wait until DRM start to have similar issues in order to see how they'll handle it in practice, in order to evaluate about returning to use such model again.
Anyway, this is post-media summit discussions. For the report, we should stick with what it was discussed there.
Some opined that we do not have a bottleneck in reviewing patches and getting them merged whilst others thought this was not the case. It is certainly true that a very large number of patches (around 500 in the last kernel release) went in through the media tree.
It still appears that there would be more patches and more drivers to get in if the throughput was higher.
I'm not so sure about that (if we expect good quality patches), specially while we don't have any automatic testing tool to double check some stuff.
I agree. To help improving the process from here, we do need automated testing. I don't think anyone has really even argued against adding automated testing.
Considering the amount of coverage in the meeting as well as the interest in general, it's just a question of time until we have something quite usable.
As a result of those discussions, One of the things that we've agreed there is to give trees at LinuxTV for more active developers that we trust enough to skip a sub-maintainer's review.
We used to have more active developer involvement. Anyway, I'll add a note on this.
We also agreed to try to improve the tooling at linuxtv.org, in order to try to improve our processes (although this discussion was actually split on other topics, like KernelCI and linuxtv infra).
How about:
It was also agreed to try to improve tooling at linuxtv.org to streamline the workflow. (Ed. note: also see testing related topics below.)
OK!
Current status of testing on the media tree - Sakari
The common practice in media subsystem development is that developers do test their patches before submitting them. This is an unwritten rule: sometimes patches end up not being tested after making slight changes to them, or they have been tested on a different kernel version. The developer may also simply forget to test the patch.
Besides this, it is not uncommon that changing the kernel configuration or switching to a different architecture will cause a compilation warning or an error.
The 0-day bot will catch some of these errors before the patches are merged, but that testing does not fully cover all the possible cases. There are some common pain points in V4L2-related Kconfig options (plain V4L2, MC or MC + subdev uAPI); newly submitted drivers may in fact require one of these, but the developer may not have realised that and so this ends up not being taken into account in Kconfig.
Once the review is done, and after being applied to the sub-maintainer tree, a patch is applied to Mauro's local tree and Mauro performs additional tests on it. These tests currently prevent a fair number of problems reaching a wider audience than the media developers.
On the other hand, whenever an issue is found, the patch will have to be fixed by the sub-maintainer or the developer. This is hardly ideal, as the problem has existed usually for a month or two before being spotted --- by a program. These checks should be instead performed on the patch when it's submitted.
Automated testing - Ezequiel Garcia
Ideal Continuous Integration process consists of the following steps:
- patch submission
- review and approval
- merge
The core question is "what level of quality standards do we want to enforce". The maintenance process should be modelled around this question, and not the other way around. Automated testing can be a part of enforcing the quality standards.
There are three steps:
- Define the quality standard
- Define how to quantify quality in respect to the standard
- Define how to enforce the standards
On the tooling side, an uAPI test tool exists. It's called v4l2-compliance, and new drivers are required to pass the v4l2-compliance test. It has quite a few favourable properties:
- Complete in terms of the uAPI coverage
- Quick and easy to run
- Nice output format for humans & scripts
There are some issues as well:
- No codec support (stateful or stateless)
- No SDR or touch support
- Frequently updated (distribution shipped v4l2-compliance useless)
- Only one contributor
Ezequiel noted that some people think that v4l2-compliance is changing too often but Hans responded that this is a necessity. The API gets amended occasionally and the existing API gets new tests. Mauro proposed moving v4l2-compliance to the kernel source tree but Hans preferred keeping it separate. That way it's easier to develop it.
To address the problem of only a single contributor, it was suggested that people implementing new APIs would need to provide the tests for v4l2-compliance as well. To achieve this, the v4l2-compliance codebase needs some cleanup to make it easier to contribute. The codebase is larger and there is no documentation.
V4l2-compliance also covers MC, V4L2 and V4L2 sub-device uAPIs.
DVB will require its own test tooling; it is not covered by v4l2-compliance. In order to facilitate automated testing, a virtual DVB driver would be useful as well. The task was added to the list of projects needing volunteers:
URL:https://linuxtv.org/wiki/index.php/Media_Open_Source_Projects:_Looking_for_Volunteers
There are some other test tools that could cover V4L2 but at the moment it seems somewhat far-fetched any of them would be used to test V4L2 in the near future:
- kselftest
- kunit
- gst-validate
- ktf (https://github.com/oracle/ktf, http://heim.ifi.uio.no/~knuto/ktf/)
KernelCI is a test automation system that supports automated compile and boot testing. As a newly added feature, additional tests may be implemented. This is what Collabora has implemented, effectively the current demo system runs v4l2-compliance on virtual drivers in a virtual machines (LAVA slaves).
A sample of the current test report is here:
URL:https://www.mail-archive.com/linux-media@vger.kernel.org/msg135787.html
The established way to run KernelCI tests is off the head of the branches of the stable and development kernel trees, including linux-next. This is not useful as such to support automated testing of patches for the media tree: the patches need to be tested before they are merged, not after merging.
In the discusion that followed among a slightly smaller group of people, it was suggested that tests could be run from select developer kernel trees, from any branch. If a developer needs long-term storage, (s)he could have another tree which would not be subject automated test builds. Alternatively, the branch name could be used as a basis for triggering an automated build, but this could end up being too restrictive.
Merging the next rc1 by the maintainer would be no special case: the branch would be tested in similar way than the developer branches containing patches, and tests should need to pass before pushing the content to the media tree master branch.
Ezequiel wished that people would reply to his e-mail to express their wishes on the testing needs (see sample report above).
Stateless codecs - Hans Verkuil
Support for stateless codecs will be merged for v4.20 with an Allwinner staging codec driver.
The earlier stateless codec discussion ended up concluding that the bitstream parsing is application specific, so there will be no need for a generic implementation that was previously foreseen. The question that remains is: should there be a simple parser for compliance testing?
All main applications support libva which was developed as the codec API to be used with Intel GPUs. A libVA frontend was written to support the Cedrus stateless V4L2 decoder driver. It remains to be seen whether the same implementation could be used as such for the other stateless codec drivers or whether changes, or in the worst case a parallel implementation, would be needed.
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-codec-userspace.pdf
New versions of the old IOCTLs - Hans Verkuil
V4L2 is an old API with shifting focus in terms of functionality and hardware supported. While there has been lots of changes to the two during the existence of V4L2, some of the API is unchanged since the old times. While the API is usable for the purpose, it is needlessly clunky: it is often not obvious how an IOCTL is related to the task at hand (such as using S_PARM to set the frame interval) or the API does not use year 2038-safe timestamps (struct v4l2_buffer). These APIs deserve to be updated.
- VIDIOC_*_PARM
In the case of VIDIOC_G_PARM and VIDIOC_S_PARM, the IOCTLs are only used to set and get the frame interval.
In this case, what can be done, is to add a new IOCTL definition, with the same IOCTL number and with binary-equivalent IOCTL argument struct that only contains the field for the frame rate itself. This is binary-compatible with the existing code and no compatibility code will be needed. The new IOCTLs will be called VIDIOC_G_FRAME_INTERVAL and VIDIOC_S_FRAME_INTERVAL.
- VIDIOC_ENUM_FRAME_INTERVALS
Besides discrete set of supported frame intervals, VIDIOC_ENUM_FRAME_INTERVALS has stepwise frame interval as well. Stepwise could be removed as the Qualcomm venus codec and uvc (100 ns units) are the only users. Additionally, the buffer type should be added to struct v4l2_frmivalenum.
There was also a discussion related to enumerating frame intervals in units of ns vs. fractional seconds. The reasoning using a fraction is that this way the frame interval on many standards can be conveyed precisely. Somebody recalled "flick", that is is the common denominator of the frame rates on all TV standards. Drivers could simply move to use the flick as the denominator, to make frame interval reporting uniform across the drivers.
- struct v4l2_buffer
struct v4l2_buffer is an age-old struct. There are a few issues in it:
- The timestamp is not 2038-safe.
- The multi-plane implementation is a mess.
- Differing implementation for the end single-plane and multi-plane APIs is confusing for both applications and drivers.
The proposal is to create a new v4l2_buffer struct. The differences to the old one would be:
- __u64 timestamps. These are 2038-safe. The timestamp source is maintained, i.e. the type remains CLOCK_MONOTONIC apart from certain drivers (e.g. UVC) that lets the user choose the timestamp.
- Put the planes right to struct v4l2_buffer. The plane struct would also be changed; the new plane struct would be called v4l2_ext_plane.
- While at it, the plane description can be improved:
- The start of data from the beginning of the plane memory.
- Add width and height to the buffer? This would make image size changes easier for the codec. (Ed. note: pixel format as well. But this approach could only partially support what the request API is for.)
- Unify single- and multi-planar APIs.
The new struct could be called v4l2_ext_buffer.
As the new IOCTL argument struct will have has different syntax as well as
s/have has/have/
Will fix.
Thanks.
semantics, it deserves to be named differently. Compatibility code will be needed to convert the users of the old IOCTLs to the new struct used internally by the kernel and drivers, and then back to the user.
- struct v4l2_create_buffers
Of the format, only the pix.fmt.sizeimage field is effectively used by the drivers supporting VIDIOC_CREATE_BUFS. This could be simplified, by just providing the desired buffer size instead of the entire v4l2_format struct. The user would be instructed to use TRY_FMT to obtain that buffer size.
The need to delete buffers seems to have eventually surfaced. That was expected, but it wasn't known when this would happen. As the buffer index range would become non-contiguous, it should be possible to create buffers one by one only, as otherwise the indices of the additional buffers would no longer be communicated to the user unambiguously.
So there would be new IOCTLs:
- VIDIOC_CREATE_BUF - Create a single buffer of given size (plus other non-format related aspects)
- VIDIOC_DELETE_BUF - Delete a single buffer
- VIDIOC_DELETE_ALL_BUFS - Delete all buffers
The naming still requires some work. The opposite of create is "destroy", not "delete".
- struct v4l2_pix_format vs. struct v4l2_pix_format_mplane
Working with the two structs depending on whether the format is multi-planar or not is painful. While we're doing changes in the area, the two could be unified as well.
(Editor note: this could be still orthogonal to the buffers, so it could be done separately as well. We'll see.)
I suspect that those "editor note" (as any post-meeting notes) don't belong to the final report.
I wanted to make the report more readable for people who are not actively working on V4L2 development and are thus likely not able to make such connections.
I understand, but the focus of the report is on what happened there.
Feel free to do any personal views about what happened there, or about any post-meeting notes, but please keep them outside of the report's body, as the report should be as objective as possible about what was discussed there.
But yeah, perhaps this could be done seprarately. Let's discuss it when actual patches gets posted.
Slides: URL:https://www.linuxtv.org/downloads/presentations/media_summit_2018/media-new-ioctls.pdf
I guess there was an action plan for that, based on the discussions (maybe some of them ended by being merged with the presentation on the above?).
Hans,
Did you take any notes about the actions to be taken? I found very helpful to have an action plan item below the topics where we made such plan.
Fault tolerant V4L2 - Kieran Bingham
Kieran presented a system where the media hardware complex consisted of eight more or less independent camera sensors that naturally end up being within a single media device.
The current implementation, as well as the API, necessitates that all devices in a media device probe successfully before the entire media device is exposed to the user. Otherwise the user would see with a partial view of the device, without the knowledge it is such.
To address the problem, additional information need to be provided to the user space. In particular:
Events on the media device to tell the graph has changed.
Graph version number is incremented at graph change (already implemented).
The property API could be applicable --- placeholders for entities that have not yet appeared?
- Alternative: known entities that have failed to probe created in the media graph and marked "disable" or "failed".
Query the state of media graph completeness.
That way, even when the devices in a media controller device appear one by one, the user space will be able to have all the necessary information on the registration state of the device.
Complex cameras - Mauro Chehab
Some new laptops integrate a raw Bayer camera + ISP instead of a USB webcam. This is expected to increase, as the solution is generally cheaper and results in better quality images --- as long as all the pieces of the puzzle are in place, including the proprietary 3A library.
Still, such devices need to be supported.
(Ed. note: there were two talks related to this topic given in the ELc-E.)
URL:https://www.youtube.com/watch?v=KpaNNJr92CY&index=31&list=PLbzoR-pLrL6qThA7SAbhVfuMbjZsJX1CY URL:https://www.youtube.com/watch?v=GIhV7tiUji0&index=60&list=PLbzoR-pLrL6qThA7SAbhVfuMbjZsJX1CY
In this specific case, it is worth to keep the note, as those presentations happened at ELC-Eu and were explicitly mentioned there during the discussions.
Development process - All
Topic-wise this is continuation of the "Tooling for sub-system tree maintenance", "Current status of testing on the media tree" and "Automated testing" topics above.
The question here is whether there's something that could be improved in the media development process and if so, how could that be done.
What came up was a suggestion to have multi-committer tree in a similar manner as the DRM developers do. This was seen to be more interesting for developers than simply being asked to review patches.
It certainly does raise the need for more precise rules for what may be committed to the multi-committer tree, when etc.
It was also requested that experienced driver maintainers would send pull requests on patches to their drivers instead of going through a sub-maintainer (pre-agreed with the relevant (sub)maintainer). This would take some work away from sub-maintainers, but not the maintainer.
No firm decisions were reached in this topic. Perhaps this could be tried out?
We did decide to experment the "experienced driver" maintainership model.
Btw, I already added an account for one such developer :-)
Ok, so we're actually trying this out. Great! :-)
There was also a request to document the sub-maintainer names in the wiki so that it'd be easier for people to figure out who to ping if their patches do not get merged.
I'm ok with that, but, after the LPC, I suspect that the best is to document it in sync with the per-subsystem profile. I'm waiting for Don to submit an updated patchset, in order to rebase our subsystem's profile.
Ok. There's a list in the wiki but I think few people end up finding it when they needed it. :-I
linuxtv.org hosting - All
Mauro noted that linuxtv.org is currently hosted in a virtual machine somewhere in a German university. The administrator of the virtual machine has not been involved with Video4Linux for some time but has been kind to provide us the hosting over the years.
It has been recognised that there is a need to find a new hosting location for the virtual machine. There is also a question of the domain name linuxtv.org. Discussion followed.
What could be agreed on rather immediately was that the domain name should be owned by "us". "Us" is not a legal entity at the moment, and a practical arrangement to achieve that could be to find a new association to own the domain name.
The hosting of the virtual machine could possibly be handled by the same association. In practice this would likely mean a virtual machine on a hosting provider. Ideally this would be paid for by a company or a group of companies.
No decisions were reached on the topic.
There was actually one decision: to talk with Linux Foundation about that. Laurent was against, but the majority was ok with the idea.
I remember Laurent was not the only one expressing concerns related to using such hosting providers. I rather remember hearing this from quite a few people in the discussion. Either way, the decisions related to e.g. hosting will be taken later when more information is available, including what LF has to offer.
I only remember Laurent raising such concerns at the meeting.
How about replacing the original last paragraph with:
Mauro will discuss with LF to find out what they can offer. Concerns were expressed over other organisations providing us with hosting we are not in charge of ourselves. Other options related to domain ownership and hosting will be researched as well.
Sounds OK to me.
Tuesday's stateless codec discussion
Hans presented a summary of this in his stateless codec status presentation, here are a bit more details.
We had a discussion (first in the Microsoft sponsor suite, then at the bar)
I don't think that the room location is relevant for the report :-)
:-D
Also, we should split this on a separate report, as this was another meeting and not all people listed above participated on it.
Works for me.
on how to support user space for the stateless codecs better. The expected outcome of that would be a rough understanding how a stateless codec user space library would look like.
The raw notes are available here:
URL:http://www.retiisi.org.uk/~sailus/v4l2/notes/osseu18-codecs.html
Attendees
Alexandre Courbot Chris Healy Ezequiel Garcia Hans Verkuil Kieran Bingham Laurent Pinchart Maxime Ripard Mauro Carvalho Chehab Nicolas Dufresne Niklas Söderlund Philip Zabell Sakari Ailus Tomasz Figa Victor Jáquez
Buffer management
Nicolas reported an issue in V4L2 buffer management. The V4L2 decouples the buffers from the format, and assumes all queued buffers (at a given point of time) have the same format. (Ed. note: the request API could be used to address this, but that particular features is not yet supported.)
- User space library
The existing projects generally integrate their own bitstream parsers for codecs. There are subtle reasons why that tends to be the case, instead of using more generic parsers. There are differences in error handling, for instance, or other matters of policy, the variation which could be difficult to fully offer using a generic API.
Maxime noted that VLC recently released a new parser meant to be used as a library, and that could be useful. Nicolas believes that we'd need a parser library independent of any other code base to avoid pulling in extra libraries and this parser would need to be maintained. It could be difficult to find the volunteers to do that.
Does ChromeOS have its own parser? Alexandre believes it does, but little was known beyond that.
There's also the language problem: ffmpeg and gstreamer are written in C, the ChomeOS parser in C++, VLC is moving to Rust. What do we pick, how do we ensure interoperability?
- libVA re-use
As a short-term solution, implementing a generic wrapper using the V4L2 stateless codec API to offer libVA API would enable generic applications to use the V4L2 stateless codec drivers as most applications already support libVA.
70 % of the applications use FFMPEG, which has a software codec API that is nearly identical to the V4L2 statless codec API. It would be trivial for applications to switch to V4L2 natively.
Mauro would like us to explain our plans to Intel to avoid surprises later on.
To be clear: it was said at the meeting that libVA is sponsored an maintained by Intel. If we're willing to use it, we should sync with them, in order to avoid unexpected surprises if they change it in a way that would cause problems for the V4L2 stateless coded implementation.
How about, instead of the original:
We need to explain our plans to libVA maintainers to better coordinate libVA API development in a way the V4L2 stateless codecs are taken into account.
Works for me.
- Source code hosting
libva is hosted on freedesktop. Should we host the libva-v4l2-codec backend there, or host it on linuxtv.org? Hans would prefer linuxtv.org as it's "closer to our kernel implementation".
- Backend support in libva
libva loads backends in order, and picks the first one that reports it can support the platform. There is also an environment variable that can specify a backend. Ezequiel enquired how to support platforms that would have multiple hardware codecs. libva doesn't seem to support this at the moment. Nicolas reported that there's an Intel SoC that have both an Intel graphics core and a Vega64 graphics core that both have a codec.
Hans said that a platform that expose multiple codecs will likely be used for specialized applications, and requiring those to implement codec support directly is acceptable. Our main focus should be to support the common case.
- Vendor support
NVidia is following our progress and is interested in using the V4L2 stateless API. On the userspace side, vdpau is pretty much dead, they have moved to nvdec. OMX is being phasing out, in particular that is taking place for RaspberryPi now.
- Tooling
bootlin has developed a debugging tool called v4l2-request-test (https://github.com/bootlin/v4l2-request-test) that has been very useful to debug the codec driver without going through the full userspace stack. This is worth mentioning and integrating.
- API discussions
Using buffer indices as handles to reference frames
This has been proposed by Tomasz, and Hans has serious concerns, he believes that having userspace predict what buffer indices will be used in the future is very fragile and would prefer using a separate 64-bit cookie associated with v4l2_buffers.
Using capture buffer indices as reference frame handles requires predicting the buffer index on the capture queue which the output queue frames will be decoded into. We could use the output queue buffer index instead, but that wouldn't work with multi-slice decoding (multiple output buffers for a single capture buffer). Using a cookie set by userspace on the output side, then copied to the capture queue by the driver, solves that problem. All slices queued on the output queue for the same decoded picture will have the same cookie value (userspace will have to ensure that).
Tomasz would prefer a buffer index-based solution, to avoid keeping a cookie-index map in userspace. Due to how V4L2 works, enqueuing a new dmabuf handle on the capture side for a V4L2 buffer with a given index will effectively delete the corresponding cookie, so userspace would need to ensure it doesn't overwrite buffers; (Tomasz: To clarify, I don't see the significant benefit of using cookies over indices. It makes it easier for user space, because it doesn't have to predict the CAPTURE buffers, but still is error prone because of the buffer requeuing problem. For now it would be good to see how it translates into real code, though. In the meantime I can try to find a better idea.)
Once we have a final version for both Tuesday meeting and the Linux Media Summit, I'll post it at the "news" section of linuxtv, add the group photo and the links for the presentation and for our nightly dinner.
Sounds good.
Thanks, Mauro
-
Ezequiel Garcia -
Laurent Pinchart -
Mauro Carvalho Chehab -
Ricardo Ribalda Delgado -
Sakari Ailus -
Tomasz Figa