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From: Greg KH on 30 Mar 2010 20:30
2.6.31-stable review patch. If anyone has any objections, please let us know.

------------------

From: Suresh Siddha <suresh.b.siddha(a)intel.com>

commit 2992e545ea006992ec9dc91c4fa996ce1e15f921 upstream.

Thomas Schlichter reported:
> 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, the
> kernel does fall back to uncached mmap. Then libpciaccess thinks it succeeded
> mapping with write combining enabled and does not set up suited MTRR entries.
> ;-(

Instead of silently mapping pci mmap region as UC minus in the case
of !pat_enabled and wc request, we can return error. Eric Anholt mentioned
that caller (like X) typically follows up with UC minus pci mmap request and
if there is a free mtrr slot, caller will manage adding WC mtrr.

Jesse Barnes says:
> Older versions of libpciaccess will behave better if we do it that way
> (iirc it only allocates an MTRR if the resource_wc file doesn't exist or
> fails to get mapped).

Reported-by: Thomas Schlichter <thomas.schlichter(a)web.de>
Signed-off-by: Thomas Schlichter <thomas.schlichter(a)web.de>
Signed-off-by: Suresh Siddha <suresh.b.siddha(a)intel.com>
Acked-by: Eric Anholt <eric(a)anholt.net>
Acked-by: Jesse Barnes <jbarnes(a)virtuousgeek.org>
Signed-off-by: Jesse Barnes <jbarnes(a)virtuousgeek.org>
Signed-off-by: Greg Kroah-Hartman <gregkh(a)suse.de>

---
arch/x86/pci/i386.c | 9 +++++++++
1 file changed, 9 insertions(+)

--- a/arch/x86/pci/i386.c
+++ b/arch/x86/pci/i386.c
@@ -282,6 +282,15 @@ int pci_mmap_page_range(struct pci_dev *
return -EINVAL;

prot = pgprot_val(vma->vm_page_prot);
+
+ /*
+ * Return error if pat is not enabled and write_combine is requested.
+ * Caller can followup with UC MINUS request and add a WC mtrr if there
+ * is a free mtrr slot.
+ */
+ if (!pat_enabled && write_combine)
+ return -EINVAL;
+
if (pat_enabled && write_combine)
prot |= _PAGE_CACHE_WC;
else if (pat_enabled || boot_cpu_data.x86 > 3)


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