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

[Jaroslav Kysela <perex@perex.cz>]

On 04. 09. 24 11:07, 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.
> 
> 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.

The snd_pcm_direct_slave_recover() function should be called for both dmix 
instances. It calls snd_pcm_prepare() for the "driver" PCM, so the driver 
should recover from suspend in this case, too.

See the "some buggy drivers" comment in snd_pcm_direct_slave_recover(). It 
looks like a driver issue, the "resume" flag mangling is just a workaround.

> SND_PCM_INFO_RESUME flag has impact on the flow of dmix resume. In my opinion
> the first resumed dmix instance should make sure slave pcm can be recovered
> properly no matter it's the first opened instance or secondary opened instance.
> Do you know why the secondary opened instance clear the SND_PCM_INFO_RESUME
> flag? Can we do the following modification?
> 
> diff --git a/src/pcm/pcm_direct.c b/src/pcm/pcm_direct.c
> @@ -1183,8 +1226,6 @@ 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;

Another option is to fix the buggy drivers and remove the workround (or make 
it configurable) from alsa-lib (revert commit 
6d1d620eadf32c6d963468ce56ff52cc3a2f32e2).

						Jaroslav

-- 
Jaroslav Kysela <perex@perex.cz>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.