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x86: work around PAGE_KERNEL_WC not getting WC in iomap_atomic_prot_pfn.

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In the absence of PAT, PAGE_KERNEL_WC ends up mapping to a memory type that
gets UC behavior even in the presence of a WC MTRR covering the area in
question.  By swapping to PAGE_KERNEL_UC_MINUS, we can get the actual
behavior the caller wanted (WC if you can manage it, UC otherwise).

This recovers the 40% performance improvement of using WC in the DRM
to upload vertex data.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
This commit is contained in:
Eric Anholt 2009-01-23 14:14:21 -08:00 • committed by Ingo Molnar
commit ef5fa0ab24
1 changed files with 10 additions and 0 deletions
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10
arch/x86/mm/iomap_32.c
View file

@ -17,6 +17,7 @@
*/
#include <asm/iomap.h>
#include <asm/pat.h>
#include <linux/module.h>
/* Map 'pfn' using fixed map 'type' and protections 'prot'
@ -29,6 +30,15 @@ iomap_atomic_prot_pfn(unsigned long pfn, enum km_type type, pgprot_t prot)
pagefault_disable();
/*
* For non-PAT systems, promote PAGE_KERNEL_WC to PAGE_KERNEL_UC_MINUS.
* PAGE_KERNEL_WC maps to PWT, which translates to uncached if the
* MTRR is UC or WC. UC_MINUS gets the real intention, of the
* user, which is "WC if the MTRR is WC, UC if you can't do that."
*/
if (!pat_enabled && pgprot_val(prot) == pgprot_val(PAGE_KERNEL_WC))
prot = PAGE_KERNEL_UC_MINUS;
idx = type + KM_TYPE_NR*smp_processor_id();
vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);
set_pte(kmap_pte-idx, pfn_pte(pfn, prot));