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

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Date:	Thu, 28 Jun 2012 14:57:11 +0200
From:	Takashi Iwai <tiwai@...e.de>
To:	Laxman Dewangan <ldewangan@...dia.com>
Cc:	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Lars-Peter Clausen <lars@...afoo.de>,
	"alsa-devel@...a-project.org" <alsa-devel@...a-project.org>,
	Stephen Warren <swarren@...dia.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

At Thu, 28 Jun 2012 18:05:53 +0530,
Laxman Dewangan wrote:
> 
> On Thursday 28 June 2012 05:58 PM, Mark Brown wrote:
> > * PGP Signed by an unknown key
> >
> > On Thu, Jun 28, 2012 at 02:30:26PM +0200, Lars-Peter Clausen wrote:
> >> On 06/28/2012 02:18 PM, Mark Brown wrote:
> >>> Oh, dear - this means we don't actually have a generic API at all.  Is
> >>> there any effort being made to make this generally available?
> >> Yes there is, and in fact in next/master there is generic
> >> dma_mmap_writecombine. I'm not quite sure how it behaves on non ARM archs
> >> though. But in theory it should fallback to a normal coherent mapping. Marek
> >> did a lot of work on this, I've put him on Cc.
> 
> So can we put the function snd_pcm_lib_writecombine_mmap() in the 
> pcm_native.c and only export this api for ARM i.e.  under macro  #ifdef 
> CONFIG_ARM so that ARM based SOCs like Tegra/epa3xx/mxs/spear can 
> directly use that one?
> Or, wait for this common API until all ARCH support it?

I think it's fine to put it first in ALSA side with some ifdef.
A similar trick is already found for snd_pcm_lib_mmap_iomem.
See include/sound/pcm.h.

But, actually it's a still question what if an architecture doesn't
support the mmap of writecombine at all.  The proposed patch doesn't
allow you to know whether writecombine-mmap is possible or not on the
running architecture until you really try to call it and fail.
It's a missing piece, IMO.


Takashi
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