linux-kernel - Re: [RFC 1/3 v3] mm: iommu: An API to unify IOMMU, CPU and device memory management

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Message-Id: <20100720190904N.fujita.tomonori@lab.ntt.co.jp>
Date:	Tue, 20 Jul 2010 19:09:31 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	linux@....linux.org.uk
Cc:	zpfeffer@...eaurora.org, fujita.tomonori@....ntt.co.jp,
	ebiederm@...ssion.com, linux-arch@...r.kernel.org,
	dwalker@...eaurora.org, mel@....ul.ie,
	linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, andi@...stfloor.org,
	linux-omap@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC 1/3 v3] mm: iommu: An API to unify IOMMU, CPU and device
	memory management

On Mon, 19 Jul 2010 09:22:13 +0100
Russell King - ARM Linux <linux@....linux.org.uk> wrote:

> > If I want to share the buffer with another device I have to
> > make a copy of the entire thing then fix up the virtual mappings for
> > the other device I'm sharing with.
> 
> This is something the DMA API doesn't do - probably because there hasn't
> been a requirement for it.
> 
> One of the issues for drivers is that by separating the mapped scatterlist
> from the input buffer scatterlist, it creates something else for them to
> allocate, which causes an additional failure point - and as all users sit
> well with the current API, there's little reason to change especially
> given the number of drivers which would need to be updated.

Agreed. There was the discussion about separating 'dma_addr and dma_len' from
scatterlist struct but I don't think that it's worth doing so.


> I'm just proving that it's not as hard as you seem to be making out.

Agreed again.
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