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_RESUMEļ¼snd_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.
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
.
The snd_pcm_resume() gets called no matter which instance, just the first one who tries to recover the suspended state. (And it's called internally at updating the various state, not necessarily an explicit recovery call.)
Unfortunately if secondary opened instance resumes first it doesn't has SND_PCM_INFO_RESUME which causes snd_pcm_resume() never be called.
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;I don't think so. The clearance of the RESUME flag here is correct. dmix doesn't support the hardware resume feature. It does its own. (And this flag is merely a info for apps, which isn't really evaluated except for the code in dmix workaround there.)
Takashi
I think dmix should know what state the real driver is. If driver requires that app should do snd_pcm_resume() how can dmix get this information?
Many thanks for answering these questions.
Regards, Chancel Liu