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

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Message-ID: <20120912120028.GC28448@n2100.arm.linux.org.uk>
Date:	Wed, 12 Sep 2012 13:00:28 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Peter Ujfalusi <peter.ujfalusi@...com>
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 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...
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