On 21. 08. 24 17:05, Takashi Iwai wrote:
On Wed, 21 Aug 2024 16:59:41 +0200,
Jaroslav Kysela wrote:
On 21. 08. 24 16:44, Takashi Iwai wrote:
On Wed, 21 Aug 2024 16:27:43 +0200,
Zeno Endemann wrote:
Takashi Iwai wrote on 13.08.24 16:05:
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.
Well, I think I've waited a decent amount of time now for more comments.
How do we proceed?
I'm still of the opinion that the removal is the most sensible solution,
so if we agree I could prepare a V2 where I just improve the commit message
a bit further.
But if we don't have a good enough consensus on this, I'd need some guidance
which alternate path should be taken to at least fix the bug of bad stop
trigger timestamps for hda devices (e.g. should I try to fix it also for
soc/intel/skylake without any testing? That seems to me the only other place
that should be affected, apart from the generic pci hda code).
IIUC, the achievement of the timestamp at the exact timing was the
goal of that change (which caused a regression unfortunately), so
keeping that feature may still make sense. I'd rather try to fix in
HD-audio side at first.
If Pierre agrees with the removal of the local timestamp call, we can
revert to there afterwards, too.
What about a patch bellow. I'll send it with proper formatting, when we decide to go with it. Perhaps, the latched flag may be reset when start is false, too.
It's similar like what I had in mind, too.
(My version was to drop the call of snd_pcm_gettime() from
snd_hdac_stream_timecounter_init() but call it at each place,
instead.)
It would be probably better to change snd_hdac_stream_timecounter_init()
arguments to catch all callers even for random backports. Also bonus is to
have the timestamping in one place. But both ways are fine.
Jaroslav
--
Jaroslav Kysela <perex@xxxxxxxx>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.
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