devmem, x86: PAT Change /dev/mem mmap with O_SYNC to use UC_MINUS

All kernel mappings like ioremap(), etc uses UC_MINUS as the type. /dev/mem
mappings with /dev/mem being opened with O_SYNC however was using UC,
resulting in a conflict with /dev/mem mmap failing. This seems to be
affecting some apps (one being flashrom) which are using O_SYNC and which were
working before.

Switch /dev/mem with O_SYNC also to UC_MINUS.
Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/mm/pat.c

@@ -390,14 +390,6 @@ int free_memtype(u64 start, u64 end)
 }
 
 
-/*
- * /dev/mem mmap interface. The memtype used for mapping varies:
- * - Use UC for mappings with O_SYNC flag
- * - Without O_SYNC flag, if there is any conflict in reserve_memtype,
- *   inherit the memtype from existing mapping.
- * - Else use UC_MINUS memtype (for backward compatibility with existing
- *   X drivers.
- */
 pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
 				unsigned long size, pgprot_t vma_prot)
 {
@@ -435,14 +427,14 @@ int phys_mem_access_prot_allowed(struct file *file, unsigned long pfn,
 				unsigned long size, pgprot_t *vma_prot)
 {
 	u64 offset = ((u64) pfn) << PAGE_SHIFT;
-	unsigned long flags = _PAGE_CACHE_UC_MINUS;
+	unsigned long flags = -1;
 	int retval;
 
 	if (!range_is_allowed(pfn, size))
 		return 0;
 
 	if (file->f_flags & O_SYNC) {
-		flags = _PAGE_CACHE_UC;
+		flags = _PAGE_CACHE_UC_MINUS;
 	}
 
 #ifdef CONFIG_X86_32
@@ -465,13 +457,14 @@ int phys_mem_access_prot_allowed(struct file *file, unsigned long pfn,
 #endif
 
 	/*
-	 * With O_SYNC, we can only take UC mapping. Fail if we cannot.
+	 * With O_SYNC, we can only take UC_MINUS mapping. Fail if we cannot.
+	 *
 	 * Without O_SYNC, we want to get
 	 * - WB for WB-able memory and no other conflicting mappings
 	 * - UC_MINUS for non-WB-able memory with no other conflicting mappings
 	 * - Inherit from confliting mappings otherwise
 	 */
-	if (flags != _PAGE_CACHE_UC_MINUS) {
+	if (flags != -1) {
 		retval = reserve_memtype(offset, offset + size, flags, NULL);
 	} else {
 		retval = reserve_memtype(offset, offset + size, -1, &flags);