[U-Boot] Newbie question

[Jeroen Hofstee]

Hello Donald,

On 23-09-14 05:03, Donald Dade wrote:
> I recently bought a Jetson TK1 with the intent of learning all about ARM
> board bring up; essentially everything that happens between the reset
> vector and the spawning of the init process (in Linux). I've got a good
> understanding of this for x86, and I'd like to similarly understand what
> happens with an ARM SoC.
>
> I've been having a rough go of it, for example I cannot even find in
> explicit terms what the reset vector address is. I began to think that
> maybe I'm presupposing too many x86 artifacts and that that's not at all
> how ARM works. x86 has a lot of cruft left over from the 70s, after all.
>
> So I know that there is a u-boot port for the Jetson, so I cloned the
> source, started swimming through it, and quickly started to drown :)
>
> So, my questions are these:
>
> Can anyone briefly explain how the build system works, i.e. how the source
> is configured to build for Jetson

The supported boards have a defconfig file in /configs nowadays. In your
case it is jetson-tk1_defconfig. (or ./MAKEALL -l arm | grep jetson)

# build
export CROSS_COMPILE=arm-linux-gnueabi-
make jetson-tk1_defconfig
make all

You should end up with two binaries,  spl/u-boot-spl.bin and u-boot.bin.
The former loads the latter one. spl is build from the same sources tree
but the object files are placed in the spl directory (and tweaked for 
size).

  and how I find where u-boot's entry point is,


- Look at the output to see which start.S is compiled.
- find start.o
- objdump the obtained elf file (with the .bin) and search for reset

There is no universal memory map / startup sequence for ARM if that is
what you were looking for. You need google or a datasheet to figure out
how (might depend on gpio e.g.) and where code execution begins on
the target.

Note versatileqemu_defconfig can be run with qemu / gdb, so you can
just step through it. You need to load symbols for the location u-boot
relocates itself to as well. Google knows how to do that.

> so I can just look at what it does?

A small note. There are two "boards" at the moment for ARM, if grep
points you to arch/arm/lib/board.c ignore it since this boards uses
common/board_f.c / common/board_r.c.

Good luck,

Regards,
Jeroen