linux-kernel - Re: [PATCH] [media] uvcvideo: Fix clock param realtime setting

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Date:	Fri, 28 Mar 2014 22:50:52 -0400
From:	Olivier Langlois <olivier@...llion01.com>
To:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:	m.chehab@...sung.com, linux-media@...r.kernel.org,
	linux-kernel@...r.kernel.org, Stable <stable@...r.kernel.org>
Subject: Re: [PATCH] [media] uvcvideo: Fix clock param realtime setting


> > > 
> > > 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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