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

Alexander Strasser eclipse7 at gmx.net
Wed Jun 5 23:04:38 EEST 2019

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.

 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;


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