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

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Message-Id: <201007281950.20847.arnd@arndb.de>
Date:	Wed, 28 Jul 2010 19:50:20 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	stepanm@...eaurora.org, linux-arm-msm@...r.kernel.org,
	dwalker@...eaurora.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] arm: msm: Add System MMU support.

On Wednesday 28 July 2010, stepanm@...eaurora.org wrote:
> > On Wednesday 28 July 2010 00:41:06 Stepan Moskovchenko wrote:
> >> Add support for the System MMUs found on the 8x60 and 8x72
> >> families of Qualcomm chips. These SMMUs allow virtualization
> >> of the address space used by most of the multimedia cores
> >> on these chips.
> >
> > How is this different from an IOMMU?
> >
> > From a very brief look, it seems that you should be using the
> > existing dma-mapping APIs here instead of making up your own.
> >
> > 	Arnd
> 
> 
> These are just SMMU APIs, and the DMA-mapping API is one layer above this.
> 
> We have our own SMMU API for the MSM SoCs because we have muliple IOMMUs,
> each one having multiple contexts, or even having multiple instances of
> the same context. Our usage model is also quite a bit different from how
> the DMA APIs are set up. I believe only two IOMMU drivers actually make
> use of the DMA API (Intel and AMD) and the other ones (OMAP and other
> SoCs) have their own APIs for their specific use cases.

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.

If the OMAP developers got this wrong, that's not your problem :-)

	Arnd
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