[U-Boot] Board specific files

[Alexander Dahl <ada@thorsis.com>]

Hello Pascal,

Am Mittwoch, 3. April 2019, 09:08:57 CEST schrieb Linder Pascal:
> I am new to U-Boot and I am trying to understand what I need to code for
> using my own board with the bootloader.

Welcome. :-)

I try to describe my view on this, anyone correct me, if something is wrong.

> The device tree files I have already found under ./arch/arm/dts/. Can I just
> add my device tree under this directory?

Yes. Note: you probably can not just copy a dts file from the Linux Kernel. 
From what I discovered when adding a new board lately, those files are 
slightly different.

> Also, what is the difference between the configuration files under
> ./configs/ and ./include/configs/? Do I need to add a file in both of the
> named directories?

For setting board options and more, U-Boot uses the Kconfig system from the 
Linux kernel (adapted to the needs of U-Boot). In the old days everything was 
set as preprocessor define in a board specific header in ./include/configs but 
more and more options have been converted over the years.

Today you still need that header in ./include/configs but fewer things are set 
there with each new U-Boot release.

The files in ./configs/ are default configs for a board to have an initial 
working ./.config configuration file for that Kconfig approach. Although these 
defconfigs are not strictly necessary, it's very convenient as a starting 
point when compiling an image for a board. Some embedded build systems use 
those defconfigs.

> If someone knows a well documented guide to add a new board to the mainline,
> I would be very grateful.

I see two steps here. Getting all the things in place for a new board to boot 
is one, the other one with a different set of challenges is getting that into 
mainline U-Boot. 

For the first I would advise to look into ./README and ./doc/ (keep in mind, 
information there might be outdated) and read lots of code of other boards, 
preferably boards with a similar SoC added lately. 

For getting your patches into mainline, the guide on submitting patches in the 
Linux kernel docs [1] mostly applies to U-Boot as well.

Greets
Alex

[1] https://www.kernel.org/doc/html/latest/process/submitting-patches.html