[vlc-devel] [PATCH v3 2/6] clock: add vlc_clock_main_SetInputDrift

[Rémi Denis-Courmont]

Le maanantaina 15. maaliskuuta 2021, 13.06.45 EET Denis Charmet a écrit :
> On 2021-03-12 16:08, Thomas Guillem wrote:
> > ---
> > 
> >  src/clock/clock.c | 15 +++++++++++++--
> >  src/clock/clock.h |  8 ++++++++
> >  2 files changed, 21 insertions(+), 2 deletions(-)
> > 
> > diff --git a/src/clock/clock.c b/src/clock/clock.c
> > index cfbb5eb6f6d..0bb2a9fe375 100644
> > --- a/src/clock/clock.c
> > +++ b/src/clock/clock.c
> > @@ -45,6 +45,7 @@ struct vlc_clock_main_t
> > 
> >      average_t coeff_avg; /* Moving average to smooth out the instant
> > 
> > coeff */
> > 
> >      double rate;
> >      double coeff;
> > 
> > +    vlc_tick_t input_drift;
> > 
> >      vlc_tick_t offset;
> >      vlc_tick_t delay;
> > 
> > @@ -88,6 +89,7 @@ static vlc_tick_t
> > main_stream_to_system(vlc_clock_main_t *main_clock,
> > 
> >  static void vlc_clock_main_reset(vlc_clock_main_t *main_clock)
> >  {
> >  
> >      AvgReset(&main_clock->coeff_avg);
> > 
> > +    main_clock->input_drift = 0;
> > 
> >      main_clock->coeff = 1.0f;
> >      main_clock->rate = 1.0f;
> >      main_clock->offset = VLC_TICK_INVALID;
> > 
> > @@ -266,7 +268,8 @@ static vlc_tick_t
> > vlc_clock_slave_to_system_locked(vlc_clock_t *clock,
> > 
> >          system = vlc_clock_monotonic_to_system_locked(clock, now, ts,
> > 
> > rate);
> > 
> >      }
> > 
> > -    return system + (clock->delay - main_clock->delay) * rate;
> > +    return system + main_clock->input_drift +
> 
> You cannot do that. The whole point of the main clock is to be the
> reference used to calculate the different drifts of other clocks. The
> main clock cannot drift it's the monotonic clock if you start doing that
> you are basically going back to the PCR master of the previous versions.

I think it's probably even worse than that. Because this adds the drift 
directly, this will lead to a permanently unstable clock, that on average will 
be roughly the input clock.

I don't expect anything good to come out of that.

-- 
Rémi Denis-Courmont
http://www.remlab.net/