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Re: [Xen-devel] Re: [RFC] Make AGP work with IOMMU

To: Dave Airlie <airlied@xxxxxxxxx>
Subject: Re: [Xen-devel] Re: [RFC] Make AGP work with IOMMU
From: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
Date: Wed, 05 Aug 2009 08:08:51 +0100
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Zhenyu Wang <zhenyu.z.wang@xxxxxxxxx>, Joerg Roedel <joerg.roedel@xxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx, Keir Fraser <kier.fraser@xxxxxxxxxxxxx>
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On Wed, 2009-08-05 at 16:57 +1000, Dave Airlie wrote:
> On Wed, Aug 5, 2009 at 4:44 PM, David Woodhouse<dwmw2@xxxxxxxxxxxxx> wrote:
> > On Tue, 2009-08-04 at 16:52 -0700, Jeremy Fitzhardinge wrote:
> >> And just when I have patches to use them for their original purpose...
> >>
> >> Looking back over the thread, are you saying that most users are already
> >> using the DMA API correctly for AGP accesses?  If that's true then we
> >> should be just fine.
> >
> > No, not 'most users'. But perhaps we should.
> >
> > All we've done so far is make intel-agp use the DMA API correctly. And
> > that's conditional on CONFIG_DMAR. But we could make it unconditional,
> > and make the other drivers do it too. The code is all fairly generic.
> >
> > Without an IOMMU, the overhead would be fairly minimal.
> >
> I'm not sure how prevalent Xen is on AGP systems, I'm going to guess
> not enough that anyone cares. Intel integrated chips are the only AGP
> codebase user that are made any more, and they don't really use AGP
> its just a legacy of the previous designs.

On the other hand, extending the existing code so that it uses the DMA
API correctly _unconditionally_ rather than only for the Intel driver
would be fairly simple and should be harmless...


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