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: <20100722043034.GC22559@codeaurora.org>
Date:	Wed, 21 Jul 2010 21:30:34 -0700
From:	Zach Pfeffer <zpfeffer@...eaurora.org>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	linux@....linux.org.uk, 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 Wed, Jul 21, 2010 at 10:44:37AM +0900, FUJITA Tomonori wrote:
> On Tue, 20 Jul 2010 15:20:01 -0700
> Zach Pfeffer <zpfeffer@...eaurora.org> wrote:
> 
> > > I'm not saying that it's reasonable to pass (or even allocate) a 1MB
> > > buffer via the DMA API.
> > 
> > But given a bunch of large chunks of memory, is there any API that can
> > manage them (asked this on the other thread as well)?
> 
> What is the problem about mapping a 1MB buffer with the DMA API?
> 
> Possibly, an IOMMU can't find space for 1MB but it's not the problem
> of the DMA API.

This goes to the nub of the issue. We need a lot of 1 MB physically
contiguous chunks. The system is going to fragment and we'll never get
our 12 1 MB chunks that we'll need, since the DMA API allocator uses
the system pool it will never succeed. For this reason we reserve a
pool of 1 MB chunks (and 16 MB, 64 KB etc...) to satisfy our
requests. This same use case is seen on most embedded "media" engines
that are getting built today.
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