Question about linux boot procedure (head_64.S)

Question about linux boot procedure (head_64.S)

[Lee HongWoo]

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Hi ~

This is a boot flow of linux kernel under the arch/powerpc/kernel and I'm
using pasemi cpu.

__start  (in head_64.S)
  ---> __start_initialization_multiplatform (in head_64.S)
    ---> __boot_from_prom (in head_64.S)
       ---> prom_init ( in prom_init.c)
         ---> __start ???

And I don't understand where __start is called, because I can find __start
only in head_64.S.
If it calls __start in head_64.S, it's a recursive call.

Can anybody explain about this precedure ?

Thanks in advance.

HongWoo.

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Question about linux boot procedure (head_64.S)

[Lee HongWoo]

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Hi ~

This is a boot flow of linux kernel under the arch/powerpc/kernel and I'm
using pasemi cpu.

__start  (in head_64.S)
  ---> __start_initialization_multiplatform (in head_64.S)
    ---> __boot_from_prom (in head_64.S)
       ---> prom_init ( in prom_init.c)
         ---> __start ???

And I don't understand where __start is called, because I can find __start
only in head_64.S.
If it calls __start in head_64.S, it's a recursive call.

Can anybody explain about this precedure ?

Thanks in advance.

HongWoo.

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Re: Question about linux boot procedure (head_64.S)

[Geoff Levand]

On 09/01/2009 03:58 AM, Lee HongWoo wrote:
> __start  (in head_64.S)
>   ---> __start_initialization_multiplatform (in head_64.S)
>     ---> __boot_from_prom (in head_64.S)
>        ---> prom_init ( in prom_init.c)
>          ---> __start ???
> 
> And I don't understand where __start is called, because I can find __start
> only in head_64.S.
> If it calls __start in head_64.S, it's a recursive call.
> 
> Can anybody explain about this precedure ?

In the general case, __start is the entry point of the kernel.
It is where the bootloader or boot wrapper program jumps to
when it transfers control to the kernel.

-Geoff

Re: Question about linux boot procedure (head_64.S)

[Lee HongWoo]

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On Wed, Sep 2, 2009 at 2:32 AM, Geoff Levand <geoffrey.levand@am.sony.com>wrote:

> On 09/01/2009 03:58 AM, Lee HongWoo wrote:
> > __start  (in head_64.S)
> >   ---> __start_initialization_multiplatform (in head_64.S)
> >     ---> __boot_from_prom (in head_64.S)
> >        ---> prom_init ( in prom_init.c)
> >          ---> __start ???
> >
> > And I don't understand where __start is called, because I can find
> __start
> > only in head_64.S.
> > If it calls __start in head_64.S, it's a recursive call.
> >
> > Can anybody explain about this precedure ?
>
> In the general case, __start is the entry point of the kernel.
> It is where the bootloader or boot wrapper program jumps to
> when it transfers control to the kernel.
>
> -Geoff
>
>
Thanks Geoff,

I believe __start is the entry point of the kernel in this case.
And the entry point is __GLOBAL(__start) in the head_64.S.

What I asked is where or what __start is called in the prom_init.c
__start(hdr, KERNELBASE + offset, 0);
Below is the simple function call flow of linux kernel boot procedure.

file : head_64.S

_GLOBAL(__start)
        /* NOP this out unconditionally */
BEGIN_FTR_SECTION
        b       .__start_initialization_multiplatform
END_FTR_SECTION(0, 1)

....

_GLOBAL(__start_initialization_multiplatform)
        /*
         * Are we booted from a PROM Of-type client-interface ?
         */
        cmpldi  cr0,r5,0
        bne     .__boot_from_prom               /* yes -> prom */

....

_STATIC(__boot_from_prom)
        /* Save parameters */
        mr      r31,r3
        mr      r30,r4
        mr      r29,r5
        mr      r28,r6
        mr      r27,r7

        /*
         * Align the stack to 16-byte boundary
         * Depending on the size and layout of the ELF sections in the
initial
         * boot binary, the stack pointer will be unalignet on PowerMac
         */
        rldicr  r1,r1,0,59

        /* Make sure we are running in 64 bits mode */
        bl      .enable_64b_mode

        /* put a relocation offset into r3 */
        bl      .reloc_offset

        LOAD_REG_IMMEDIATE(r2,__toc_start)
        addi    r2,r2,0x4000
        addi    r2,r2,0x4000

        /* Relocate the TOC from a virt addr to a real addr */
        add     r2,r2,r3

        /* Restore parameters */
        mr      r3,r31
        mr      r4,r30
        mr      r5,r29
        mr      r6,r28
        mr      r7,r27

        /* Do all of the interaction with OF client interface */
        bl      .prom_init
        /* We never return */
        trap


file : prom_init.c

unsigned long __init prom_init(unsigned long r3, unsigned long r4,
                               unsigned long pp,
                               unsigned long r6, unsigned long r7)
{
    ...
    ...
        __start(hdr, KERNELBASE + offset, 0);

        return 0;
}


HongWoo.

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Re: Question about linux boot procedure (head_64.S)

[Geoff Levand]

On 09/01/2009 05:25 PM, Lee HongWoo wrote:
> On Wed, Sep 2, 2009 at 2:32 AM, Geoff Levand <geoffrey.levand@am.sony.com>wrote:
>> In the general case, __start is the entry point of the kernel.
>> It is where the bootloader or boot wrapper program jumps to
>> when it transfers control to the kernel.
> 
> I believe __start is the entry point of the kernel in this case.
> And the entry point is __GLOBAL(__start) in the head_64.S.
> 
> What I asked is where or what __start is called in the prom_init.c
> __start(hdr, KERNELBASE + offset, 0);

I think Michael answered this.  Just FYI, more info about the powerpc
boot is in the kernel source file:

  Documentation/powerpc/booting-without-of.txt

-Geoff

Re: Question about linux boot procedure (head_64.S)

[Michael Ellerman]

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On Tue, 2009-09-01 at 19:58 +0900, Lee HongWoo wrote:
> Hi ~ 
> 
> This is a boot flow of linux kernel under the arch/powerpc/kernel and
> I'm using pasemi cpu. 
> 
> __start  (in head_64.S) 
>   ---> __start_initialization_multiplatform (in head_64.S) 
>     ---> __boot_from_prom (in head_64.S) 
>        ---> prom_init ( in prom_init.c) 
>          ---> __start ???
> 
> And I don't understand where __start is called, because I can find
> __start only in head_64.S. 
> If it calls __start in head_64.S, it's a recursive call. 
> 
> Can anybody explain about this precedure ? 

It calls __start() with different arguments. They are checked in
__start_initialization_multiplatform:

308         /*
309          * Are we booted from a PROM Of-type client-interface ?             
310          */
311         cmpldi  cr0,r5,0
312         beq     1f
313         b       .__boot_from_prom               /* yes -> prom */


The first time through __start we are running under OF. The kernel can
detect this based on the arguments it is passed (r5 in particular).

prom_init() deals with talking to OF and flattening the OF device tree.
We then call back into __start but this time r5 is 0:

2555         __start(hdr, kbase, 0);                                            

So the second time through we don't call into prom_init(), instead the
kernel continues using the flattened device tree.

The startup is structured this way so that the kernel can boot either
from OF (in which case we call prom_init()), or directly with a
flattened device tree.

cheers

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