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

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Date:	Tue, 7 May 2013 09:45:54 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Alex Deucher <alexdeucher@...il.com>
Cc:	Jerome Glisse <j.glisse@...il.com>, linux-fbdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 3/7] drm: Update drm_addmap and drm_mmap to use PAT WC
 instead of MTRRs

On Tue, May 7, 2013 at 7:08 AM, Alex Deucher <alexdeucher@...il.com> wrote:
> On Mon, May 6, 2013 at 7:39 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Mon, May 6, 2013 at 4:04 PM, Jerome Glisse <j.glisse@...il.com> wrote:
>>> 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.
>>
>> 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?
>
> A lot of old chips used to put the registers and framebuffer in the
> same BAR.  IIRC, the xserver and later libpciaccess had workarounds to
> deal with this.

I think I read the code wrong (so my patch is garbage).  Maybe there's
actually no problem -- if DRM_AGP and DRM_FRAME_BUFFER are always WC,
DRM_REGISTERS is only WC if explicitly requested, and DRM_SHM is
always WB, so everything should be covered.

Anything using libpciaccess ought to be unaffected by my changes -- I
don't want to change /proc/mtrr or the sysfs stuff.  The only possible
issue is if there's a memtype conflict, but that's nothing new.

--Andy
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