[PATCH] mtrr: allow uncached type for PCI prefetchable memory

[Alex Hung]

Buglink: https://bugs.launchpad.net/bugs/1781920

Intel® 64 and IA-32 Architectures Software Developer's Manual seems to
suggest uncachable memory can be used for memory-mapped I/O.

Details can be found in Section 11.3.2 - Choosing a Memory Type

Signed-off-by: Alex Hung <alex.hung at canonical.com>
---
 src/bios/mtrr/mtrr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/bios/mtrr/mtrr.c b/src/bios/mtrr/mtrr.c
index 4948b55a..a614ffbb 100644
--- a/src/bios/mtrr/mtrr.c
+++ b/src/bios/mtrr/mtrr.c
@@ -351,7 +351,7 @@ static int guess_cache_type(
 
 		if (pref) {
 			*must = 0;
-			*mustnot = WRITE_BACK | UNCACHED;
+			*mustnot = WRITE_BACK;
 		} else {
 			*must = UNCACHED;
 			*mustnot = (~UNCACHED) & (~DEFAULT);
-- 
2.17.1