Re: A question about VIDIOC_DQBUF

[Andrea <audetto@tiscali.it>]

Andrea wrote:
> Hi,
> 
> I would like to understand better the way VIDIOC_DQBUF works.
> 
...
> 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.

Moreover:

- One can only VIDIOC_DQBUF after calling STREAMON. Before it should return -EINVAL? Block?
- After calling STREAMOFF, 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 is not needed because STREAMOFF flushes the buffer queue. Does it not?

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

pwc should return -EINVAL to all ioctl calls after STREAMOFF?

Could someone please tell me where I am right and where I am wrong...

What is the reference implementation? vivi? em28xx?

Andrea

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