Re: [alsa-devel] [PATCH] dma: pl330: Add support for DMA_PAUSE command

[Lars-Peter Clausen]

On 05/19/2014 10:37 AM, Takashi Iwai wrote:
> At Fri, 16 May 2014 12:51:51 +0200,
> Lars-Peter Clausen wrote:
> > On 05/16/2014 07:49 AM, Takashi Iwai wrote:
> > > At Thu, 15 May 2014 21:21:12 +0200,
> > > Lars-Peter Clausen wrote:
> > > > On 05/15/2014 02:01 PM, Tushar Behera wrote:
> > > > > On 14 May 2014 17:54, Lars-Peter Clausen <lars@xxxxxxxxxx> wrote:
> > > > > > On 05/14/2014 02:07 PM, Tushar Behera wrote:
> > > > > > > On 14 May 2014 17:29, Jassi Brar <jassisinghbrar@xxxxxxxxx> wrote:
> > > > > > > > On Wed, May 14, 2014 at 8:53 AM, Tushar Behera <tushar.behera@xxxxxxxxxx>
> > > > > > > > wrote:
> > > > > > > > > While playing back audio, pmc_dmaengine requests the DMA channel to
> > > > > > > > > stop DMA transmission through DMA_PAUSE command.
> > > > > > > > >
> > > > > > > > > Currently PL330 driver doesn't support DMA pause command, leaving
> > > > > > > > > the DMA state inconsistent when the system resumes. Instead, it would
> > > > > > > > > be better to terminate the DMA transfer during suspend and restart
> > > > > > > > > again during resume.
> > > > > > > > >
> > > > > > > > > Tested with audio playback across a suspend-resume cycle.
> > > > > > > > What is pmc_dmaengine? How does DMA_PAUSE help, when there is no
> > > > > > > > DMA_RESUME?
> > > > > > >
> > > > > > > Sorry, it is a typo.
> > > > > > >
> > > > > > > sound/core/pcm_dmaengine.c:snd_dmaengine_pcm_trigger() -->
> > > > > > > dmaengine_pause() is called during system suspend.
> > > > > >
> > > > > >
> > > > > > It is only called if the DMA driver has support for pausing and resuming DMA
> > > > > > transfers. Or at least that is the intention.
> > > > > >
> > > > > > - Lars
> > > > >
> > > > > During suspend, snd_dmaengine_pcm_trigger():SNDRV_PCM_TRIGGER_SUSPEND
> > > > > is called which unconditionally calls dmaengine_pause(). Should we
> > > > > update snd_dmaengine_pcm_trigger() to check for DMA pause/resume
> > > > > support and call dmaengine_pause() or dmaengine_terminate_all()
> > > > > accordingly?
> > > >
> > > > As far as I understand it we do not have to do anything for TRIGGER_SUSPEND
> > > > if we do not set the SNDRV_PCM_INFO_RESUME flag. It looks like
> > > > TRIGGER_SUSPEND is called unconditionally during suspend. But since the
> > > > error code is ignored it should be fine if we just call dmaengine_pause()
> > > > and that return -ENOSYS or similar.
> > >
> > > Well, TRIGGER_SUSPEND is issued by PCM core no matter whether
> > > SNDRV_PCM_INFO_RESUME flag is set or not.  The resume behavior depends
> > > on the flag (SNDRV_PCM_TRIGGER_RESUME is issued if the flag is set,
> > > otherwise the normal setup is done by alsa-lib at resume), but the
> > > suspend is always triggered in the same way.
> > >
> > > So, PCM core assumes that the driver stops the stream somehow by
> > > SNDRV_PCM_TRIGGER_SUSPEND.
> >
> > Ok, so we should call terminate_all() when we get a
> > SNDRV_PCM_TRIGGER_SUSPEND if the dmaengine driver does not support pauseing
> > the transfer. Thanks for the clarification.
> >
> > On a related note it seems like it is still possible to get
> > PAUSE_PUSH/PAUSE_RELEASE events even if SNDRV_PCM_INFO_PAUSE is not set. Do
> > you have an idea what should be the right thing to do in such a case? Return
> > -ENOSYS?
>
> That's weird.  We have already a check in snd_pcm_pre_pause() in
> pcm_native.c to filter only for substreams with SNDRV_PCM_INFO_PAUSE
> flag.
>
> Could you check in which code path it is triggered?

Uhm, yes, I was only looking at the code, but couldn't find the check that 
makes sure that the PAUSE_PUSH/PAUSE_RELEASE events are only triggered if 
SNDRV_PCM_INFO_PAUSE is set. Looks like I need a pair of glasses.

Thanks and sorry for the noise,
- Lars

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