checking atags for boot parameters

checking atags for boot parameters

[Christopher Harvey]

 I'm suspicious that the bootloader I'm using isn't passing atags to the 
 kernel properly.

 I'm getting a prefetch exception right after the "uncompressing 
 linux....done" message, does it make sense that this could happen 
 because mem=128M isn't getting passed properly?

 Basically what I want to do is have the kernel print out the boot 
 parameters along with the "uncompressing linux" message.

 thanks,
 Christopher

checking atags for boot parameters

[Mulyadi Santosa]

Hi Christ... :)

On Tue, Jun 21, 2011 at 21:20, Christopher Harvey
<chris@basementcode.com> wrote:
> ?I'm suspicious that the bootloader I'm using isn't passing atags to the
> ?kernel properly.

which bootloader?

> ?I'm getting a prefetch exception right after the "uncompressing
> ?linux....done" message, does it make sense that this could happen
> ?because mem=128M isn't getting passed properly?

uhm, fairly unlikely for me.... must be something else... could you
tell us which arch it is and which kernel version (along with patches
from certain tree, if any) that yield that error message?

-- 
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com

checking atags for boot parameters

[Christopher Harvey]

 On Wed, 22 Jun 2011 00:14:50 +0700, Mulyadi Santosa 
 <mulyadi.santosa@gmail.com> wrote:
> Hi Christ... :)
 It's Chris.
>
> On Tue, Jun 21, 2011 at 21:20, Christopher Harvey
> <chris@basementcode.com> wrote:
>> ?I'm suspicious that the bootloader I'm using isn't passing atags to 
>> the
>> ?kernel properly.
>
> which bootloader?
 uboot
>
>> ?I'm getting a prefetch exception right after the "uncompressing
>> ?linux....done" message, does it make sense that this could happen
>> ?because mem=128M isn't getting passed properly?
>
> uhm, fairly unlikely for me.... must be something else... could you
> tell us which arch it is and which kernel version (along with patches
> from certain tree, if any) that yield that error message?
 It's not an error message, but with difficulty I was able to trace some 
 of the code and it ends up in a prefetch exception handler. I am not 
 able to find out from where however, and the line stepping is a bit 
 sketchy. No major patches to the kernel, and it runs in QEMU without 
 error. Adding uboot to QEMU then booting the same kernel yields the 
 problems.

 It would be nice to find out what physical address the kernel is 
 uncompressed to as well, I'm as of yet unable to properly trace after 
 decompression.

 -C

checking atags for boot parameters

[Gavin Guo]

> ?It's not an error message, but with difficulty I was able to trace some
> ?of the code and it ends up in a prefetch exception handler. I am not
> ?able to find out from where however, and the line stepping is a bit
> ?sketchy. No major patches to the kernel, and it runs in QEMU without
> ?error. Adding uboot to QEMU then booting the same kernel yields the
> ?problems.

To see what's happened, you can check the register about exceptional
virtual address in your platform, then up to the instruction caused
the exception. As to the atags parameter, searching MACHINE_START
under the platform you use, then the struct member, named boot_params,
below the MACHINE_START should be found. According to the assigned
value of boot_params, the address which uboot stick the parameter in
can be examine to see if the value is the same as the one assigned in
uboot.

> ?It would be nice to find out what physical address the kernel is
> ?uncompressed to as well, I'm as of yet unable to properly trace after
> ?decompression.

Referring to what I replied in another thread:
You can see that in /arch/arm/kernel/head.S, the Kernel startup entry
point is put in "ENTRY(stext)" above that is a line .section
".text.head", "ax" which says that the Kernel startup code is
allocated in .text.head section. And also you can find the following
at the beginning of the /arch/arm/kernel/vmlinux.lds.S:

ENTRY(stext)

#ifndef __ARMEB__
jiffies = jiffies_64;
#else
jiffies = jiffies_64 + 4;
#endif

SECTIONS
{
#ifdef CONFIG_XIP_KERNEL
       . = XIP_VIRT_ADDR(CONFIG_XIP_PHYS_ADDR);
#else
       . = PAGE_OFFSET + TEXT_OFFSET;
#endif
       .text.head : {
               _stext = .;
               _sinittext = .;
               *(.text.head)
       }

Obviously, ".text.head" section begins with TEXT_OFFSET + PAGE_OFFSET.
So, what is TEXT_OFFSET? It is defined in arch/arm/Makefile as
TEXT_OFFSET := $(textofs-y) where you can also find that textofs-y is
defined as "textofs-y       := 0x00008000". PAGE_OFFSET is defined
under configs/bcmring_defconfig:CONFIG_PAGE_OFFSET=0xC0000000, here
bcmring_defconfig is just an example. You can find other defconfig
also has CONFIG_PAGE_OFFSET too. The other trick is objdumpping the
vmlinux under kernel root, then you can see the kernel startup address
in the beginning of the first line.

Gavin Guo
OS kernel engineer
Andestech