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

[Dave Airlie]

>>
>> 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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