lkml.org 
[lkml]   [2009]   [Jul]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: Linux 2.6.27.27


On Wed, 22 Jul 2009, Krzysztof Oledzki wrote:

>
>
> On Wed, 22 Jul 2009, 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;
>>>> + 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).
>
> More fun.
>
> Actually, the problem is indepenent from -fno-strict-overflow and also exists
> with -fwrapv. It is probably caused by "unsigned char i" to "int i"
> translation and the variables reordering.
>
> With "unsigned char i" before "unsigned char csum = 0":
> edid_checksum loop: edid[0]=0, csum=0
> edid_checksum loop: edid[1]=255, csum=0
> edid_checksum loop: edid[2]=255, csum=255
> -edid_checksum loop: edid[3]=255, csum=254
> -edid_checksum loop: edid[4]=255, csum=253
> -edid_checksum loop: edid[5]=255, csum=252
> -edid_checksum loop: edid[6]=255, csum=251
> -edid_checksum loop: edid[7]=0, csum=250
> -edid_checksum loop: edid[8]=6, csum=250
> -edid_checksum loop: edid[9]=207, csum=0
> (...)
> -edid_checksum debug: csum=0, all_null=255, err=1 -> OK
>
> With "int i" after "unsigned char csum = 0":
> edid_checksum loop: edid[0]=0, csum=0
> edid_checksum loop: edid[1]=255, csum=0
> edid_checksum loop: edid[2]=255, csum=255
> +edid_checksum loop: edid[3]=255, csum=510
> +edid_checksum loop: edid[4]=255, csum=765
> +edid_checksum loop: edid[5]=255, csum=1020
> +edid_checksum loop: edid[6]=255, csum=1275
> +edid_checksum loop: edid[7]=0, csum=1530
> +edid_checksum loop: edid[8]=6, csum=1530
> +edid_checksum loop: edid[9]=207, csum=1536
> (...)
> +edid_checksum debug: csum=6400, all_null=255, err=0 -> ERROR
>
> Funny. ;)

OK. I'm just blind. The oryginal patch is wrong. Instead of:

- 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);


It should be:

- unsigned char i, csum = 0, all_null = 0;
- int err = 0, fix = check_edid(edid);
+ unsigned char csum = 0, all_null = 0;
+ int i, err = 0, fix = check_edid(edid);

The patch should not change "unsigned char (...) csum" to "unsigned (...) csum".

Best regards,

Krzysztof Olędzki
\
 
 \ /
  Last update: 2009-07-22 12:49    [W:0.258 / U:0.484 seconds]
©2003-2014 Jasper Spaans. Advertise on this site