linux-kernel - [PATCH] X86: MM: Add PAT Type write-through in combination with mtrr

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Date:	Sun, 27 Oct 2013 13:55:25 +0100
From:	Andreas Werner <wernerandy@....de>
To:	tglx@...utronix.de
Cc:	mingo@...hat.com, hpa@...or.com, x86@...nel.org,
	dave@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	wernerandy@....de
Subject: [PATCH] X86: MM: Add PAT Type write-through in combination with mtrr

This patch adds the Write-through memory type in combination with mtrr.
If you call ioremap_cache to request cachable memory (write-back) the
function tries to set the PAT to write-back only if the mtrr setting of
the requested region is also marked as Write-Back.
If the mttr regions are marked e.g. as Write-through or with other
types, the function will always return UC- memory.

If you check the Intel document " IA-32 SDM vol 3a table Effective
Memory Type", there
are many other combinations possible.

This patch will only add the following combination:
  PAT=Write-Back + MTRR=Write-Through = Effective Memory of
Write-Through

Tested on - Intel (R) Atom E680 (Tunnel Creek)
          - Intel (R) Core(TM)2 Duo

Signed-off-by: Andreas Werner <wernerandy@....de>
---
 arch/x86/mm/pat.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
index 6574388..9cfe107 100644
--- a/arch/x86/mm/pat.c
+++ b/arch/x86/mm/pat.c
@@ -149,10 +149,15 @@ 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;
+		switch (mtrr_type) {
+		case MTRR_TYPE_WRBACK:
+		case MTRR_TYPE_WRTHROUGH:
+		  return _PAGE_CACHE_WB;
+
+		default:
+		  return _PAGE_CACHE_UC_MINUS;
+		}
 	}
 
 	return req_type;
-- 
1.8.4

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