linux-kernel - Re: Debugging APM - cat /proc/apm produces oops

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Message-Id: <200607242351.37578.linux@rainbow-software.org>
Date:	Mon, 24 Jul 2006 23:51:37 +0200
From:	Ondrej Zary <linux@...nbow-software.org>
To:	Stephen Rothwell <sfr@...b.auug.org.au>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Debugging APM - cat /proc/apm produces oops

On Sunday 23 July 2006 17:06, Stephen Rothwell wrote:
> On Sun, 23 Jul 2006 16:30:53 +0200 Ondrej Zary <linux@...nbow-software.org> 
wrote:

> >  printing eip:
> > 00002f9d
> > *pre = 00000000
> > Oops: 0002 [#4]
> > Modules linked in:
> > CPU:    0
> > EIP:    00c0:[<00002f9d>]    Not tainted VLI
>
>           ^^^^
> This is the APM BIOS 16 bit code segment.

Looking at BIOS disassembly:
2F97: push bp
2F98: mov bp,sp
2F9A: add sp,-2
2F9D: mov [bp][-2],bx    <-- it oopses here

I realized that I can modify the BIOS easily as it's stored in shadow RAM. So 
I replaced the offending MOV with three NOPs and tested again. This time it 
oopsed at 0x2FAD:
2FAD: cmp w,[bp][-2],1
2FB1: je 2FCB

that jump was taken during my single stepping, so I NOPped out the CMP and 
replaced JE with JMPS. Then booted Linux and APM seems to work fine - battery 
percentage and remaining time is there as well as AC power status.
There seems to be 4 these operations:
mov [bp][-2],bx
cmp w,[bp][-2],1
cmp w,[bp][-2],8002
cmp w,[bp][-2],8001
but I've hit only the first two of them. I wonder what's that for (especially 
when it works without that).

-- 
Ondrej Zary
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