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;