From mboxrd@z Thu Jan  1 00:00:00 1970
From: zpfeffer@codeaurora.org (Zach Pfeffer)
Date: Wed, 21 Jul 2010 21:30:34 -0700
Subject: [RFC 1/3 v3] mm: iommu: An API to unify IOMMU, CPU and device
	memory management
In-Reply-To: <20100721104356S.fujita.tomonori@lab.ntt.co.jp>
References: <20100715014148.GC2239@codeaurora.org>
	<20100719082213.GA7421@n2100.arm.linux.org.uk>
	<20100720221959.GC12250@codeaurora.org>
	<20100721104356S.fujita.tomonori@lab.ntt.co.jp>
Message-ID: <20100722043034.GC22559@codeaurora.org>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

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@codeaurora.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.