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

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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.