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

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Message-Id: <1255959874.2823.46.camel@sbs-t61>
Date:	Mon, 19 Oct 2009 06:44:34 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Jan Beulich <JBeulich@...ell.com>
Cc:	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

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.

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.

or Perhaps just try to add mtrr only for the pci mmap case like the 4th
patch in this series..

thanks,
suresh

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