Re: [PATCH 3/7] drm: Update drm_addmap and drm_mmap to use PAT WC instead of MTRRs

[Andy Lutomirski <luto@amacapital.net>]

On Mon, May 6, 2013 at 4:04 PM, Jerome Glisse <j.glisse@gmail.com> wrote:
> On Mon, May 6, 2013 at 5:22 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>> On Fri, May 3, 2013 at 4:00 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>> Signed-off-by: Andy Lutomirski <luto@amacapital.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?

I'd really hate to have to track fake mtrrs in the drm core to emulate
real mtrrs.

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

On the bright side, in the entire 2005 monolithic xorg tarball, the
only thing that looks at the mtrr exported to userspace appears to be
dritests.

--Andy