Commit ef5fa0ab authored by Eric Anholt's avatar Eric Anholt Committed by Ingo Molnar
Browse files

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: default avatarEric Anholt <eric@anholt.net>
Signed-off-by: default avatarH. Peter Anvin <hpa@zytor.com>
parent e1b4d114

arch/x86/mm/iomap_32.c

+10 −0
Original line number Original line Diff line number Diff line
@@ -17,6 +17,7 @@ 
 */

 */





#include <asm/iomap.h>

#include <asm/iomap.h>

#include <asm/pat.h>

#include <linux/module.h>

#include <linux/module.h>





/* Map 'pfn' using fixed map 'type' and protections 'prot'

/* 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();

	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();

	idx = type + KM_TYPE_NR*smp_processor_id();

	vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);

	vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);

	set_pte(kmap_pte-idx, pfn_pte(pfn, prot));

	set_pte(kmap_pte-idx, pfn_pte(pfn, prot));

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