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

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Message-ID: <CAH3drwYSZVrM4uWwFAxD+c0n1Q9i-et=Mwbk253E+cd+BtW9yw@mail.gmail.com>
Date:	Mon, 6 May 2013 19:04:59 -0400
From:	Jerome Glisse <j.glisse@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	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

On Mon, May 6, 2013 at 5:22 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Fri, May 3, 2013 at 4:00 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> Signed-off-by: Andy Lutomirski <luto@...capital.net>
>> ---
>>
>> This needs careful review.  I don't really know what this code does, nor
>> do I have the hardware.  (I don't understand AGP and the associated
>> caching implications.)
>
> This patch is wrong (I didn't update the matching mtrr_del), and I'm
> reworking this whole series.  But I may need some help on this one:
> why is the mtrr handle of a map (whatever a map is) exported to
> userspace via the ADD_MAP and GET_MAP ioctls?  What (if anything) is
> userspace supposed to do with it?  Do I need to return a valid MTRR
> register number?  Is there any userspace code at all that sets
> _DRM_WRITE_COMBINING in DRM_IOCTL_ADD_MAP with appropriate alignment
> and needs the MTRR, for which the drm driver doesn't already add the
> MTRR?
>
> --Andy

>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.

Sorry for the bad news, but that's what i remember on that front
thought i would need to read all the code again.

Cheers,
Jerome
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