[PATCH 5/5] x86: PAT: make pat_x_mtrr_type() more readable

[Andreas Herrmann <andreas.herrmann3@amd.com>]

I've found it inconvenient to review pat_x_mtrr_type().
Thus I slightly changed it and added some comment to make it more
readable.
I've also added BUG statements for (some) unused/unhandled PAT/MTRR types.

Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
---
 arch/x86/mm/pat.c |   30 +++++++++++++++++++++---------
 1 files changed, 21 insertions(+), 9 deletions(-)

diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
index 2f6c33d..fc0a397 100644
--- a/arch/x86/mm/pat.c
+++ b/arch/x86/mm/pat.c
@@ -185,18 +185,30 @@ static int pat_x_mtrr_type(u64 start, u64 end, unsigned long prot,
 	prot &= (~_PAGE_CACHE_MASK);
 
 	/* Currently doing intersection by hand. Optimize it later. */
-	if (pat_type == _PAGE_CACHE_WC) {
+	/*    effective type		        ret_prot
+	  pat \ mtrr  WB WC UC		pat \ mtrr  WB  WC  UC
+	  WC          WC WC WC		WC          WC  WC  WC
+	  UC-         UC WC UC		UC-         UC- UC- UC-
+	  UC          UC UC UC		UC          UC  UC  UC
+	  WB          WB WC UC		WB          WB  WC  UC
+	 */
+	if (pat_type == _PAGE_CACHE_WC)
 		*ret_prot = prot | _PAGE_CACHE_WC;
-	} else if (pat_type == _PAGE_CACHE_UC_MINUS) {
+	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) {
+	else if (pat_type == _PAGE_CACHE_UC)
 		*ret_prot = prot | _PAGE_CACHE_UC;
-	} else if (mtrr_type == MTRR_TYPE_WRCOMB) {
-		*ret_prot = prot | _PAGE_CACHE_WC;
-	} else {
-		*ret_prot = prot | _PAGE_CACHE_WB;
-	}
+	else if (pat_type == _PAGE_CACHE_WB) {
+		if (mtrr_type == MTRR_TYPE_WRBACK)
+			*ret_prot = prot | _PAGE_CACHE_WB;
+		else if (mtrr_type == MTRR_TYPE_WRCOMB)
+			*ret_prot = prot | _PAGE_CACHE_WC;
+		else if (mtrr_type == MTRR_TYPE_UNCACHABLE)
+			*ret_prot = prot | _PAGE_CACHE_UC;
+		else
+			BUG();
+	} else
+		  BUG();
 
 	return 0;
 }
-- 
1.5.5.3