[PATCH 2/2] Use MTRR for write combining mmap/ioremap if PAT is not available

[Thomas Schlichter]

X.org uses libpciaccess which tries to mmap with write combining enabled via
/sys/bus/pci/devices/*/resource0_wc. Currently, when PAT is not enabled, we
fall back to uncached mmap. Then libpciaccess thinks it succeeded mapping
with write combining anabled and does not set up suited MTRR entries. ;-(

So when falling back to uncached mapping, we better try to set up MTRR
entries automatically. To match this modified PCI mmap behavior, also
ioremap_wc and set_memory_wc are adjusted.

Signed-off-by: Thomas Schlichter <thomas.schlichter@xxxxxx>
---
 arch/x86/mm/ioremap.c  |   15 ++++++++++-----
 arch/x86/mm/pageattr.c |   10 ++++++++--
 arch/x86/pci/i386.c    |    6 ++++++
 3 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/arch/x86/mm/ioremap.c b/arch/x86/mm/ioremap.c
index 334e63c..abe40fa 100644
--- a/arch/x86/mm/ioremap.c
+++ b/arch/x86/mm/ioremap.c
@@ -21,6 +21,7 @@
 #include <asm/tlbflush.h>
 #include <asm/pgalloc.h>
 #include <asm/pat.h>
+#include <asm/mtrr.h>
 
 #include "physaddr.h"
 
@@ -268,11 +269,15 @@ EXPORT_SYMBOL(ioremap_nocache);
  */
 void __iomem *ioremap_wc(resource_size_t phys_addr, unsigned long size)
 {
-	if (pat_enabled)
-		return __ioremap_caller(phys_addr, size, _PAGE_CACHE_WC,
-					__builtin_return_address(0));
-	else
-		return ioremap_nocache(phys_addr, size);
+	if (!pat_enabled) {
+		void __iomem *ret = ioremap_nocache(phys_addr, size);
+		if (ret)
+			mtrr_add_unaligned(phys_addr, size,
+					   MTRR_TYPE_WRCOMB, false);
+		return ret;
+	}
+	return __ioremap_caller(phys_addr, size, _PAGE_CACHE_WC,
+				__builtin_return_address(0));
 }
 EXPORT_SYMBOL(ioremap_wc);
 
diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index dd38bfb..c25f697 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -23,6 +23,7 @@
 #include <asm/pgalloc.h>
 #include <asm/proto.h>
 #include <asm/pat.h>
+#include <asm/mtrr.h>
 
 /*
  * The current flushing context - we pass it instead of 5 arguments:
@@ -1010,8 +1011,13 @@ int set_memory_wc(unsigned long addr, int numpages)
 {
 	int ret;
 
-	if (!pat_enabled)
-		return set_memory_uc(addr, numpages);
+	if (!pat_enabled) {
+		ret = set_memory_uc(addr, numpages);
+		if (!ret)
+			mtrr_add_unaligned(__pa(addr), numpages * PAGE_SIZE,
+					   MTRR_TYPE_WRCOMB, false);
+		return ret;
+	}
 
 	ret = reserve_memtype(__pa(addr), __pa(addr) + numpages * PAGE_SIZE,
 		_PAGE_CACHE_WC, NULL);
diff --git a/arch/x86/pci/i386.c b/arch/x86/pci/i386.c
index b22d13b..8379e9b 100644
--- a/arch/x86/pci/i386.c
+++ b/arch/x86/pci/i386.c
@@ -33,6 +33,7 @@
 #include <linux/bootmem.h>
 
 #include <asm/pat.h>
+#include <asm/mtrr.h>
 #include <asm/e820.h>
 #include <asm/pci_x86.h>
 #include <asm/io_apic.h>
@@ -301,5 +302,10 @@ int pci_mmap_page_range(struct pci_dev *dev, struct vm_area_struct *vma,
 
 	vma->vm_ops = &pci_mmap_ops;
 
+	if (!pat_enabled && write_combine)
+		mtrr_add_unaligned(vma->vm_pgoff << PAGE_SHIFT,
+				   vma->vm_end - vma->vm_start,
+				   MTRR_TYPE_WRCOMB, false);
+
 	return 0;
 }
-- 
1.6.5


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