linux-kernel - Re: [alsa-devel] [PATCH] dma: pl330: Add support for DMA

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Message-ID: <s5h4n0mduzw.wl%tiwai@suse.de>
Date:	Mon, 19 May 2014 10:37:39 +0200
From:	Takashi Iwai <tiwai@...e.de>
To:	Lars-Peter Clausen <lars@...afoo.de>
Cc:	Linux-ALSA <alsa-devel@...a-project.org>,
	Vinod Koul <vinod.koul@...el.com>,
	Jassi Brar <jassisinghbrar@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	dmaengine@...r.kernel.org, Dan Williams <dan.j.williams@...el.com>,
	Tushar Behera <tushar.behera@...aro.org>
Subject: Re: [alsa-devel] [PATCH] dma: pl330: Add support for DMA_PAUSE	command

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@...afoo.de> wrote:
> >>>> On 05/14/2014 02:07 PM, Tushar Behera wrote:
> >>>>>
> >>>>> On 14 May 2014 17:29, Jassi Brar <jassisinghbrar@...il.com> wrote:
> >>>>>>
> >>>>>> On Wed, May 14, 2014 at 8:53 AM, Tushar Behera <tushar.behera@...aro.org>
> >>>>>> 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?


thanks,

Takashi
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