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

- From: Andy Lutomirski \<luto@amacapital.net\>
- Date: 2013-05-10 01:21:55 
- List: linux-fbdev
- Page: https://ratatoskr.run/linux-fbdev/2013/05/4047271
- Thread: https://ratatoskr.run/dri-devel/2013/05/4046967/t.md

```text
On Thu, May 9, 2013 at 4:44 PM, Jerome Glisse [ref] wrote:
> On Thu, May 9, 2013 at 3:46 PM, Andy Lutomirski [ref] 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
```