Re: [PATCH] ALSA: core: Remove trigger_tstamp_latched

[Takashi Iwai <tiwai@xxxxxxx>]

On Tue, 13 Aug 2024 15:58:13 +0200,
Zeno Endemann wrote:
> 
> Takashi Iwai wrote on 13.08.24 15:41:
> > On Tue, 13 Aug 2024 14:54:42 +0200,
> > Zeno Endemann wrote:
> >> 
> >> Pierre-Louis Bossart wrote on 13.08.24 10:04:
> >>> by focusing on the trigger timestamp I think you're looking at the wrong
> >>> side of the problem. The timestamping is improved by using the same
> >>> hardware counter for the trigger AND regular timestamp during
> >>> playback/capture. If you look at a hardware counter during
> >>> playback/capture but the start position is recorded with another method,
> >>> would you agree that there's a systematic non-reproducible offset at
> >>> each run? You want the trigger and regular timestamps to be measured in
> >>> the same way to avoid measurement differences.
> >> 
> >> I am not sure what you are talking about. I have not seen any place in the
> >> code where the trigger timestamp is taken in any other more sophisticated
> >> way than what the default is doing, i.e. calling snd_pcm_gettime. So I do
> >> not see how your custom *trigger* timestamps are done "with another method".
> >> 
> >>> I will not disagree that most applications do not need precise
> >>> timestamping, but if you want to try to enable time-of-flight
> >>> measurements for presence or gesture detection you will need higher
> >>> sampling rates and micro-second level accuracy.
> >> 
> >> I don't know, this sounds very theoretical at best to me. However I do not
> >> have the desire to try to further argue and convince you otherwise.
> >> 
> >> Do you want to propose a different solution for the stop trigger timestamp
> >> bug? That is my main goal after all.
> > 
> > Ah, I guess that the discussion drifted because of misunderstanding.
> > 
> > This isn't about the accuracy of the audio timestamp, but rather the
> > timing of trigger tstamp.  The commit 2b79d7a6bf34 ("ALSA: pcm: allow
> > for trigger_tstamp snapshot in .trigger") allowed the trigger_tstamp
> > taken in the driver's trigger callback.  But, the effectiveness of
> > this change is dubious, because the timestamp taken in the usual code
> > path in PCM core is right after the trigger callback, hence the
> > difference should be negligible -- that's the argument.
> 
> Exactly. Sorry if my communication was not clear on that.
> 
> > 
> > No matter how the fix will be, could you put the Fixes tag pointing to
> > the culprit commit(s) at the next submission?
> 
> Will do. I guess I'll have to look up which commit actually enabled the
> trigger_tstamp_latched in hda, as 2b79d7a6bf34 has no driver using that
> yet, so is not technically the culprit?

You can take the HD-audio side, the commit ed610af86a71 ("ALSA: hda:
read trigger_timestamp immediately after starting DMA") instead, too.
Maybe it doesn't matter much which commit is chosen; both should
appear in the same kernel version.


thanks,

Takashi