Input and output devices support this I/O method when the
V4L2_CAP_STREAMING flag in the
capabilities field of struct v4l2_capability
returned by the VIDIOC_QUERYCAP ioctl is set. If the particular user
pointer method (not only memory mapping) is supported must be
determined by calling the VIDIOC_REQBUFS ioctl.
This I/O method combines advantages of the read/write and
memory mapping methods. Buffers are allocated by the application
itself, and can reside for example in virtual or shared memory. Only
pointers to data are exchanged, these pointers and meta-information
are passed in struct v4l2_buffer. The driver must be switched
into user pointer I/O mode by calling the VIDIOC_REQBUFS with the
desired buffer type. No buffers are allocated beforehands,
consequently they are not indexed and cannot be queried like mapped
buffers with the VIDIOC_QUERYBUF ioctl.
Example 3.2. Initiating streaming I/O with user pointers
struct v4l2_requestbuffers reqbuf;
memset (&reqbuf, 0, sizeof (reqbuf));
reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
reqbuf.memory = V4L2_MEMORY_USERPTR;
if (ioctl (fd, VIDIOC_REQBUFS, &reqbuf) == -1) {
if (errno == EINVAL)
printf ("Video capturing or user pointer streaming is not supported\n");
else
perror ("VIDIOC_REQBUFS");
exit (EXIT_FAILURE);
}
Buffer addresses and sizes are passed on the fly with the
VIDIOC_QBUF ioctl. Although buffers are commonly cycled,
applications can pass different addresses and sizes at each
VIDIOC_QBUF call. If required by the hardware the
driver swaps memory pages within physical memory to create a
continuous area of memory. This happens transparently to the
application in the virtual memory subsystem of the kernel. When buffer
pages have been swapped out to disk they are brought back and finally
locked in physical memory for DMA.[17]
Filled or displayed buffers are dequeued with the
VIDIOC_DQBUF ioctl. The driver can unlock the memory pages at any
time between the completion of the DMA and this ioctl. The memory is
also unlocked when VIDIOC_STREAMOFF is called, VIDIOC_REQBUFS, or
when the device is closed. Applications must take care not to free
buffers without dequeuing. For once, the buffers remain locked until
further, wasting physical memory. Second the driver will not be
notified when the memory is returned to the application's free list
and subsequently reused for other purposes, possibly completing the
requested DMA and overwriting valuable data.
For capturing applications it is customary to enqueue a
number of empty buffers, to start capturing and enter the read loop.
Here the application waits until a filled buffer can be dequeued, and
re-enqueues the buffer when the data is no longer needed. Output
applications fill and enqueue buffers, when enough buffers are stacked
up output is started. In the write loop, when the application
runs out of free buffers it must wait until an empty buffer can be
dequeued and reused. Two methods exist to suspend execution of the
application until one or more buffers can be dequeued. By default
VIDIOC_DQBUF blocks when no buffer is in the
outgoing queue. When the O_NONBLOCK flag was
given to the open() function, VIDIOC_DQBUF
returns immediately with an EAGAIN error code when no buffer is available. The
select() or poll() function are always available.
To start and stop capturing or output applications call the
VIDIOC_STREAMON and VIDIOC_STREAMOFF ioctl. Note
VIDIOC_STREAMOFF removes all buffers from both
queues and unlocks all buffers as a side effect. Since there is no
notion of doing anything "now" on a multitasking system, if an
application needs to synchronize with another event it should examine
the struct v4l2_buffer timestamp of captured
buffers, or set the field before enqueuing buffers for output.
Drivers implementing user pointer I/O must
support the VIDIOC_REQBUFS,
VIDIOC_QBUF, VIDIOC_DQBUF,
VIDIOC_STREAMON and
VIDIOC_STREAMOFF ioctl, the
select() and poll() function.[18]
[17] We expect that frequently used buffers are typically not swapped out. Anyway, the process of swapping, locking or generating scatter-gather lists may be time consuming. The delay can be masked by the depth of the incoming buffer queue, and perhaps by maintaining caches assuming a buffer will be soon enqueued again. On the other hand, to optimize memory usage drivers can limit the number of buffers locked in advance and recycle the most recently used buffers first. Of course, the pages of empty buffers in the incoming queue need not be saved to disk. Output buffers must be saved on the incoming and outgoing queue because an application may share them with other processes.
[18] At the driver level select() and
poll() are the same, and
select() is too important to be optional. The
rest should be evident.