x86 PAT: fix performance drop for glx, use UC minus for ioremap(), ioremap_nocache() and pci_mmap_page_range()

Use UC_MINUS for ioremap(), ioremap_nocache() instead of strong UC.
Once all the X drivers move to ioremap_wc(), we can go back to strong
UC semantics for ioremap() and ioremap_nocache().

To avoid attribute aliasing issues, pci_mmap_page_range() will also
use UC_MINUS for default non write-combining mapping request.

Next steps:
	a) change all the video drivers using ioremap() or ioremap_nocache()
	   and adding WC MTTR using mttr_add() to ioremap_wc()

	b) for strict usage, we can go back to strong uc semantics
	   for ioremap() and ioremap_nocache() after some grace period for
	   completing step-a.

	c) user level X server needs to use the appropriate method for setting
	   up WC mapping (like using resourceX_wc sysfs file instead of
	   adding MTRR for WC and using /dev/mem or resourceX under /sys)

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 9 insertions, 3 deletions

diff --git a/arch/x86/pci/i386.c b/arch/x86/pci/i386.c
index 94f6c73a53d..8af0f0bae2a 100644
--- a/arch/x86/pci/i386.c
+++ b/arch/x86/pci/i386.c
@@ -301,6 +301,13 @@ int pci_mmap_page_range(struct pci_dev *dev, struct vm_area_struct *vma,
 	prot = pgprot_val(vma->vm_page_prot);
 	if (pat_wc_enabled && write_combine)
 		prot |= _PAGE_CACHE_WC;
+	else if (pat_wc_enabled)
+		/*
+		 * ioremap() and ioremap_nocache() defaults to UC MINUS for now.
+		 * To avoid attribute conflicts, request UC MINUS here
+		 * aswell.
+		 */
+		prot |= _PAGE_CACHE_UC_MINUS;
 	else if (boot_cpu_data.x86 > 3)
 		prot |= _PAGE_CACHE_UC;
 
@@ -319,9 +326,8 @@ int pci_mmap_page_range(struct pci_dev *dev, struct vm_area_struct *vma,
 		 * - request is uncached, return cannot be write-combine
 		 * - request is write-combine, return cannot be write-back
 		 */
-		if ((flags == _PAGE_CACHE_UC &&
-		     (new_flags == _PAGE_CACHE_WB ||
-		      new_flags == _PAGE_CACHE_WC)) ||
+		if ((flags == _PAGE_CACHE_UC_MINUS &&
+		     (new_flags == _PAGE_CACHE_WB)) ||
 		    (flags == _PAGE_CACHE_WC &&
 		     new_flags == _PAGE_CACHE_WB)) {
 			free_memtype(addr, addr+len);