linux-kernel - Re: Linux 2.6.27.27

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Date:	Wed, 22 Jul 2009 10:32:19 +0200 (CEST)
From:	Krzysztof Oledzki <olel@....pl>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	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, 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;
>> +	int i, err = 0, fix = check_edid(edid);
>>
>> 	if (fix)
>> 		fix_edid(edid, fix);
>
> Wow! You guys rock! ;)
>
> Indeed, this simple change is enough to make my kernel bootable. However, 
> there is still something wrong. My console is now 80x30 instead of 128x48:
>
> -Console: switching to colour frame buffer device 128x48
> +Console: switching to colour frame buffer device 80x30
>
> So, it looks like the loop may be still miscompiled.
>
> The kernel is here:
> http://noc.axelspringer.pl/no-strict-overflow-vs-wrapv/vmlinux-fno-strict-overflow-fixed.bz2

OK, by adding a simple debug printk I'm now sure that the problem is 
indeed in the loop:

--- linux-2.6.27.27-a/drivers/video/fbmon.c	2009-07-20 05:45:22.000000000 +0200
+++ linux-2.6.27.27-b/drivers/video/fbmon.c	2009-07-22 09:45:34.000000000 +0200
@@ -272,6 +272,8 @@
  		err = 1;
  	}

+	printk("edid_checksum debug: csum=%u, all_null=%u, err=%d\n", csum, all_null, err);
+
  	return err;
  }


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).

Best regards,

 				Krzysztof Olędzki