[FFmpeg-devel] v4l2: bug #1570 and possible solution

[Stefano Sabatini]

On date Friday 2013-02-08 11:11:45 +0100, Giorgio Vazzana encoded:
> Hello,
> 
> I am working at fixing bug #1570. I have added some av_log() to see
> what is going on (see patch #1). The problem seems to be that code
> outside libavdevice/v4l2.c tries to read more buffers (that is, video
> frames, in v4l2 language) than we have obtained, causing the last
> ioctl call with VIDIOC_DQBUF to hang:
> 
> holden at rye:~/src/ffmpeg$ ./ffplay -loglevel debug -f video4linux2
> -channel 1 -video_size 640x480 /dev/video0
> ffplay version N-49703-g0d194ee Copyright (c) 2003-2013 the FFmpeg developers
>   built on Feb  8 2013 10:12:28 with gcc 4.4.3 (Ubuntu 4.4.3-4ubuntu5.1)
>   configuration: --enable-gpl --enable-x11grab --enable-libxvid
> --enable-libmp3lame --enable-libtheora --enable-libvorbis
> --enable-libopus --enable-libvpx --enable-libx264
>   libavutil      52. 17.101 / 52. 17.101
>   libavcodec     54. 91.102 / 54. 91.102
>   libavformat    54. 61.104 / 54. 61.104
>   libavdevice    54.  3.103 / 54.  3.103
>   libavfilter     3. 35.101 /  3. 35.101
>   libswscale      2.  2.100 /  2.  2.100
>   libswresample   0. 17.102 /  0. 17.102
>   libpostproc    52.  2.100 / 52.  2.100
> [video4linux2,v4l2 @ 0x2e2aea0] fd:5 capabilities:5010015
> [video4linux2,v4l2 @ 0x2e2aea0] Selecting input_channel: 1
> [video4linux2,v4l2 @ 0x2e2aea0] input_channel: 1, input_name: Composite1
> [video4linux2,v4l2 @ 0x2e2aea0] The V4L2 driver is using the interlaced mode
> [video4linux2,v4l2 @ 0x2e2aea0] Current standard: PAL, id: 255,
> frameperiod: 1/25
> [video4linux2,v4l2 @ 0x2e2aea0] V4L2 buffer obtained: 8
> [video4linux2,v4l2 @ 0x2e2aea0] Buffer 0 dequeued.
> [video4linux2,v4l2 @ 0x2e2aea0] Buffer 1 dequeued.
> [video4linux2,v4l2 @ 0x2e2aea0] Buffer 2 dequeued.
> [video4linux2,v4l2 @ 0x2e2aea0] Buffer 3 dequeued.
> [video4linux2,v4l2 @ 0x2e2aea0] Buffer 4 dequeued.
> [video4linux2,v4l2 @ 0x2e2aea0] Buffer 5 dequeued.
> [video4linux2,v4l2 @ 0x2e2aea0] Buffer 6 dequeued.
> [video4linux2,v4l2 @ 0x2e2aea0] Buffer 7 dequeued.
> ^Cholden at rye:~/src/ffmpeg$
> 

> Here, we requested 256 buffers to the V4L2 driver, but we obtained
> only 8 (this number changes with video_size and pixel_format). The
> buffer are dequeued one at a time, but they are not queued again
> (nothing calls mmap_release_buffer() ), so at the 9th call the ioctl
> to dequeue another buffer blocks, and that's why I think ffplay hangs.

What does determine the number of requested buffers? Also why aren't
they dequeued?

> 
> If I reduce -probesize from the command line, everything works as
> expected. So, the problem seems to be that code outside
> libavdevice/v4l2.c tries to read more data (buffers) than we have
> negotiated.
> I have added an hack to limit probesize (see patch #2) and it solves
> the problem for me:

I have no strong objection on this, but I would like to know why there
is a problem in the first place.
-- 
FFmpeg = Fantastic and Free Magic Portable Enlightening Glue