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authorEric Anholt <eric@anholt.net>2009-01-23 14:14:21 -0800
committerIngo Molnar <mingo@elte.hu>2009-01-26 11:14:27 +0100
commitef5fa0ab24b87646c7bc98645acbb4b51fc2acd4 (patch)
tree212ee811a6271eb77e887d4f63817c01606ae8a2 /arch/x86/mm
parente1b4d1143651fb3838be1117785b6e0386fa151f (diff)
x86: work around PAGE_KERNEL_WC not getting WC in iomap_atomic_prot_pfn.
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>
Diffstat (limited to 'arch/x86/mm')
-rw-r--r--arch/x86/mm/iomap_32.c10
1 files changed, 10 insertions, 0 deletions
diff --git a/arch/x86/mm/iomap_32.c b/arch/x86/mm/iomap_32.c
index d0151d8ce45..ca53224fc56 100644
--- a/arch/x86/mm/iomap_32.c
+++ b/arch/x86/mm/iomap_32.c
@@ -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));