[linux-dvb] [PATCH 1/7] Adding empia base driver

Devin Heitmueller devin.heitmueller at gmail.com
Sat Nov 1 17:51:25 CET 2008


Hello Hans,

Thanks for getting back to me.  Responses inline:

On Sat, Nov 1, 2008 at 12:21 PM, Hans Verkuil <hverkuil at xs4all.nl> wrote:
>> As one of the half dozen people who are working on the linux-dvb
>> version of em28xx, I am against the wholesale replacement of the
>> current version with Markus's codebase.
>
> At this time I do not advocate replacing the current em28xx driver. But
> when they are both in the kernel, then I expect and hope that the best
> features of the em28xx driver are merged into the empia driver and that
> the current em28xx driver can eventually be dropped.

I'm certainly not against this approach, and having it in the mainline
will make it easier for others to contribute and improve the codebase.
 We would however have to deal with how to handle all the overlapping
product support and the increased confusion that results from users
reporting problems and figuring out which driver they are talking
about (this is already an issue today though as most users don't
understand that there are two drivers).

>> # Doesn't leverage common infrastructure such as videobuf (resulting
>> in duplicate functionality and more difficult for those who have to
>> maintain multiple drivers)
>
> Definitely a candidate to merge into Markus' driver eventually. There
> are more drivers that do not use videobuf (my own ivtv and cx18 drivers
> spring to mind).

Agreed.  If both drivers are used in parallel than this is less of an
issue.  I was just against the wholesale replacement, which would
result in moving backwards in these areas since the work was already
done in the mainline driver.

>> # Firmware blobs embedded in source - While it's easier for the user,
>> many distributions do not allow firmware blobs in the kernel due to
>> the belief that this is not GPL compatible. We would need to get
>> permission from the vendor to redistribute the firmware as a file (in
>> the V4L driver, we extract it from the Windows driver binary)
>
> From what I saw firmware blobs were only present in the xceive drivers,
> and it is my opinion that it is not a good idea to merge these into the
> kernel. Much better to fix the existing drivers. Having the empia
> driver into the kernel will actually force those fixes to be made.

Yes, I was referring to the Xceive drivers.   I agree with what you
are saying, as long as we can agree that we should not have parallel
tuner implementations in the kernel and the changes to use the
mainline tuners should be made *before* it gets imported.

>> # Ambigious licensing - some of the files have headers from companies
>> other than Empiatech which are very clearly not GPL compatible (like
>> the Micronas drx3973d driver). Also, it's not clear that even the
>> firmware blobs mentioned above are authorized to be redistributed by
>> their rightful owners (Xceive and Micronas). While Empiatech may be
>> ok with making a GPL driver, these parties have not consented to
>> having their intellectual property in the kernel (they may have
>> consented but the header files say just the opposite).
>
> Licensing should obviously be addressed. But such drivers (except for
> the xceive ones) are currently not used by the empia sources as
> submitted by Markus.

I do not believe they should be included into the codebase until the
licensing issues are addressed.  Having the code in the kernel is a
liability risk, even if it is not used by anything right now.

>> # It has its own xc3028 and xc5000 tuner driver. I don't know whether
>> his driver is better than the one in V4L. Presumably he has the
>> datasheets for those parts, but on the other hand the V4L driver
>> allows loading of the firmware externally. The V4L drivers are also
>> used by devices beyond the em28xx and may have functionality required
>> by other companies products.
>
> For the record: other devs have datasheets and sources as well for these
> devices.

Yes, I know.  Markus has suggested that his versions of the drivers
are better because they are based on the reference code.  The xc5000
driver aside (where the mainline driver is also based on the Xceive
reference code with proper licensing and attribution), I do not
believe he has ever offered any technical basis for his assertion.

>> # What I'll call "Black magic" - lots of arbitrary code without any
>> explanation as to what it is doing or why. Why does the DVB init
>> routine write 0x77 to register 0x12? What does that do? A combination
>> of poor use of constants and commented code combined with a lack of
>> access to the datasheets leaves this a mystery. You just have to
>> "trust that it's doing the right thing because it works"
>
> This is not an uncommon occurence when datasheets are not public.
> Hopefully Markus can address such problems when the driver is in the
> kernel. It's IMHO not a blocking issue.

He has had the opportunity to do this in his own tree, and has thus
far not done it because, as he put it in email to me "nobody cares
about this".  I disagree with this assertion personally as someone who
has had to fix bugs in the mainline driver and it would have been very
helpful to at least have commented what some of the code is doing.  He
has the datasheets, and has made a conscious decision to not describe
what the code is doing.

>> # He's the only one who has access to the datasheets, so there is
>> limited opportunity for peer review. The community driver is based on
>> reverse engineering, and we can pass around USB traces we collect to
>> justify/explain design decisions. How do you question a design when
>> the basis of answers is essentially "because the secret document that
>> I can't show you says so"?
>
> There are lots of drivers that are based on NDAs (e.g. my cx18 driver).
> The code is public, but the datasheets aren't. That's actually much
> better than to rely on reverse engineering. Of course, you get the best
> results if the datasheets are also public, but that's sadly not always
> possible. Often active developers can all get NDAs, so that multiple
> devs have access to datasheets (again, that's the case for the cx18
> driver).
>
> I see this as an advantage, not a disadvantage.

I understand the value of datasheets, as I am in this situation myself
with several devices.  However, in many cases a well written driver
will have good comments as to what it is doing (super secret
algorithms aside).  In fact, now that I have access to some of the
Empia datasheets, I have some patches for the mainline driver that
better document some of these cases.

>> I'm sorry if the sharing of my views on this matter create more
>> animosity within the community, as that is the exact opposite of what
>> I want.
>
> This is I think the last chance to get Markus' driver into the kernel.
> If this fails again, then there is no other choice but to fork it all.
> But for the end-users it's so much better if Markus would maintain the
> empia driver since he has the datasheets and hardware.
>
> Forget the history, and see this as a new driver. I think I presented a
> reasonable roadmap for it to be merged.

Sure, and if Markus is willing to compromise on things like the tuner
drivers, then this would be good for everybody.  Past experience has
suggested that he was unwilling to compromise on anything (based on my
attempts in the past), so if things have changed then I would be
thrilled to work with him.

Devin

-- 
Devin J. Heitmueller
http://www.devinheitmueller.com
AIM: devinheitmueller



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