[FFmpeg-devel] [PATCH] lavd/v4l2: produce a 0 byte packet when a dequeued buffer's size is unexpected

[Alexander Strasser]

On 2019-06-05 22:04 +0200, Alexander Strasser wrote:
> From: Stephan Hilb <stephan at ecshi.net>
>
> Behave like we do for V4L2_BUF_FLAG_ERROR, implemented in commit 28f20d2ff4 .
>
> For some devices (probably also related to the V4L driver implementation)
> it happens that when invoking the ioctl DQBUF, the returned buffer is not
> of the expected size. 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
>
> For the ffmpeg CLI tool this means it will stop capturing and exit.
>
> The described behaviour was observed at least with one OmniVision USB
> web cam and with some stk1160 devices.
>
> If you search the web for the error message, you will find quite a few
> instances of this problem. Some of them experienced on other devices.
>
> Probably fixes ticket #4795
>
> Signed-off-by: Alexander Strasser <eclipse7 at gmx.net>
> ---
>
> This is exactly Stephan's patch except for the commit message.
>
> @Stephan: I hope you are OK with my wording in the new message.
>
> I contacted Giorgio off-list and also put him in Bcc for this email.
>
> He previously reacted, but he probably doesn't have enough time. So if
> there are no objections I intent to commit in roughly a week if no
> more issues are found and no objections are raised.

Will push soon.

The current behavior surely causes problems for some users.
So it's better drop the short frame's data, log a warning
and have applications continue capturing instead of aborting.

It's also analog to how v4l2 indev behaves when it receives
corrupted data (V4L2_BUF_FLAG_ERROR).


  Alexander

>  libavdevice/v4l2.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/libavdevice/v4l2.c b/libavdevice/v4l2.c
> index a9a0ed324d..446a243cf8 100644
> --- a/libavdevice/v4l2.c
> +++ b/libavdevice/v4l2.c
> @@ -538,11 +538,10 @@ static int mmap_read_frame(AVFormatContext *ctx, AVPacket *pkt)
>              s->frame_size = buf.bytesused;
>
>          if (s->frame_size > 0 && buf.bytesused != s->frame_size) {
> -            av_log(ctx, AV_LOG_ERROR,
> +            av_log(ctx, AV_LOG_WARNING,
>                     "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;
> +            buf.bytesused = 0;
>          }
>      }
>
> --