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

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Message-ID: <s5hha4qp92i.wl%tiwai@suse.de>
Date:	Fri, 16 May 2014 07:49:09 +0200
From:	Takashi Iwai <tiwai@...e.de>
To:	Lars-Peter Clausen <lars@...afoo.de>
Cc:	Tushar Behera <tushar.behera@...aro.org>,
	Jassi Brar <jassisinghbrar@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	dmaengine@...r.kernel.org, Vinod Koul <vinod.koul@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Linux-ALSA <alsa-devel@...a-project.org>
Subject: Re: [PATCH] dma: pl330: Add support for DMA_PAUSE command

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.


Takashi
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