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SubjectRe: [PATCH] [media] uvcvideo: Fix clock param realtime setting
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> > >
> > > 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 ?
>

This isn't stupid. I had the same though after replying you.
Intuitively, I would think that monotonic clock is a better choice for
multimedia. I am just speculating but I would say that ffmpeg decided to
use realtime clock as the standard clock throughout the project for
portability purposes since it is a cross-platform project.

Now you know how I ended up trying the clock=realtime option.
IMHO, if the option is there, it should be working but just removing it
could also be a valid option.

I feel that this could bring some new problems if it stays there
because, I'll be honest and say that I didn't test the driver behavior
when the time goes backward....





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