[FFmpeg-devel] [PATCH] libavdevice/v4l2: Return EAGAIN when receiving a frame of unexpected size

[Alexander Strasser]

On 2019-03-17 18:25 +0100, Nicolas George wrote:
> Alexander Strasser (12019-03-17):
> > I had found that when capturing video for some hours from
> > USB Camera-B4.09.24.1 (Manufacturer: OmniVision Technologies, Inc.),
> > sometimes when invoking the ioctl DQBUF, the returned buffer is not
> > filled with the size we expect for the raw video frame.
> 
> Which seems like a kernel bug, does it not?

Seems like a kernel bug, yes. Probably driver specific. Though I am not
100% sure it is strictly a bug. Might be that this is the best a driver
for a particular hardware can do.

In any case at least 2 different and probably more devices exhibited the
described behaviour, which was never a permanent failure. I suspect some
drivers will always exhibit this behavior sometimes. That's why I think
we should handle this particular case more graceful.


> > To avoid this, return AVERROR(EAGAIN) instead.
> 
> Returning EAGAIN when not in non-blocking mode is invalid. FFERROR_REDO
> would be the correct code in that case.

Maybe using EAGAIN wasn't a good choice :(

Using FFERROR_REDO sounds promising. I will try that.

I tested the EAGAIN version and it worked. I also tested returning a
zero-sized packet, like in the case where V4L flags the data as corrupt,
that worked too. See commit 28f20d2ff487aa589643d8f70eaf614b78839685

Do you think the zero-sized packet would be better than returning
FFERROR_REDO?


Thanks for your comments!

  Alexander