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

[Arun Raghavan]

On Wed, 4 Apr 2018, at 12:49 PM, Takashi Iwai wrote:
> 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.

Is there a resolution for this?

Cheers,
Arun