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

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Message-ID: <4EBA98A1.90902@ericsson.com>
Date:	Wed, 9 Nov 2011 16:13:37 +0100
From:	Niklas Söderlund <niklas.soderlund@...csson.com>
To:	Borislav Petkov <bp@...64.org>
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

On 11/09/2011 03:42 PM, Borislav Petkov wrote:
> On Wed, Nov 09, 2011 at 03:32:58PM +0100, Niklas Söderlund wrote:
>> Use a lookup table to calculate the size of a chip select. The old
>> method of calculating size return erroneous values for some memory
>> configurations.
>
> Can you elaborate more on those erroneous values, please?
>
> Thanks.
>

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.

Best Regards
// Niklas
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