1 files changed, 24 insertions, 25 deletions

diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
index de3a99812450..183fdd36d3bd 100644
--- a/arch/x86/mm/pat.c
+++ b/arch/x86/mm/pat.c
@@ -151,32 +151,33 @@ static int pat_x_mtrr_type(u64 start, u64 end, unsigned long prot,
 	unsigned long pat_type;
 	u8 mtrr_type;
 
-	mtrr_type = mtrr_type_lookup(start, end);
-	if (mtrr_type == 0xFF) {		/* MTRR not enabled */
-		*ret_prot = prot;
-		return 0;
-	}
-	if (mtrr_type == 0xFE) {		/* MTRR match error */
-		*ret_prot = _PAGE_CACHE_UC;
-		return -1;
-	}
-	if (mtrr_type != MTRR_TYPE_UNCACHABLE &&
-	    mtrr_type != MTRR_TYPE_WRBACK &&
-	    mtrr_type != MTRR_TYPE_WRCOMB) {	/* MTRR type unhandled */
-		*ret_prot = _PAGE_CACHE_UC;
-		return -1;
-	}
-
 	pat_type = prot & _PAGE_CACHE_MASK;
 	prot &= (~_PAGE_CACHE_MASK);
 
-	/* Currently doing intersection by hand. Optimize it later. */
+	/*
+	 * We return the PAT request directly for types where PAT takes
+	 * precedence with respect to MTRR and for UC_MINUS.
+	 * Consistency checks with other PAT requests is done later
+	 * while going through memtype list.
+	 */
 	if (pat_type == _PAGE_CACHE_WC) {
 		*ret_prot = prot | _PAGE_CACHE_WC;
+		return 0;
 	} else if (pat_type == _PAGE_CACHE_UC_MINUS) {
 		*ret_prot = prot | _PAGE_CACHE_UC_MINUS;
-	} else if (pat_type == _PAGE_CACHE_UC ||
-	           mtrr_type == MTRR_TYPE_UNCACHABLE) {
+		return 0;
+	} else if (pat_type == _PAGE_CACHE_UC) {
+		*ret_prot = prot | _PAGE_CACHE_UC;
+		return 0;
+	}
+
+	/*
+	 * Look for MTRR hint to get the effective type in case where PAT
+	 * request is for WB.
+	 */
+	mtrr_type = mtrr_type_lookup(start, end);
+
+	if (mtrr_type == MTRR_TYPE_UNCACHABLE) {
 		*ret_prot = prot | _PAGE_CACHE_UC;
 	} else if (mtrr_type == MTRR_TYPE_WRCOMB) {
 		*ret_prot = prot | _PAGE_CACHE_WC;
@@ -233,14 +234,12 @@ int reserve_memtype(u64 start, u64 end, unsigned long req_type,
 
 	if (req_type == -1) {
 		/*
-		 * Special case where caller wants to inherit from mtrr or
-		 * existing pat mapping, defaulting to UC_MINUS in case of
-		 * no match.
+		 * Call mtrr_lookup to get the type hint. This is an
+		 * optimization for /dev/mem mmap'ers into WB memory (BIOS
+		 * tools and ACPI tools). Use WB request for WB memory and use
+		 * UC_MINUS otherwise.
 		 */
 		u8 mtrr_type = mtrr_type_lookup(start, end);
-		if (mtrr_type == 0xFE) { /* MTRR match error */
-			err = -1;
-		}
 
 		if (mtrr_type == MTRR_TYPE_WRBACK) {
 			req_type = _PAGE_CACHE_WB;