linux-kernel - Re: IOMMU and graphics cards

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Date:	Tue, 7 Jul 2009 10:59:04 +0200
From:	Joerg Roedel <joerg.roedel@....com>
To:	Dave Airlie <airlied@...ux.ie>
CC:	David Woodhouse <dwmw2@...radead.org>,
	Joerg Roedel <joro@...tes.org>,
	Zhenyu Wang <zhenyu.z.wang@...el.com>,
	linux-kernel@...r.kernel.org, fujita.tomonori@....ntt.co.jp,
	iommu@...ts.linux-foundation.org, mingo@...hat.com,
	David Miller <davem@...emloft.net>, eric@...olt.net
Subject: Re: IOMMU and graphics cards

On Mon, Jul 06, 2009 at 10:35:59PM +0100, Dave Airlie wrote:
> 
> 
> On Mon, 6 Jul 2009, David Woodhouse wrote:
> 
> > On Mon, 2009-07-06 at 15:11 +0200, Joerg Roedel wrote:
> > > Ok, cool, that sounds good. Which in-kernel DRM drivers break with IOMMU
> > > for you? I'll may probably add a similar temporary workaround for AMD
> > > IOMMU too...
> > 
> > The Intel one definitely broke -- I don't know about the others. There
> > are some old patches at http://people.freedesktop.org/~zhen/agp-mm-*
> > which make it look like _all_ AGP drivers are broken.
> > 
> > I wouldn't bother adding the workaround -- as I said, I'm planning to
> > rip it out of 2.6.32 (and in linux-next as soon as it's reasonable to do
> > so). Let's just let them fix it.
> 
> cc'ing Eric,
> 
> My memory of this is graphics becomes totally useless and can be 10x-50x 
> slower. I think ripping this out without the person doing the ripping 
> taking responsiblity for doing speed regression testing is totally insane.

Are you sure that using the dma-api has such an performance impact? I've
heard from other people that switching to dma-api with amd iommu had no
significant performance impact.

> I personally have no IOMMU hw from Intel or AMD and nobody has seen it fit 
> to supply me with any at any point in time, I'm not on the correct gravy 
> train. So I suspect the people with the hw will have to do the work and 
> the regression testing.

You just need to switch to dma-api. You don't need an iommu to test.
Most bugs in such code can also be found and eliminated using the
dma-api debugging interface.

	Joerg

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