linux-kernel - Re: [PATCH 11/11] ASoC: omap-pcm: Convert to use dmaengine

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Date:	Wed, 12 Sep 2012 15:53:47 +0300
From:	Peter Ujfalusi <peter.ujfalusi@...com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Liam Girdwood <lrg@...com>, Tony Lindgren <tony@...mide.com>,
	Vinod Koul <vinod.koul@...el.com>, Dan Williams <djbw@...com>,
	Jarkko Nikula <jarkko.nikula@...mer.com>,
	alsa-devel@...a-project.org, linux-omap@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Janusz Krzysztofik <jkrzyszt@....icnet.pl>,
	Ricardo Neri <ricardo.neri@...com>
Subject: Re: [PATCH 11/11] ASoC: omap-pcm: Convert to use dmaengine

On 09/12/2012 03:00 PM, Russell King - ARM Linux wrote:
> On Wed, Sep 12, 2012 at 02:47:07PM +0300, Peter Ujfalusi wrote:
>> 2. Pause/Resume
>>    OMAP DMA engine backend does not support pausing and resuming
>>    an in-progress transfer.  It is unclear from the specs what
>>    effect clearing the enable bit has on the DMA position of a
>>    destination synchronized transfer, and whether the transfer
>>    can be restarted from the exact point that it was paused (or
>>    whether the data in the FIFO read from memory is simply
>>    discarded.)
> 
> It's worth noting that this comment (which was in my original patch)
> is there to spark _comment_ and _discussion_ and should not make its
> way into the final version of these patches.
> 
> Given that suspend/resume is important on OMAP platforms, it's something
> that needs to be resolved - in a way that complies with what ALSA expects.
> I do not believe that the way the existing drivers do this is compliant
> as the manuals imply that stopping memory->peripheral transfers results
> in data being discarded from the DMA's FIFOs.  As I understand it, ALSA
> requires no data to be discarded.
> 
> As we have no way to know how much data may be discarded from the DMA
> FIFO...

I need to look at this, but at first look we do wait for the drain in
omap_stop_dma(). We used to use omap_stop_dma/omap_start_dma for pause/resume
operations.
But sDMA also have a bit: CDPi: PAUSE_LINK_LIST which should do what we are
looking for.
Need to read the relevant parts of the TRM, but AFAIK we are using normal mode
with linked list (self linking).
I already have the patch for this, I just need to test it on HW.

-- 
Péter
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