linux-kernel - Re: MTRR use in drivers

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Message-ID: <51C69732.1010906@gmail.com>
Date:	Sun, 23 Jun 2013 08:35:30 +0200
From:	Brice Goglin <brice.goglin@...il.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Airlie <airlied@...ux.ie>,
	dri-devel@...ts.freedesktop.org
Subject: Re: MTRR use in drivers

Le 21/06/2013 07:00, H. Peter Anvin a écrit :
> An awful lot of drivers, mostly DRI drivers, are still mucking with
> MTRRs directly as opposed to using ioremap_wc() or similar interfaces.
> In addition to the architecture dependency, this is really undesirable
> because MTRRs are a limited resource, whereas page table attributes are not.
>
> Furthermore, this perpetuates the need for the horrific hack known as
> "MTRR cleanup".
>
> What, if anything, can we do to clean up this mess?
>
> 	-hpa
>

The first network driver that used ioremap_wc() back in 2008 (myri10ge)
had to keep using MTRR because ioremap_wc() silently falls back to
ioremap_nocache() when PAT is disabled.

I asked about this in https://lkml.org/lkml/2008/5/31/42 and there was
some talk about putting the MTRR addition in the nocache fallback path
but I guess nobody implemented the idea.

Brice

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