linux-kernel - Re: Regression in 2.6.23-pre Was: Problems with 2.6.23-rc6 on AMD Geode LX800

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Message-ID: <46FAC958.9000501@zytor.com>
Date:	Wed, 26 Sep 2007 14:04:24 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jordan Crouse <jordan.crouse@....com>
CC:	Joerg Pommnitz <pommnitz@...oo.com>, cebbert@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: Regression in 2.6.23-pre Was: Problems with 2.6.23-rc6 on AMD
 Geode LX800

Jordan Crouse wrote:
> On 26/09/07 12:14 -0700, H. Peter Anvin wrote:
>> Please try the following debug patch to let us know what is going on.
>>
>> 	-hpa
> 
>> diff --git a/arch/i386/boot/memory.c b/arch/i386/boot/memory.c
>> index 1a2e62d..a0ccf29 100644
>> --- a/arch/i386/boot/memory.c
>> +++ b/arch/i386/boot/memory.c
>> @@ -33,6 +33,12 @@ static int detect_memory_e820(void)
>>  		      "=m" (*desc)
>>  		    : "D" (desc), "a" (0xe820));
>>  
>> +		printf("e820: err %d id 0x%08x next %u %08x:%08x %u\n",
>> +		       err, id, next,
>> +		       (unsigned int)desc->addr,
>> +		       (unsigned int)desc->size,
>> +		       desc->type);
>> +
>>  		if (err || id != SMAP)
>>  			break;
> 
> Okay, we have clarity.   Here is the output
> 
> e820: err 0 id 0x534d4150 next 15476 00000000:0009fc00 1
> e820: err 0 id 0x534d4150 next 15496 0009fc00:00000400 2
> e820: err 0 id 0x534d4150 next 15516 000e0000:00020000 2
> e820: err 0 id 0x0e7b0000 next 11536 00100000:0e6b0000 1
> 
> In the last entry,  id is obviously wrong (it should be 'SMAP' or
> 0x534d4150).  This is the BIOS bug.
> 
> Here's the reason why this bothers us now.  In the old assembly code,
> if the returned ID wasn't equal to 'SMAP', we jumped straight to the e801
> code.  In the new code in memory.c, if id != SMAP, we break out of the
> int15 loop, and return boot_params.e820_entries, which in our case is
> 3.  detect_memory() considers this to be successful, and no attempt to
> parse e801 is made.
> 
> So thats where the problem is - in the old code with the buggy BIOS, we
> punted to reading the e801 information, and that was enough to keep us 
> going.   In the new code, we allow a partial table to be used, and we
> blow up.
> 
> Attached is a patch to fix this - it returns -1 on error, and only sets
> boot_params.e820_entries to be non-zero if we have something useful
> in it.  This punts the detection to the e801 code, which then is
> then successful.
> 
> This fixes the problem with the DB800, and so it probably should
> with the other Geode platforms affected by this.
> 
> Many thanks to hpa for the guiding hand.
> 

This patch is obviously wrong.  There are a lot of e820 BIOSen out there
that terminate with CF=1, and that is a legitimate termination condition
for e820.  Now, as far as what to do when id != SMAP, it probably is
still the right thing to do; since the BOS vendor couldn't get something
that elementary correct, we shouldn't trust the data.

I'll write up a corrected patch.

	-hpa
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