linux-kernel - Re: IOMMU and graphics cards

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Date:	Thu, 7 May 2009 14:00:08 +0200
From:	Joerg Roedel <joerg.roedel@....com>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
CC:	dwmw2@...radead.org, linux-kernel@...r.kernel.org,
	iommu@...ts.linux-foundation.org, mingo@...hat.com,
	airlied@...ux.ie
Subject: Re: IOMMU and graphics cards

On Thu, May 07, 2009 at 07:23:33PM +0900, FUJITA Tomonori wrote:
> On Thu, 7 May 2009 12:01:14 +0200
> Joerg Roedel <joerg.roedel@....com> wrote:
> 
> > > Do you meant that AMD IOMMU code use the bitmap each device to manage
> > > address space so enlarging the DMA address space wastes memory?
> > 
> > Its not only the bitmap. There are also the page tables that would waste
> > memory if the aperture in the AMD IOMMU driver is enlarged to, say, 4GB.
> > Most devices don't need such a large amount of DMA address space. As I
> > found out VT-d does not have this problem because they implemented
> > another allocator which can cover all possible memory.
> > For the AMD IOMMU I currently implement a kind of a dynamically growing
> > bitmap allocator to fix this problem.
> 
> Yeah, I know the deference. Well, I like the AMD IOMMU driver's
> property that dma mapping doesn't fail in OOM but I understand why you
> want such daynamically growing.

Well, I will try to minimize the impact. Current plan is to preallocate
bitmap and page tables for 128MB of DMA address space. Everything beyond
that will be allocated on demand.

> > > >  But unless this problem isn't solved the
> > > > drivers won't be fixed, I guess.
> > > > I guess the DRM code in the kernel may have the same problem with IOMMU
> > > > enabled?
> > > 
> > > Looks like the DRM code uses the DMA API.
> > 
> > The proprietary drivers make problems so far. For the ATI one I am in
> > contact with the developers to try to fix it. But I can't do anything
> > about the other proprietary driver I am aware of :-(
> 
> I don't know anything about the graphic drivers but are there any
> other proprietary drivers except for ATI and AMD? Fixing only two
> drivers to make the majority happy?

At least Nvidia comes to my mind. Not sure about other graphic card
vendors...

Joerg

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