linux-kernel - Re: [PATCH 3/7] drm: Update drm_addmap and drm

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Date:	Tue, 7 May 2013 13:09:48 +1000
From:	Dave Airlie <airlied@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Jerome Glisse <j.glisse@...il.com>, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org
Subject: Re: [PATCH 3/7] drm: Update drm_addmap and drm_mmap to use PAT WC
 instead of MTRRs

>>
>> From memory, even on pat system we need mtrr for VRAM is PCI BAR. We
>> cover it with a write combine MTRR. The whole ioctl is use by some ddx
>> or maybe even directly the XServer to do this mtrr mess in userspace.
>
> Egads!  So we have a _DRM_WRITE_COMBINING flag, which will continue to
> work fine, but almost nothing uses it.
>
> I'm amazed this stuff works in the current code, though.  Apparently
> the memory type (WC or UC) of a drm mapping is determined by the mtrr
> the driver set, but if one part of the BAR is textures or the
> framebuffer and another part is an outgoing command ring, won't one of
> them end up with the wrong memory type?

Nobody sane puts the command ring in VRAM.

Dave.
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