linux-kernel - Re: [RFC/PATCH v2 0/8] Clean up write-combining MTRR addition

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Date:	Thu, 9 May 2013 18:21:33 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Jerome Glisse <j.glisse@...il.com>
Cc:	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	linux-fbdev@...r.kernel.org,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Alex Deucher <alexdeucher@...il.com>,
	Dave Airlie <airlied@...il.com>
Subject: Re: [RFC/PATCH v2 0/8] Clean up write-combining MTRR addition

On Thu, May 9, 2013 at 4:44 PM, Jerome Glisse <j.glisse@...il.com> wrote:
> On Thu, May 9, 2013 at 3:46 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> A fair number of drivers (mostly graphics) add write-combining MTRRs.
>> Most ignore errors and most add the MTRR even on PAT systems which don't
>> need to use MTRRs.
>
> This comment is wrong, as i said we need MTRR on PAT system for VRAM.

I didn't follow it last time.

 - If userspace is setting an MTRR directly, then it will work exactly
as before -- this patch has no effect on the userspace MTRR APIs.

 - If userspace uses the drm map interface with DRM_FRAME_BUFFER or
DRM_WRITE_COMBINING, there won't be an MTRR but the range will still
be WC.

 - If userspace uses GEM or TTM, then everything should still use WC
(TTM has explicit handling for this, which I presume is correct, and
i915, the major GEM user, already doesn't set an MTRR).

 - If userspace does not map the range, then either the kernel driver
is buggy or it doesn't matter because there's no map.

Is there a case I've missed?

--Andy
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