[RFC PATCH -tip] x86/pat: fix pat_x_mtrr_type to its essentials

[Xiaotian Feng <dfeng@redhat.com>]

Due to the IA-32 SDM vol 3a "Effective Memory Type" tables, the PAT
and mtrr relations should be:
	|	UC	UC-	WC	WB	[PAT]
--------+--------------------------------------
UC	|	UC	UC(UC-)	WC	UC(UC-)
WC	|	UC	WC(UC-)	WC	WC(UC-)
WB	|	UC	UC(UC-)	WC	WB
[MTRR]
* In () is the current return value.

So, commit b6ff32, Fix pat_x_mtrr_type() to use UC_MINUS when the
mtrr type return UC. But it also made mtrr type WC use UC_MINUS.
This is not reasonable.

Signed-off-by: Xiaotian Feng <dfeng@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
---
 arch/x86/mm/pat.c |    9 +++++----
 1 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
index c37fd51..92e9627 100644
--- a/arch/x86/mm/pat.c
+++ b/arch/x86/mm/pat.c
@@ -228,10 +228,11 @@ static unsigned long pat_x_mtrr_type(u64 start, u64 end, unsigned long req_type)
 		u8 mtrr_type;
 
 		mtrr_type = mtrr_type_lookup(start, end);
-		if (mtrr_type != MTRR_TYPE_WRBACK)
-			return _PAGE_CACHE_UC_MINUS;
-
-		return _PAGE_CACHE_WB;
+		if (mtrr_type == MTRR_TYPE_WRBACK)
+			return _PAGE_CACHE_WB;
+		else if (mtrr_type == MTRR_TYPE_WRCOMB)
+			return _PAGE_CACHE_WC;
+		return _PAGE_CACHE_UC_MINUS;
 	}
 
 	return req_type;
-- 
1.6.5.2