[pulseaudio-discuss] [PATCH] alsa-util: Set ALSA report_delay flag in pa_alsa_safe_delay()

[Takashi Iwai]

On Wed, 04 Apr 2018 08:45:41 +0200,
Georg Chini wrote:
> 
> On 04.04.2018 08:01, Takashi Iwai wrote:
> > On Wed, 04 Apr 2018 00:35:24 +0200,
> > Pierre-Louis Bossart wrote:
> >> On 4/2/18 3:14 PM, Georg Chini wrote:
> >>> On 02.04.2018 21:35, Pierre-Louis Bossart wrote:
> >>>>
> >>>> On 04/02/2018 07:54 AM, Georg Chini wrote:
> >>>>> The current code does not call snd_pcm_status_set_audio_htstamp_config()
> >>>>> to configure the way timestamps are updated in ALSA. This leads to
> >>>>> incorrect time stamps in the status object returned by snd_pcm_status(),
> >>>>> so the computed latencies are wrong.
> >>>>>
> >>>>> This patch uses snd_pcm_status_set_audio_htstamp_config() to set the
> >>>>> ALSA report_delay flag to 1 before the call to snd_pcm_status(). With
> >>>>> this, time stamps are updated as expected.
> >>>>> ---
> >>>>>    src/modules/alsa/alsa-util.c | 7 +++++++
> >>>>>    1 file changed, 7 insertions(+)
> >>>>>
> >>>>> diff --git a/src/modules/alsa/alsa-util.c b/src/modules/alsa/alsa-util.c
> >>>>> index 61fb4903..b91a0e98 100644
> >>>>> --- a/src/modules/alsa/alsa-util.c
> >>>>> +++ b/src/modules/alsa/alsa-util.c
> >>>>> @@ -1187,6 +1187,7 @@ int pa_alsa_safe_delay(snd_pcm_t *pcm,
> >>>>> snd_pcm_status_t *status, snd_pcm_sframes
> >>>>>        size_t abs_k;
> >>>>>        int err;
> >>>>>        snd_pcm_sframes_t avail = 0;
> >>>>> +    snd_pcm_audio_tstamp_config_t tstamp_config;
> >>>>>          pa_assert(pcm);
> >>>>>        pa_assert(delay);
> >>>>> @@ -1200,6 +1201,12 @@ int pa_alsa_safe_delay(snd_pcm_t *pcm,
> >>>>> snd_pcm_status_t *status, snd_pcm_sframes
> >>>>>         * avail, delay and timestamp values in a single kernel call
> >>>>> to improve
> >>>>>         * timer-based scheduling */
> >>>>>    +    /* The time stamp configuration needs to be set so that the
> >>>>> +     * ALSA code will use the internal delay reported by the driver */
> >>>>> +    tstamp_config.type_requested = 1; /* ALSA default time stamp
> >>>>> type */
> >>>>> +    tstamp_config.report_delay = 1;
> >>>>> +    snd_pcm_status_set_audio_htstamp_config(status, &tstamp_config);
> >>>>> +
> >>>> are you sure it's necessary or is this possibly a misunderstanding
> >>>> of what audio_tstamps are?
> >>>>
> >>>> this command is only for the audio timestamp, and to the best of my
> >>>> knowledge you are not using the results using one of the
> >>>> snd_pcm_status_get_audio_htstamp_* commands
> >>>>
> >>>> the typical usage (see alsa-lib/test/audio_time.c) is this:
> >>>>
> >>>>      snd_pcm_status_set_audio_htstamp_config(status, audio_tstamp_config);
> >>>>
> >>>>      if ((err = snd_pcm_status(handle, status)) < 0) {
> >>>>          printf("Stream status error: %s\n", snd_strerror(err));
> >>>>          exit(0);
> >>>>      }
> >>>>      snd_pcm_status_get_trigger_htstamp(status, trigger_timestamp);
> >>>>      snd_pcm_status_get_htstamp(status, timestamp);
> >>>>      snd_pcm_status_get_audio_htstamp(status, audio_timestamp);
> >>>>      snd_pcm_status_get_audio_htstamp_report(status, audio_tstamp_report);
> >>>>
> >>>> if you are not using the _get_audio_hstamp() then the config has
> >>>> essentially no effect, and the delay is available separately in the
> >>>> status command as before.
> >>>>
> >>>>>        if ((err = snd_pcm_status(pcm, status)) < 0)
> >>>>>            return err;
> >>> See this bug report, why it is needed:
> >>>
> >>> https://bugzilla.kernel.org/show_bug.cgi?id=199235
> >>>
> >>> It finally turned out that there was not a bug but just the flag missing.
> >>> We are using  snd_pcm_status_get_htstamp() with the time smoother
> >>> to calculate sink/source latency.
> >> Humm, that looks more like a bug in the fix (20e3f9 'ALSA: pcm: update
> >> tstamp only in audio_tstamp changed'). I don't think we intended that
> >> changes in the way the audio_tstamp is calculated (with or without
> >> delay) would impact when the system timestamp is updated. I am pretty
> >> sure we only wanted to update the timestamp when the hw_ptr changed
> >> with this fix so as to go back to the traditional behavior before
> >> kernel 4.1.
> > The fact here is that hwptr still remains same but only the delay
> > changes.  As the prior-4.1 traditional behavior, the timestamp isn't
> > updated in such a case.  Before the commit, the timestamp is always
> > updated no matter whether hwptr is updated or not, and it broke
> > applications.
> >
> > So, the question is how we should look at the timestamp.  The fix is
> > based on the assumption that tstamp is always coupled with
> > audio_tstamp, where the latter calculation depends on the tstamp
> > config flag.
> >
> > OTOH, if we take rather audio_tstamp always coupled with the
> > hwptr+delay, we may fix the code in a different way, too.  But this
> > would need to remember the last delay, and moreover, I don't think
> > it's better interpretation -- if we read tstamp in that way, what the
> > heck audio_tstamp is for?
> >
> >
> > thanks,
> >
> > Takashi
> 
> As already said in the bug discussion, I think the right way to fix
> the issue is to exclude runtime->delay from the delay estimation
> when report_delay is not set.

No.  The tstamp_config is only for timestamp behavior, as its name
stands.  The runtime->delay had been always calculated even before the
htimestamp was introduced, hence such a change will break old
applications again.

An alternative "fix" would be to change the default tstamp_config to
set report_delay=1 in alsa-lib.  It has also a potential risk of
breakage, but maybe safer than excluding runtime->delay as suggested.


thanks,

Takashi