[alsa-devel] [PATCH V2] ASoC: soc-pcm: BE dai needs prepare when pause release after resume

[Takashi Iwai]

On Wed, 08 May 2019 18:30:08 +0200,
Ranjani Sridharan wrote:
> 
> On Wed, 2019-05-08 at 10:32 +0800, libin.yang at intel.com wrote:
> > From: Libin Yang <libin.yang at intel.com>
> > 
> > If playback/capture is paused and system enters S3, after system
> > returns
> > from suspend, BE dai needs to call prepare() callback when
> > playback/capture
> > is released from pause if RESUME_INFO flag is not set.
> Hi Takashi,
> 
> This is a question for you. We've run into the problem of not being
> able to do a pause-release after the system resumes from S3 after we
> removed INFO_RESUME from the SOF driver. 
> 
> Apparently, with this flag removed, when the user does a pause release
> after resuming from S3, the prepare() callback gets invoked for the FE
> but doesnt happen for the the BE. Could you please guide us on whether
> this is the right approach and if not, suggest an alternative?

Hm, it's a good question.  Currently the PCM core doesn't care about
the paused stream wrt PM by the assumption that the paused / stopped
stream doesn't need a special resume treatment.  But, generally
speaking, the pause-release won't work for a hardware that doesn't
support the full resume, either.  For example, the legacy HD-audio may
restart from some wrong position if resumed from the pause.

Maybe this problem hasn't been seen just because the pause function is
rarely used.

So, the safe behavior would be to let the stream being SUSPENDED state
at snd_pcm_stream_suspend() when it's in the PAUSED and has no
INFO_RESUME capability.  Then the application does re-prepare the
stream like the running one.

But the question is what's expected at next.  Should the application
re-start?  But it was paused.  Should PCM core automatically move to
pause?  But most hardware can't move the pointer to any random
position.

My gut feeling is just to treat like a normal error-restart,
i.e. re-prepare / re-start.  But I'm open and would like to hear more
opinions.


thanks,

Takashi