Re: [PATCH/RFC v2 1/2] v4l: vb2: Don't return POLLERR during transient buffer underruns

[Hans Verkuil <hverkuil@xs4all.nl>]

On 06/06/2014 11:50 AM, Hans de Goede wrote:
> Hi,
> 
> On 06/05/2014 02:23 PM, Laurent Pinchart wrote:
>> The V4L2 specification states that
>>
>> "When the application did not call VIDIOC_QBUF or VIDIOC_STREAMON yet
>> the poll() function succeeds, but sets the POLLERR flag in the revents
>> field."
>>
>> The vb2_poll() function sets POLLERR when the queued buffers list is
>> empty, regardless of whether this is caused by the stream not being
>> active yet, or by a transient buffer underrun.
>>
>> Bring the implementation in line with the specification by returning
>> POLLERR only when the queue is not streaming. Buffer underruns during
>> streaming are not treated specially anymore and just result in poll()
>> blocking until the next event.
> 
> After your patch the implementation is still not inline with the spec,
> queuing buffers, then starting a thread doing the poll, then doing the
> streamon in the main thread will still cause the poll to return POLLERR,
> even though buffers are queued, which according to the spec should be enough
> for the poll to block.
> 
> The correct check would be:
> 
> if (list_empty(&q->queued_list) && !vb2_is_streaming(q))
> 	eturn res | POLLERR;

Good catch! I should have seen that :-(

v4l2-compliance should certainly be extended to test this as well.

Regards,

	Hans

> 
> Regards,
> 
> Hans
> 
> 
>>
>> Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
>> Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
>> ---
>>  drivers/media/v4l2-core/videobuf2-core.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/media/v4l2-core/videobuf2-core.c b/drivers/media/v4l2-core/videobuf2-core.c
>> index 349e659..fd428e0 100644
>> --- a/drivers/media/v4l2-core/videobuf2-core.c
>> +++ b/drivers/media/v4l2-core/videobuf2-core.c
>> @@ -2533,9 +2533,9 @@ unsigned int vb2_poll(struct vb2_queue *q, struct file *file, poll_table *wait)
>>  	}
>>  
>>  	/*
>> -	 * There is nothing to wait for if no buffers have already been queued.
>> +	 * There is nothing to wait for if the queue isn't streaming.
>>  	 */
>> -	if (list_empty(&q->queued_list))
>> +	if (!vb2_is_streaming(q))
>>  		return res | POLLERR;
>>  
>>  	if (list_empty(&q->done_list))
>>