Hi Olivier, On Friday 28 March 2014 17:27:08 Olivier Langlois wrote: > On Fri, 2014-03-28 at 17:20 +0100, Laurent Pinchart wrote: > > On Friday 28 March 2014 01:42:38 Olivier Langlois wrote: > > > timestamps in v4l2 buffers returned to userspace are updated in > > > uvc_video_clock_update() which uses timestamps fetched from > > > uvc_video_clock_decode() by calling unconditionally ktime_get_ts(). > > > > > > Hence setting the module clock param to realtime have no effect > > > before this patch. > > > > > > This has been tested with ffmpeg: > > > > > > ffmpeg -y -f v4l2 -input_format yuyv422 -video_size 640x480 -framerate > > > 30 -i /dev/video0 \ -f alsa -acodec pcm_s16le -ar 16000 -ac 1 -i > > > default \ > > > -c:v libx264 -preset ultrafast \ > > > -c:a libfdk_aac \ > > > out.mkv > > > > > > and inspecting the v4l2 input starting timestamp. > > > > > > Signed-off-by: Olivier Langlois <olivier@xxxxxxxxxxxxxx> > > > Cc: Stable <stable@xxxxxxxxxxxxxxx> > > > > Before applying this, I'm curious, do you have a use case for realtime > > time stamps ? > > Yes. ffmpeg uses wall clock time to create timestamps for audio packets from > ALSA device. OK. I suppose I shouldn't drop support for the realtime clock like I wanted to then :-) > There is a bug in ffmpeg describing problems to synchronize audio and > the video from a v4l2 webcam. > > https://trac.ffmpeg.org/ticket/692 > > To workaround this issue, ffmpeg devs added a switch to convert back > monotonic to realtime. From ffmpeg/libavdevice/v4l2.c: > > -ts <int> .D.... set type of timestamps for > grabbed frames (from 0 to 2) (default 0) > default .D.... use timestamps from the kernel > abs .D.... use absolute timestamps (wall > clock) > mono2abs .D.... force conversion from monotonic > to absolute timestamps > > If the v4l2 driver is able to send realtime ts, it is easier synchronize > in userspace if all inputs use the same clock. That might be a stupid question, but shouldn't ALSA use the monotonic clock instead ? -- Regards, Laurent Pinchart -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
