linux-kernel - Re: [PATCH v2 03/15] dmaengine: Add no_wakeup parameter to dmaengine_prep_dma

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Date:	Fri, 14 Sep 2012 14:20:30 +0530
From:	Vinod Koul <vinod.koul@...ux.intel.com>
To:	Lars-Peter Clausen <lars@...afoo.de>
Cc:	Peter Ujfalusi <peter.ujfalusi@...com>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Liam Girdwood <lrg@...com>, Tony Lindgren <tony@...mide.com>,
	Russell King <rmk+kernel@....linux.org.uk>,
	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 v2 03/15] dmaengine: Add no_wakeup parameter to
 dmaengine_prep_dma_cyclic()

On Fri, 2012-09-14 at 10:13 +0200, Lars-Peter Clausen wrote:
> On 09/14/2012 05:26 AM, Vinod Koul wrote:
> > On Thu, 2012-09-13 at 17:27 +0200, Lars-Peter Clausen wrote:
> >> Hi,
> >>
> >> Hm... Do you think it would work as well if we implement this by
> >> setting the
> >> callback for the descriptor to NULL? If the callback is NULL there is
> >> nothing to at the end of a transfer/period and the dma engine driver
> >> may
> >> choose to disable interrupts. This would also benefit non cyclic
> >> transfers
> >> where the callback is NULL and we do not need add the new parameter to
> >> dmaengine_prep_dma_cyclic.
> > That will work too.... BUT the idea of no_wake mode in ALSA is that we
> > should not have any interrupts, so anything which is going to cause
> > interrupts to AP in undesired. The interrupts still happen and it just
> > that dmaengine driver is not notifying client.
> 
> Well, the idea was that the driver would disable interrupts if there is no
> callback to call, since there would be nothing to do in the interrupt
> handler anyway. But I guess the flags approach should work fine as well.
Yes we _could_ do that, but this relies on dmaengine driver to have this
implicit understanding. Anyone using dmaengine library in ASoC may or
may not be aware of this, so i would consider it hackish.

Using this flag explicitly makes everyone aware what the intended
behaviour is.

-- 
~Vinod

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