linux-kernel - Re: [PATCH] amd64_edac: fix size decoding error on K8

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Date:	Wed, 9 Nov 2011 21:35:46 +0100
From:	Borislav Petkov <bp@...64.org>
To:	Niklas Söderlund 
	<niklas.soderlund@...csson.com>
Cc:	"dougthompson@...ssion.com" <dougthompson@...ssion.com>,
	"linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] amd64_edac: fix size decoding error on K8

Hi Niklas,

On Wed, Nov 09, 2011 at 04:13:37PM +0100, Niklas Söderlund wrote:
> The decoding table from the documentation:
> 
> cs_mode		size (mb)
> 0		32
> 1		64
> 2		128
> 3		128
> 4		256
> 5		512
> 6		256
> 7		512
> 8		1024
> 9		1024
> 10		2048
> 
> The original code:
> 
> if (cs_mode == 3 || cs_mode == 8)
> 	return 32 << (cs_mode - 1);
> else
> 	return 32 << cs_mode;
> 
> The code tries to compensate for the irregularity in the decoding
> table but fails for cs_mode greater or equal to 4. It fails because:
> 
> - It do not consider the cumulating effect of reoccurring sizes for
> all values of cs_mode. It only tries for the special cases and it
> only succeeds in one.
> 
> - It do not consider the out of order values of cs_mode values 4-7:
> 256, 512, 256, 512.

Agreed, thanks for reporting this. However, I removed the static tables
at the time because I didn't want to carry unnecessary weight around for
no reason (tables were static back then). And I know, they're small but
dead weight like that tends to accumulate.

Your approach is ok with creating the lookup table on the stack but how
about the following instead - it is much smaller and it is only code,
no need for stack memory at all. It should give you the proper mapping,
IINM.

--
diff --git a/drivers/edac/amd64_edac.c b/drivers/edac/amd64_edac.c
index c9eee6d33e9a..3702f20d784e 100644
--- a/drivers/edac/amd64_edac.c
+++ b/drivers/edac/amd64_edac.c
@@ -1132,12 +1132,12 @@ static int k8_dbam_to_chip_select(struct amd64_pvt *pvt, u8 dct,
 		return ddr2_cs_size(cs_mode, dclr & WIDTH_128);
 	}
 	else if (pvt->ext_model >= K8_REV_D) {
+		unsigned diff;
 		WARN_ON(cs_mode > 10);
 
-		if (cs_mode == 3 || cs_mode == 8)
-			return 32 << (cs_mode - 1);
-		else
-			return 32 << cs_mode;
+		diff = cs_mode / 3 + (unsigned)(cs_mode > 5);
+
+		return 32 << (cs_mode - diff);
 	}
 	else {
 		WARN_ON(cs_mode > 6);
--

Thanks.

-- 
Regards/Gruss,
Boris.

Advanced Micro Devices GmbH
Einsteinring 24, 85609 Dornach
GM: Alberto Bozzo
Reg: Dornach, Landkreis Muenchen
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