[U-Boot] U-Boot on ARM9

[U-Boot] U-Boot on ARM9

[Roman Mashak]

Hello.

I have a few questions on arm926 initialization code, located in
${U_BOOT}/cpu/arm926ejs/start.S:
1) what is the point of deliberately setting up SVC mode, as it's
already in this mode right upon the reset?
2) I can't quite understand what 'CONFIG_USE_IRQ' is used for. It is
defined on just a few targets.
3) The stack set up -- commonly startup code initializes stacks for
all modes, but it's not clear how it's done in 'start.S'. Seems like
somehow stacks are run-time configured, but I didn't perceive  how.

Thanks in advance for comments.

-- 
Roman Mashak

[U-Boot] U-Boot on ARM9

[Hebbar]

Hi,

>>
>>I have a few questions on arm926 initialization code, located in
>>${U_BOOT}/cpu/arm926ejs/start.S:
>>1) what is the point of deliberately setting up SVC mode, as it's
>>already in this mode right upon the reset?
What if the target board has a custom bootmonitor & you use U-Boot as
secondary bootloader. This is a precautionary measure.

>>2) I can't quite understand what 'CONFIG_USE_IRQ' is used for. It is
>>defined on just a few targets.
Generally bootloader doesnt use IRQ for running. But if u need to use
IRQ/FIQ u need to setup stacks for the Same. And also When Interrupt occurs
you need to set sp to pint to proper stacks, save registers & then jump to
irq handler. U_Boot provides a common envelope for all interrupt related
functions. 


>>3) The stack set up -- commonly startup code initializes stacks for
>>all modes, but it's not clear how it's done in 'start.S'. Seems like
>>somehow stacks are run-time configured, but I didn't perceive  how.
U-boot Initially sets up Stack for SVC Mode. 

Upon Interrupt Triggers, Stacks are setup for the IRQ/FIQ Modes. 

This is done in the ${U_BOOT}/cpu/arm926ejs/start.S

for SVC Mode ---> @ line 173
for IRQ Mode --->  @ line 327
for FIQ Mode --->  @ line 331

Hope this would clear your doubts.

Regards
Gururaja
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[U-Boot] U-Boot on ARM9

[Roman Mashak]

Hello.

Thank you for your quick response.

2008/9/22 Hebbar <gururajakr@sanyo.co.in>:
[skip]

> Generally bootloader doesnt use IRQ for running. But if u need to use
> IRQ/FIQ u need to setup stacks for the Same.

Right, because U-Boot is basically needed to initialize hardware and
prepaare it to launch OS on it. IRQ/FIQ may be necessary to run
standalone code within bootlloader ?

-- 
Roman Mashak