linux-kernel - Re: [RFC Patch] use MTRR for write combining if PAT is not available

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Date:	Mon, 19 Oct 2009 15:54:22 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Jan Beulich <JBeulich@...ell.com>,
	Thomas Schlichter <thomas.schlichter@....de>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Robert Hancock <hancockrwd@...il.com>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Tejun Heo <tj@...nel.org>, "x86@...nel.org" <x86@...nel.org>,
	Yinghai Lu <yinghai@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	"dri-devel@...ts.sourceforge.net" <dri-devel@...ts.sourceforge.net>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"jbarnes@...tuousgeek.org" <jbarnes@...tuousgeek.org>,
	Thomas Hellstrom <thellstrom@...are.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC Patch] use MTRR for write combining if PAT is not
	available


* Suresh Siddha <suresh.b.siddha@...el.com> wrote:

> On Mon, 2009-10-19 at 02:16 -0700, Jan Beulich wrote:
> > Functionality-wise this looks fine to me
> 
> If we are going to make ioremap() and set_memory_wc() add mtrr's in 
> non-pat case, then we need to delete the added mtrr(s) in the 
> corresponding iounmap() and set_memory_wb() aswell.
> 
> hmm, this is becoming too complex. The way i915 and other graphics 
> drivers are using set_memory_wc(), it is def a bad idea to start 
> adding mtrr's behind the back for non-pat case.

Touching MTRRs beyond working around basic bugs like non-cached RAM 
sounds like madness. The interactions with PAT are ... countless.

> Can't we just force PAT option always and we probably don't care about 
> ioremap_wc() on processors were PAT doesn't get enabled because of 
> known errata.

We can make PAT configurability dependent on EMBEDDED-y - mind sending a 
patch for that?

	Ingo
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