linux-kernel - Re: [alsa-devel] [PATCH] ASoC: snd_dmaengine: add common api for pcm

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Date:	Fri, 29 Jun 2012 18:42:23 +0200
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Takashi Iwai <tiwai@...e.de>
CC:	Laxman Dewangan <ldewangan@...dia.com>,
	"alsa-devel@...a-project.org" <alsa-devel@...a-project.org>,
	Stephen Warren <swarren@...dia.com>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"lrg@...com" <lrg@...com>,
	Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [alsa-devel] [PATCH] ASoC: snd_dmaengine: add common api for
 pcm_mmap

On 06/28/2012 03:51 PM, Takashi Iwai wrote:
> At Thu, 28 Jun 2012 19:06:07 +0530,
> Laxman Dewangan wrote:
> [...]
> 
> But, still we need to be careful about this.  As mentioned, there is
> no flag to know the possibility of writecombine mmap beforehand.
> It'd be nice if we have either a compile-time or a run-time flag /
> function to check that.  Then the driver can also expose the mmap
> capability to user-space depending on the flag.
> 

If writecombine is not supported it will fallback to a "normal" mmap.
Here's an excerpt from the DMA mapping documentation:

DMA_ATTR_WRITE_COMBINE
----------------------

DMA_ATTR_WRITE_COMBINE specifies that writes to the mapping may be
buffered to improve performance.

Since it is optional for platforms to implement DMA_ATTR_WRITE_COMBINE,
those that do not will simply ignore the attribute and exhibit default
behavior.

- Lars
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