linux-kernel - Re: [patch 3/4] x86: PAT Update validate_pat

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Message-ID: <48B4E66C.2080806@myri.com>
Date:	Wed, 27 Aug 2008 07:30:20 +0200
From:	Brice Goglin <brice@...i.com>
To:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
CC:	Dave Airlie <airlied@...il.com>, "mingo@...e.hu" <mingo@...e.hu>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
Subject: Re: [patch 3/4] x86: PAT Update validate_pat_support for intel CPUs

Pallipadi, Venkatesh wrote:
> Default MTRR being UC for all reserved regions and drivers
> wanting WC is very common situation and performance will be
> affected when that ends up being UC access.
> With pat disabled, drivers can set MTRR with WC in this case
> and get the expected performance.
>   

Are you saying that drivers are supposed to check if PAT is disabled
before deciding whether they need to setup a MTRR WC? If so, how to do
we check that? Unless we get a way to know whether ioremap_wc() returned
WC or UC, I will always keep the MTRR WC in myri10ge.

Brice

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