x86/PCI/PAT: return EINVAL for pci mmap WC request for !pat_enabled

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: NThomas Schlichter <thomas.schlichter@web.de>
Signed-off-by: NThomas Schlichter <thomas.schlichter@web.de>
Signed-off-by: NSuresh Siddha <suresh.b.siddha@intel.com>
Acked-by: NEric Anholt <eric@anholt.net>
Acked-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>

arch/x86/pci/i386.c

@@ -280,6 +280,15 @@ int pci_mmap_page_range(struct pci_dev *dev, struct vm_area_struct *vma,
 		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)