Re: [EXT] Re: Suspend/resume Issue on pcm_dmix.c in alsa-lib

[Takashi Iwai <tiwai@suse.de>]

On Thu, 05 Sep 2024 13:01:11 +0200,
Chancel Liu wrote:
> 
> > > > > Hi Takashi,
> > > > >
> > > > > Thanks for your reply and suggestions. Finally we have found the root
> > cause.
> > > > > Seems it's related to both drivers and alsa-lib.
> > > > >
> > > > > When two dmix clients run in parallel we get two direct dmix instances.
> > > > > 1st dmix instance:
> > > > > snd_pcm_dmix_open()
> > > > >       snd_pcm_direct_initialize_slave()
> > > > >               save_slave_setting()
> > > > > Since the driver we are using has SND_PCM_INFO_RESUME flag,
> > > > > dmix->spcm->info has this flag. Then this flag is cleared in
> > > > dmix->shmptr->s.info.
> > > > >
> > > > > 2nd dmix instance:
> > > > > snd_pcm_dmix_open()
> > > > >       snd_pcm_direct_open_secondary_client()
> > > > >               copy_slave_setting()
> > > > > 2nd dmix->spcm->info is copied from dmix->shmptr->s.info so it doesn'
> > > > > has this flag.
> > > > >
> > > > > If 1st dmix instance resumes firstly it should implement recovery of
> > > > > slave pcm in snd_pcm_direct_slave_recover(). Because 1st
> > > > > dmix->spcm->info has
> > > > > SND_PCM_INFO_RESUME，snd_pcm_resume(direct->spcm) can be called
> > > > > correctly to resume slave pcm.
> > > >
> > > > ... and immediately stop the stream, then prepare and restart as a usual
> > > > restart.
> > > >
> > > > > However if 2nd dmix instance resumes firstly,
> > > > > snd_pcm_resume(direct->spcm) will not be called because it's
> > > > > spcm->info doesn't has SND_PCM_INFO_RESUME flag. The 1st dmix
> > instance
> > > > > assumes someone else already did recovery so
> > > > > snd_pcm_resume(direct->spcm) won't be called neither. In result the
> > > > > slave pcm fails to resume.
> > > >
> > > > Something wrong happening here, then.
> > > >
> > > > In dmix, there is no hardware resume at all, but it's always a restart of the
> > > > stream.  The call of snd_pcm_resume() is only temporarily for
> > inconsistencies
> > > > that can be a problem on some drivers (IIRC dmaengine stuff).  That said,
> > > > dmix does a kind of fake resume, stops and restarts the stream cleanly on
> > the
> > > > first instance.  On the second instance, it's already recovered, hence it
> > bails
> > > > out.
> > > >
> > > > If poll() hangs on the second instance, there can be some other problem.
> > > > Maybe the resume -> stop -> restart sequence doesn't work with your
> > driver
> > > > well?
> > > >
> > >
> > > Our dma driver will do PAUSE in system suspend and requires doing RESUME
> > in
> > > system resume. Current problem is that snd_pcm_resume() is not called by
> > both
> > > 1st instance and 2nd instance.
> > 
> > That's weird.  Are you really testing with the latest alsa-lib code?
> > 
> > If application doesn't call snd_pcm_resume(), it means that the PCM
> > state isn't set to SUSPENDED, so it pretends as if still running.
> > 
> > Or if you mean that snd_pcm_resume() to the slave PCM isn't called
> > (even though snd_pcm_resume() is called for the dmix PCM), check
> > whether snd_pcm_direct_slave_recover() gets called, especially at the
> > point:
> > 
> >         /* some buggy drivers require the device resumed before prepared;
> >          * when a device has RESUME flag and is in SUSPENDED state,
> > resume
> >          * here but immediately drop to bring it to a sane active state.
> >          */
> >         if (state == SND_PCM_STATE_SUSPENDED &&
> >             (direct->spcm->info & SND_PCM_INFO_RESUME)) {
> >                 snd_pcm_resume(direct->spcm);
> >                 snd_pcm_drop(direct->spcm);
> >                 snd_pcm_direct_timer_stop(direct);
> >                 snd_pcm_direct_clear_timer_queue(direct);
> >         }
> > 
> > Try to put debug prints or catch via breakpoint whether this code path
> > is executed.
> > 
> > Also, does the issue happen with the latest 6.11-rc kernel, too?
> > If yes, what if you drop SNDRV_PCM_INFO_RESUME bit flag in the driver
> > side?  Does the problem persist, or it works?
> > 
> 
> I'm working on kernel 6.6 and alsa-lib v1.2.11. It's not so outdated I think and
> then I will try to switch on the latest version.
> 
> Indeed I did some debug on this part. Please see my comments inline.
> 
> int snd_pcm_direct_slave_recover(snd_pcm_direct_t *direct)
> {
> 	...
> 	
> 	/* [Chancel]
> 	 * When two dmix clients run in parallel we get two direct dmix instances.
> 	 * 1st dmix->spcm->info has SND_PCM_INFO_RESUME flag but 2nd dmix doesn't.

OK, that must be the cause.  It's because the second open copies the
saved shmem->s.info into spcm->info at its open time while we already
dropped the INFO_RESUME bit.  All the rest behavior are side effect of
this inconsistency.

I guess dropping the INFO_RESUME bit at hw_params and hw_refine should
work instead.  A totally untested fix is below.

(And I believe the drop of INFO_PAUSE should be handled similarly,
 too, instead of dropping spcm->info bit there.)


Takashi

--- a/src/pcm/pcm_direct.c
+++ b/src/pcm/pcm_direct.c
@@ -1018,6 +1018,7 @@ int snd_pcm_direct_hw_refine(snd_pcm_t *pcm, snd_pcm_hw_params_t *params)
 	}
 	dshare->timer_ticks = hw_param_interval(params, SND_PCM_HW_PARAM_PERIOD_SIZE)->max / dshare->slave_period_size;
 	params->info = dshare->shmptr->s.info;
+	params->info &= ~SND_PCM_INFO_RESUME;
 #ifdef REFINE_DEBUG
 	snd_output_puts(log, "DMIX REFINE (end):\n");
 	snd_pcm_hw_params_dump(params, log);
@@ -1031,6 +1032,7 @@ int snd_pcm_direct_hw_params(snd_pcm_t *pcm, snd_pcm_hw_params_t * params)
 	snd_pcm_direct_t *dmix = pcm->private_data;
 
 	params->info = dmix->shmptr->s.info;
+	params->info &= ~SND_PCM_INFO_RESUME;
 	params->rate_num = dmix->shmptr->s.rate;
 	params->rate_den = 1;
 	params->fifo_size = 0;
@@ -1183,8 +1185,6 @@ static void save_slave_setting(snd_pcm_direct_t *dmix, snd_pcm_t *spcm)
 	COPY_SLAVE(buffer_time);
 	COPY_SLAVE(sample_bits);
 	COPY_SLAVE(frame_bits);
-
-	dmix->shmptr->s.info &= ~SND_PCM_INFO_RESUME;
 }
 
 #undef COPY_SLAVE