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SubjectRe: 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
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