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