linux-kernel - Re: Linux 2.6.27.27

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Date:	Wed, 22 Jul 2009 11:58:09 +0200
From:	Jens Rosenboom <jens@...a.mcbone.net>
To:	Krzysztof Oledzki <olel@....pl>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Troy Moure <twmoure@...pr.net>, Greg KH <gregkh@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>, stable@...nel.org,
	lwn@....net, Ian Lance Taylor <iant@...gle.com>
Subject: Re: Linux 2.6.27.27

On Wed, 2009-07-22 at 10:32 +0200, Krzysztof Oledzki wrote:
> 
> On Wed, 22 Jul 2009, Krzysztof Oledzki wrote:
> 
> >
> >
> > On Tue, 21 Jul 2009, Linus Torvalds wrote:
> >
> >> 
> >> 
> >> On Tue, 21 Jul 2009, Linus Torvalds wrote:
> >>> 
> >>> Anyway, I bet we can work around the compiler bug by just changing the
> >>> type of "i" from "unsigned char" to be a plain "int".
> >> 
> >> IOW, like this.
> >>
> >> 		Linus
> >> 
> >> ---
> >> drivers/video/fbmon.c |    4 ++--
> >> 1 files changed, 2 insertions(+), 2 deletions(-)
> >> 
> >> diff --git a/drivers/video/fbmon.c b/drivers/video/fbmon.c
> >> index 5c1a2c0..af4a15c 100644
> >> --- a/drivers/video/fbmon.c
> >> +++ b/drivers/video/fbmon.c
> >> @@ -256,8 +256,8 @@ static void fix_edid(unsigned char *edid, int fix)
> >> 
> >> static int edid_checksum(unsigned char *edid)
> >> {
> >> -	unsigned char i, csum = 0, all_null = 0;
> >> -	int err = 0, fix = check_edid(edid);
> >> +	unsigned csum = 0, all_null = 0;

I guess Linus shouldn't have deleted the "char" here...

[...]

> Here is a diff between a good and a bad kernel:
> 
> -edid_checksum debug: csum=0, all_null=255, err=1
> -edid_checksum debug: csum=0, all_null=255, err=1
> -Console: switching to colour frame buffer device 128x48
> +edid_checksum debug: csum=6400, all_null=255, err=0
> +Console: switching to colour frame buffer device 80x30
> 
> In the good one the function is called twice and it returns err=1 (==OK). 
> In the bad kernel it returns 0 because csum!=0x00 (==6400).

6400 is a multiple of 256, so as unsigned char it is ==0.

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