[alsa-devel] [PATCH] ASoC: Intel: Bsw: Read sst DSP DMA pointer after period elapse occurs

[Fang, Yang A]

> -----Original Message-----
> From: Mark Brown [mailto:broonie at kernel.org]
> Sent: Tuesday, August 18, 2015 5:35 PM
> To: Fang, Yang A
> Cc: lgirdwood at gmail.com; alsa-devel at alsa-project.org;
> dgreid at chromium.org; Nujella, Sathyanarayana;
> kevin.strasser at linux.intel.com; Sripathi, Srinivas; Iriawan, Denny; Jain,
> Praveen K; Koul, Vinod; Mirche, Dinesh
> Subject: Re: [PATCH] ASoC: Intel: Bsw: Read sst DSP DMA pointer after
> period elapse occurs
> 
> On Tue, Aug 18, 2015 at 05:33:33PM +0000, Fang, Yang A wrote:
> 
> > > > We should not read sst hw_ptr and pcm_delay in
> > > > sst_platform_pcm_pointer Since it will be ricky if
> > > > sst_platform_pcm_pointer is called while dsp is updating the
> > > > timestamp.Now read sst hw_ptr after period elapse interrupt occurs.
> 
> > > So I guess there's still some risk that we could run into problems
> > > here if we take too long to service the interrupt and/or the period
> > > is very short?  Is there anything we can do to improve that?
> 
> >  I think before and after the patch. There is no difference in term of
> > handling the period elapse interrupt . because before the patch
> > sst_period_elapsed calls the snd_pcm_period_elapsed which in turn
> > calling the sst_platform_pcm_pointer which calls the stream_read_tstamp.
> 
> I'm not sure I understand what the patch fixes then?  My concern is that
> we're just moving the timing around which changes but doesn't address the
> race condition.

We are seeing the issue during the long time playback.  
The ring_buffer_counter (part of snd_sst_tstamp  struct )supposes only increasing by the firmware  once firmware fetches  one period size of data.
 It will update the ring_buffer_counter in the mailbox and trigger a period elapse interrupt.  We have found the ring_buffer_counter got decreased 
during long time playback if we  call stream_read_tstamp to read mailbox  in sst_platform_pcm_pointer function.

There are two cases sst_platform_pcm_pointer will be getting called. One is called by sst_period_elapsed function.
 The second case is triggered by the userspace ioctl . The second case supposes reading the same case  ring_buffer_counter  value as last  read 
when period elapse interrupt  occurs . However we saw it somehow decreases.This causes audio stutter.