linux-kernel - Re: [PATCH 3/7] ARM: Samsung: update/rewrite Samsung SYSMMU (IOMMU) driver

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Message-Id: <201103111700.17373.arnd@arndb.de>
Date:	Fri, 11 Mar 2011 17:00:17 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Marek Szyprowski <m.szyprowski@...sung.com>
Cc:	linux-arm-kernel@...ts.infradead.org,
	linux-samsung-soc@...r.kernel.org, linux-media@...r.kernel.org,
	Tomasz Stanislawski <t.stanislaws@...sung.com>,
	k.debski@...sung.com, kgene.kim@...sung.com,
	kyungmin.park@...sung.com,
	Sylwester Nawrocki <s.nawrocki@...sung.com>,
	Andrzej Pietrasiewicz <andrzej.p@...sung.com>,
	" '대인기'" <inki.dae@...sung.com>,
	" '강민규'" <mk7.kang@...sung.com>,
	"'KyongHo Cho'" <pullip.cho@...sung.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/7] ARM: Samsung: update/rewrite Samsung SYSMMU (IOMMU) driver

On Friday 11 March 2011, Marek Szyprowski wrote:
> > > > does not support IOMMUs, but that could be changed by wrapping it
> > > > using the include/asm-generic/dma-mapping-common.h infrastructure.
> > >
> > > ARM dma-mapping framework also requires some additional research for better DMA
> > > support (there are still issues with multiple mappings to be resolved).
> > 
> > You mean mapping the same memory into multiple devices, or a different problem?
> 
> Mapping the same memory area multiple times with different cache settings is not
> legal on ARMv7+ systems. Currently the problems might caused by the low-memory
> kernel linear mapping and second mapping created for example by dma_alloc_coherent()
> function.

Yes, I know this problem, but I don't think the case you describe is a serious
limitation (there are more interesting cases, though): dma_map_single() etc
will create additional *bus* addresses for a physical address, not additional
virtual addresses.

dma_alloc_coherent should allocate memory that is not also mapped cached,
which is what I thought we do correctly.

	Arnd
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