[alsa-devel] [RFC PATCH] ALSA: core: do not update tstamp when hw ptr has not moved

[Takashi Iwai]

On Mon, 20 Nov 2017 14:13:32 +0100,
Henrik Eriksson wrote:
> 
> On Thu, Nov 16, 2017 at 19:49:36 +0100, Takashi Iwai wrote:
> > On Thu, 16 Nov 2017 19:11:37 +0100,
> > Pierre-Louis Bossart wrote:
> > > 
> > > On 11/16/17 8:48 AM, Takashi Iwai wrote:
> > > > On Thu, 16 Nov 2017 13:43:57 +0100,
> > > > Henrik Eriksson wrote:
> > > >>
> > > >> This reverts a change that was part of a larger commit.  That change
> > > >> broke timestamps in some application code for us as outlined in the log,
> > > >> and reverting it obviously fixes this.  Since the original change did
> > > >> not explain why the behaviour was modified I can't speak for the effects
> > > >> on that use case.
> 
> > > > So this is a regression and we need to address it.
> > > >
> > > > I *guess* that the reason to update tstamp event at the same hwptr is
> > > > that now we can get a more fine-grained tstamp.  OTOH, we have to
> > > > avoid the regression by that.
> > > >
> > > > What about the (untested) patch like below?  It skips the tstamp
> > > > update as default but follows the new standard when a better tstamp
> > > > mode is deployed.
> > > 
> > > update_audio_tstamp() only updates the audio timestamp when the type
> > > is DEFAULT already, but indeed update runtime->status->tstamp is
> > > updated unconditionally.
> > > 
> > > I have a bit of heartburn with the suggested solutions because the
> > > delay can change even with the same hw_ptr. Not updating the
> > > timestamps would be going back to the days where timing updates was
> > > dependent on DMA granularity.
> > > 
> > > Maybe we could make the tstamp update dependent on a change in
> > > audio_tstamp, e.g.
> > > 
> > > 
> > > diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
> > > index a93a4235a332..b606f3ea7a17 100644
> > > --- a/sound/core/pcm_lib.c
> > > +++ b/sound/core/pcm_lib.c
> > > @@ -248,8 +248,10 @@ static void update_audio_tstamp(struct
> > > snd_pcm_substream *substream,
> > >                                 runtime->rate);
> > >                 *audio_tstamp = ns_to_timespec(audio_nsecs);
> > >         }
> > > -       runtime->status->audio_tstamp = *audio_tstamp;
> > > -       runtime->status->tstamp = *curr_tstamp;
> > > +       if (runtime->status->audio_tstamp != *audio_tstamp) {
> 
> This should be
> 
> if (!timespec_equal(&runtime->status->audio_tstamp, audio_tstamp)) {
> 
> > > +               runtime->status->audio_tstamp = *audio_tstamp;
> > > +               runtime->status->tstamp = *curr_tstamp;
> > > +       }
> > 
> > OK for me as long as it works.
> > Henrik?
> 
> This appears to work for our use case.
> 
> How do I progress this, i.e., do I update my patch to use Pierre-Louis's
> fix?  How should I attribute that?

Care to tidy up and submit a patch with some nice description?
You can put Pierre's credit like
  Suggested-by: Pierre-Louis Bossart <pierre-louis.bossart at linux.intel.com>


thanks,

Takashi