linux-kernel - Re: [RFC PATCH 00/12] PAT 64b: PAT support for X86

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Date:	Sat, 15 Dec 2007 08:27:44 +1000
From:	"Dave Airlie" <airlied@...il.com>
To:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
Cc:	"Dave Airlie" <airlied@...net.ie>, venkatesh.pallipadi@...el.com,
	ak@....de, ebiederm@...ssion.com, rdreier@...co.com,
	torvalds@...ux-foundation.org, gregkh@...e.de, davej@...hat.com,
	mingo@...e.hu, tglx@...utronix.de, hpa@...or.com,
	akpm@...ux-foundation.org, arjan@...radead.org,
	jesse.barnes@...el.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 00/12] PAT 64b: PAT support for X86_64

On Dec 15, 2007 8:00 AM, Siddha, Suresh B <suresh.b.siddha@...el.com> wrote:
> On Fri, Dec 14, 2007 at 12:28:25AM +0000, Dave Airlie wrote:
> > Yes, the main use for GPUs is to have RAM pages mapped WC, and placed into
> > a GART on the GPU side, currently for Intel IGD we are okay as the CPU can
> > access the GPU GART aperture, but other chips exist where this isn't
> > possible, I think poulsbo and possible some of the AMD IGPs..
>
> Ok. So how is it working today on these platforms with no PAT support.
> Open source drivers use UC or WB on these platforms? As this RAM is not
> contiguous, one can't use MTRRs to set WC. Right?
>
> Well, if WC is needed for RAM, then we have to address it too.
>

It doesn't work really, which is mostly the problem :)

We mostly use UC on these pages, or WB within cache coherent domains.
mtrrs are totally useless in this situation.

Dave.
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