PAGE_CACHE_WC strikes again

[Eric Anholt]

I just tracked down what was cutting performance 10x on one of my
systems on a microbenchmark I'd just written:

--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -540,7 +540,7 @@ int drm_gem_mmap(struct file *filp, struct
vm_area_struct *vma)
 	/* FIXME: use pgprot_writecombine when available */
 	prot = pgprot_val(vma->vm_page_prot);
 #ifdef CONFIG_X86
-	prot |= _PAGE_CACHE_WC;
+	/*prot |= _PAGE_CACHE_WC;*/
 #endif
 	vma->vm_page_prot = __pgprot(prot);
 
Turns out that setting PAGE_CACHE_WC disables the WC effect of the MTRR
on my non-PAT (disabled due to CPU errata) 945GM system, and this
workaround took GTT-mapped writes from 120MB/s to 1180MB/s.

What's the right way to be setting our PTEs?  This is similar to the
workaround in ef5fa0ab24b87646c7bc98645acbb4b51fc2acd4, but to do it in
the driver as well means exporting pat_enabled, and it really seems like
PAT presence shouldn't be something the driver has to worry about --
I've got a WC MTRR and I'm asking for a WC mapping and I'm getting
uncached.  If I failed to init the MTRR, the state of the aperture I'm
mapping would be uncached.

Test code is at
git://anongit.freedesktop.org/git/xorg/app/intel-gpu-tools
under benchmarks/intel_upload_blit_large_gtt but requires libdrm master
until we get a release out in the next week or so.  Also includes a few
standalone regression tests, which may be useful for people making
changes that may affect i915.

-- 
Eric Anholt
eric@xxxxxxxxxx                         eric.anholt@xxxxxxxxx

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