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

[Alexander Strasser]

Hi all!

On 2019-03-20 19:56 +0100, Alexander Strasser wrote:
> 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.
>
> Here are two examples for such occurrences:
>
>     [video4linux2,v4l2 @ 0x258b440] Dequeued v4l2 buffer contains 609596 bytes, but 614400 were expected. Flags: 0x00000001.
>     /dev/video1: Invalid data found when processing input
>
>     [video4linux2,v4l2 @ 0x225f440] Dequeued v4l2 buffer contains 609508 bytes, but 614400 were expected. Flags: 0x00000001.
>     /dev/video1: Invalid data found when processing input
>
> The ffmpeg CLI tool will then stop capturing and exit.
>
> To avoid this, return FFERROR_REDO instead. I have not seen the
> issue appearing twice or more often in a row. It was more like
> one single error every two hours.


After thinking about the FFERROR_REDO approach some more, I think it
is not ideal in the case where a driver permanently delivers frames
of unexpected size. The FFmpeg lib call would not return to the client,
potentially freezing the running program.

So maybe the zero-sized package approach, like we do with the corrupted
frames, is more robust here? Any opinions?


Best regards,
  Alexander


>
> While searching for the above quoted error message I found a ticket
> that mentions this type of problem on our tracker.
>
> Fixes #4795
> ---
>  libavdevice/v4l2.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/libavdevice/v4l2.c b/libavdevice/v4l2.c
> index 1b9c6e7..5a1b324 100644
> --- a/libavdevice/v4l2.c
> +++ b/libavdevice/v4l2.c
> @@ -542,7 +542,7 @@ static int mmap_read_frame(AVFormatContext *ctx, AVPacket *pkt)
>                     "Dequeued v4l2 buffer contains %d bytes, but %d were expected. Flags: 0x%08X.\n",
>                     buf.bytesused, s->frame_size, buf.flags);
>              enqueue_buffer(s, &buf);
> -            return AVERROR_INVALIDDATA;
> +            return FFERROR_REDO;
>          }
>      }
>
> --