linux-kernel - Re: [PATCH 2.6.23-rc1] APM detection logic bug fix

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Message-ID: <46A52D05.4050603@zytor.com>
Date:	Mon, 23 Jul 2007 15:34:45 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mikael Pettersson <mikpe@...uu.se>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2.6.23-rc1] APM detection logic bug fix

Mikael Pettersson wrote:
> Starting with kernel 2.6.23-rc1, the i386 APM driver fails
> on several of my machines with the message:
> 
> apm: BIOS not found
> 
> This happens because of a bug in the i386 boot code rewrite
> from assembler to C. The original assembly code had the
> following code in its APM BIOS presence test (boot/setup.S):
> 
> 	andw	$0x02, %cx			# Is 32 bit supported?
> 	je	done_apm_bios			# No 32-bit, no (good) APM BIOS
> 
> That is, the code bails out if bit 2 is zero.
> 
> In the new C version, this is coded as (boot/apm.c):
> 
> 	if (cx & 0x02)		/* 32 bits supported? */
> 		return -1;
> 
> Here we see that the test has been accidentally inverted.
> 
> The fix is to negate the test. I've verified that this
> allows the APM driver to work again on my affected machines.
> 
> Signed-off-by: Mikael Pettersson <mikpe@...uu.se>
> 

Thanks, applied.

	-hpa
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