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

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Date:	Wed, 14 Jul 2010 13:11:49 -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 14, 2010 at 10:59:38AM +0900, FUJITA Tomonori wrote:
> On Tue, 13 Jul 2010 05:14:21 -0700
> Zach Pfeffer <zpfeffer@...eaurora.org> wrote:
> 
> > > You mean that you want to specify this alignment attribute every time
> > > you create an IOMMU mapping? Then you can set segment_boundary_mask
> > > every time you create an IOMMU mapping. It's odd but it should work.
> > 
> > Kinda. I want to forget about IOMMUs, devices and CPUs. I just want to
> > create a mapping that has the alignment I specify, regardless of the
> > mapper. The mapping is created on a VCM and the VCM is associated with
> > a mapper: a CPU, an IOMMU'd device or a direct mapped device.
> 
> Sounds like you can do the above with the combination of the current
> APIs, create a virtual address and then an I/O address.
> 

Yes, and that's what the implementation does - and all the other
implementations that need to do this same thing. Why not solve the
problem once?

> The above can't be a reason to add a new infrastructure includes more
> than 3,000 lines.

Right now its 3000 lines because I haven't converted to a function
pointer based implementation. Once I do that the size of the
implementation will shrink and the code will act as a lib. Users pass
buffer mappers and the lib will ease the management of of those
buffers.

>  
> 
> > > Another possible solution is extending struct dma_attrs. We could add
> > > the alignment attribute to it.
> > 
> > That may be useful, but in the current DMA-API may be seen as
> > redundant info.
> 
> If there is real requirement, we can extend the DMA-API.

If the DMA-API contained functions to allocate virtual space separate
from physical space and reworked how chained buffers functioned it
would probably work - but then things start to look like the VCM API
which does graph based map management.

> 
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