A question about VIDIOC_DQBUF

[Andrea <audetto@tiscali.it>]

Hi,

I would like to understand better the way VIDIOC_DQBUF works.

I've read the documentation at
http://www.linuxtv.org/downloads/video4linux/API/V4L2_API/spec/r8849.htm

and some tutorial at
http://lwn.net/Articles/240667/

I'm trying to check the implementation of pwc of the 2 ioctl VIDIOC_DQBUF and VIDIOC_QBUF.

Is the following correct?

- First, an application queues a buffer, then it dequeues the buffer.
- Then again, a buffer is queued and then dequeued.
- Dequeuing a buffer blocks is the buffer is not ready (unless device opened with O_NONBLOCK).
- Trying to dequeue a buffer without queuing it first is an error, and the ioctl VIDIOC_DQBUF should 
return -EINVAL.

<- end of question ->

Now, about pwc: (if the above is correct).

1) VIDIOC_DQBUF blocks always until a buffer is ready, regardless of O_NONBLOCK.
2) VIDIOC_DQBUF does not check if a buffer has been previously queued. Moreover VIDIOC_QBUF is 
almost a no-op. It has no way to check if a buffer has been queued before VIDIOC_DQBUF.

If I have understood correctly (very unlikely), this is the reason why mplayer hangs while stopping 
the stream with pwc:

		while (!ioctl(priv->video_fd, VIDIOC_DQBUF, &buf));

This code should eventually return -EINVAL, while pwc just blocks waiting for the next buffer (which 
never arrives because VIDIOC_STREAMOFF has been called).

I've read that video-buf should simplify the handling of buffers. Should I try it?

Andrea

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