[U-Boot] ARMv7 Execution Flow

From: Lokesh Vutla <lokeshvutla@ti.com>
To: u-boot@lists.denx.de
Subject: [U-Boot] ARMv7 Execution Flow
Date: Mon, 6 Jun 2016 09:18:53 +0530	[thread overview]
Message-ID: <5754F2A5.3020108@ti.com> (raw)
In-Reply-To: <CAB=FTDMhJ4dczOAQzUeHqoNrz8Kp_F8Oj+aEXt_G9ROpceOOvQ@mail.gmail.com>

Hi James,

On Saturday 04 June 2016 12:25 AM, James Pollard wrote:
> Hi,
> 
> I'm a new U-Boot user. I'm trying to follow the flow of execution for the
> ARMv7 architecture, and have found myself running into confusion.
> 
> At the end of arch/arm/cpu/armv7/start.S, lowlevel_init is called. This is
> defined (for the ARMv7 architecture in arch/arm/cpu/armv7/low_level_init.S
> (not sure if there's a reason for the discrepancy in the use of
> underscores?), and at the end of this file, s_init is called.

So, s_init is called by cpu_init_crit and after that _main will be
called. Below is the code flow for u-boot(Skipped all the #ifdefs).

- reset
	- save_boot_params
	- cpu_init_cp15:
		- Invalidate caches and apply erratas:
	- cpu_init_crit:
		- lowlevel_init
			- s_init
	- _main (arch/arm/lib/crt0.S)
		- Setup C runtime environment
		- board_init_f
		- relocate code if !SPL
		- board_init_r

> 
> As far as I can tell, s_init is only defined for some of the ARMv7 CPUs:
> the bcm281xx folder doesn't contain any files that define this function. In
> the case of the am33xx CPU it's defined in board.c, but this function seems
> to return without calling board_init_f.

s_init is not necessary to be called for all the boards. board_init_f
should take care of system init. As per the above code flow board_init_f
will be called for every board.

> 
> My questions are:
> 
>    1.  In the case of the bcm281xx, what happens? s_init doesn't seem to be
>    defined for this CPU.

Yes, board_init_f will take care of the initialization.

>    2.  In the case of the am33xx, where is board_init_f called? As far as I
>    can tell, s_init should return to lowlevel_init without having
>    called board_init_f , which then returns back to start.S, terminating
>    the execution prematurely.

This has been answered above.

> 
> I presume my analysis of the assembly must be highly incorrect?
> 
> Sorry if this is the wrong place for such a question.

Yes, this is the right place to ask :)

Thanks and regards,
Lokesh

> 
> Yours,
> 
> James Pollard
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