Re: A question about VIDIOC_DQBUF

[Andrea <audetto@tiscali.it>]

Is there anybody who could help my with the followin?
I would like to know if my interpretation of VIDIOC_DQBUF is correct.

Thanks


Andrea wrote:
> 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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