[U-Boot-Users] clarifications u-boot for arm

[U-Boot-Users] clarifications u-boot for arm

[Ram]

Hi,
    I am using a omap5912osk board which uses arm926-ejs CPU.

    I am trying to understand the flow of u-boot - what really happens
when the board is powered on till i get the u-boot prompt which
accepts commands from the user.


   I started with the file u-boot-1.1.4/cpu/arm926ejs/start.S

   Is this the place where u-boot actually starts?.


   It tries switch the cpu mode to SVC32 mode and relocates u-boot to
RAM, disables MMU
   and does so many things which i dont really understand.


   Could anyone explain it to me what really is happening out there and why?.




   Then i figured out that a function start_armboot() is called.  This
function is present in
    lib_arm/board.c is this function called by start.S?

    Where exactly it is called from?.

   There is a reference to start_armboot in start.S not sure if it is the one?.



    ldr pc, _start_armboot

_start_armboot:
    .word start_armboot

  Does this actually invoke start_armboot?.


  I have got so far. could  you please tell me am i right in my understarting?.



  I understand that start_armboot initialises the the cpu, board, dram....



  Once this is done - we can say that the board has been initialised.

  Now start_armboot provides a prompt to accept and download the kernel.


  Am i right in assuming this?.



  Any other information and pointers u provide will be helpful


  I am trying to understand u-boot sources and any help will be highly
appreciated.



  I just wanted to clarify my understanding.



   Thanks and Regards,
     sriram

[U-Boot-Users] clarifications u-boot for arm

[Wolfgang Denk]

In message <8bf247760607140029l62f6c124idee300237361518f@mail.gmail.com> you wrote:
>
>     I am trying to understand the flow of u-boot - what really happens
> when the board is powered on till i get the u-boot prompt which
> accepts commands from the user.

You've posted the same question before.

>    I started with the file u-boot-1.1.4/cpu/arm926ejs/start.S
>    Is this the place where u-boot actually starts?.

Yes.

>    It tries switch the cpu mode to SVC32 mode and relocates u-boot to
> RAM, disables MMU
>    and does so many things which i dont really understand.

I recommend toi follow the flow step by  step.  Most  functions  have
pretty descriptive names.

>    Could anyone explain it to me what really is happening out there and why?.

We initialize the system step by step as  needed  to  get  a  running
system capable of booting Linux.

>    Then i figured out that a function start_armboot() is called.  This
> function is present in
>     lib_arm/board.c is this function called by start.S?
> 
>     Where exactly it is called from?.

Use grep to find the call.

>    There is a reference to start_armboot in start.S not sure if it is the one?.

Why not?

>     ldr pc, _start_armboot
> _start_armboot:
>     .word start_armboot
> 
>   Does this actually invoke start_armboot?.

Guess what happens when you load a new address into the rpgram counter?

>   I have got so far. could  you please tell me am i right in my understarting?.

Yes.

>   I understand that start_armboot initialises the the cpu, board, dram....
>   Once this is done - we can say that the board has been initialised.

More or less.

>   Any other information and pointers u provide will be helpful

Read the README, the manual, and the sources.

Best regards,

Wolfgang Denk

-- 
Software Engineering:  Embedded and Realtime Systems,  Embedded Linux
Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: wd at denx.de
"The lesser of two evils -- is evil."             - Seymour (Sy) Leon

[U-Boot-Users] clarifications u-boot for arm

[Ram]

Hi,
   Thanks for your response. I have gone much further and would like to know If

      ldr pc, _start_armboot
     _start_armboot:
         .word start_armboot


    Does this really invoke start_armboot?.  This is because
_start_armboot is what is
    stored in pc and not start_armboot and there is also a branch
point _start_armboot (underscore start_armboot) Please see above.

   Actually the above declares a variable start_armboot and does not
invoke the function
   start_armboot. If this is true, then who calls start_armboot?


    According to me start_armboot ( ) which is present in
lib_arm/board.c never returns until u reset the board and probably
should be one of the last functions that needs to be called.

    Correct?.


   Rest agreed, What i have understood is
         First,  reset is called which set the cpu to SVC32 mode and
calls cpu_init_crit
         which inturn calls lowlevel_init to setup memory and board
specific register values.

          Then stack is setup and start_amboot is called.


          start_armboot is the point of entry where code is C. ie
(Assembly to C) begins from start_armboot.


          am i Correct?.


        This is the function which does some remaining initialisation
and then starts initialising the for pheripherals.


     am i Correct?


  I didnt understand the things relating interrupt and thier processing though.

Thanks and Regards,
sriram

[U-Boot-Users] clarifications u-boot for arm

[Russell Peterson]

Hello,

The answer to your question "who calls start_armboot?"  is answered by you
ASM code below.  Hint... take a close look at the regsiter you are loading.

:-)

Regards,

Russell


On 7/14/06, Ram <vshrirama@gmail.com> wrote:
>
> Hi,
>   Thanks for your response. I have gone much further and would like to
> know If
>
>      ldr pc, _start_armboot
>     _start_armboot:
>         .word start_armboot
>
>
>    Does this really invoke start_armboot?.  This is because
> _start_armboot is what is
>    stored in pc and not start_armboot and there is also a branch
> point _start_armboot (underscore start_armboot) Please see above.
>
>   Actually the above declares a variable start_armboot and does not
> invoke the function
>   start_armboot. If this is true, then who calls start_armboot?
>
>
>    According to me start_armboot ( ) which is present in
> lib_arm/board.c never returns until u reset the board and probably
> should be one of the last functions that needs to be called.
>
>    Correct?.
>
>
>   Rest agreed, What i have understood is
>         First,  reset is called which set the cpu to SVC32 mode and
> calls cpu_init_crit
>         which inturn calls lowlevel_init to setup memory and board
> specific register values.
>
>          Then stack is setup and start_amboot is called.
>
>
>          start_armboot is the point of entry where code is C. ie
> (Assembly to C) begins from start_armboot.
>
>
>          am i Correct?.
>
>
>        This is the function which does some remaining initialisation
> and then starts initialising the for pheripherals.
>
>
>     am i Correct?
>
>
> I didnt understand the things relating interrupt and thier processing
> though.
>
> Thanks and Regards,
> sriram
>
>
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[U-Boot-Users] clarifications u-boot for arm

[Jerry Van Baren]

See bottom posted reply.

Russell Peterson wrote:
> Hello,
>  
> The answer to your question "who calls start_armboot?"  is answered by 
> you ASM code below.  Hint... take a close look at the regsiter you are 
> loading.
>  
> :-)
>  
> Regards,
>  
> Russell
> 
>  
> On 7/14/06, *Ram* <vshrirama at gmail.com <mailto:vshrirama@gmail.com>> wrote:
> 
>     Hi,
>       Thanks for your response. I have gone much further and would like
>     to know If
> 
>          ldr pc, _start_armboot
>         _start_armboot:
>             .word start_armboot
> 
> 
>        Does this really invoke start_armboot?.  This is because
>     _start_armboot is what is
>        stored in pc and not start_armboot and there is also a branch
>     point _start_armboot (underscore start_armboot) Please see above.
> 
>       Actually the above declares a variable start_armboot and does not
>     invoke the function
>       start_armboot. If this is true, then who calls start_armboot?

[snip - I believe your observations are correct, but am not an expert]

>     Thanks and Regards,
>     sriram

Sriram:

C compiler naming convention prepends an underscore to the C function 
name so the C function start_armboot() can be referenced from assembly 
as a branch (call) to the label _start_armboot.

gvb

[U-Boot-Users] clarifications u-boot for arm

[Andreas Schweigstill]

Dear Ram!

Ram schrieb:
>     Does this really invoke start_armboot?.  This is because
> _start_armboot is what is
>     stored in pc and not start_armboot and there is also a branch
> point _start_armboot (underscore start_armboot) Please see above.

And why don't you learn the syntax and semantics of ARM assembler
before you send messages to this list?

>   I didnt understand the things relating interrupt and thier processing though.

In order to understand the interrupt related things I recommend reading
some documents about ARM architecture, your processor datasheet(s) AND
U-BOOT DOCUMENTATION. Then you will find out by yourself why it is so
difficult to understand interrupt handling on most platforms... :-) And
why don't you just hook your JTAG debugger on the interrupt entry point
and try to follow the execution flow? Then you will also notice
something very important in U-Boot...

Andreas Schweigstill

-- 
Dipl.-Phys. Andreas Schweigstill
Schweigstill IT | Embedded Systems
Schauenburgerstra?e 116, D-24118 Kiel, Germany
Phone: (+49) 431 5606-435, Fax: (+49) 431 5606-436
Mobile: (+49) 171 6921973, Web: http://www.schweigstill.de/