linux-kernel - Re: [PATCH 1/2] arm: msm: Add System MMU support.

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Message-Id: <20100729131500Q.fujita.tomonori@lab.ntt.co.jp>
Date:	Thu, 29 Jul 2010 13:15:37 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	linux@....linux.org.uk
Cc:	arnd@...db.de, linux-arm-kernel@...ts.infradead.org,
	linux-arm-msm@...r.kernel.org, dwalker@...eaurora.org,
	stepanm@...eaurora.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] arm: msm: Add System MMU support.

On Wed, 28 Jul 2010 22:21:56 +0100
Russell King - ARM Linux <linux@....linux.org.uk> wrote:

> On Wed, Jul 28, 2010 at 07:50:20PM +0200, Arnd Bergmann wrote:
> > The DMA API is extremely flexible, it works just fine with all the
> > IOMMUs that I've seen so far. Please take a look at
> > include/asm-generic/dma-mapping-common.h and its users to see how
> > to use multiple IOMMUs depending on the device.
> 
> We don't yet use those DMA API interface extensions because we haven't
> had the need.  If someone who has the need wants to put the effort in
> though...
> 
> One of the problems with it though is the abstraction of the sync*
> operations is the wrong way around for stuff like dmabounce - we want
> to be passed the base address of the buffer (so we can look this up),
> plus offset and length.  We don't want to know just the region which
> is affected.

We can't pass the base address because the DMA API callers don't pass
the base address for dma_sync_single_for_{device|cpu}.

dma_sync_single_range_for_* requires the base address but they are
obsolete.

So you need to fix dmabounce. Actually, I send you a patch to fix
dmabounce long ago (looks like not applied yet):

http://kerneltrap.org/mailarchive/linux-netdev/2010/4/5/6274046
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