[U-Boot] Port old board to Kconfig

[Tim Godfrey]

Hello!

I am working with a VAR-SOM-AM43 dev board from a company called Variscite.
The company doesn't support u-boot builds for this board after the
migration to Kconfig (maintenance happens on a 2014.07 branch), and I would
like to port the board to more recent u-boot sources.

The SoC on the dev board is a Texas Instruments AM4738 ARM processor. My
understanding of the abstraction layers is
- Architecture: ARM
- CPU: armv7
- SoC: AM33xx
- Board: varsomam43
- target build configurations

The vendor supplied Yocto layer includes the board support source files
(board.*, mux.c, etc), so my plan had been to build u-boot with the board
(and config, based on the old boards.cfg entry) being varsomam43:
- CONFIG_VARSOMAM43 (board)
- CONFIG_TARGET_VARSOMAM43 (target)

Once I had these legacy configurations working with the new source tree I
was planning to migrate the code to the Kconfig way of doing things (which
I'm still learning about).

That is my plan, but there are a few things that are unclear to me or that
I am unsure of.
- Is my plan a sensible approach?
- Do I understand the board/config layer division correctly? It is not
clear to me that I shouldn't build my board with the AM43XX board
definition (that is what the Variscite sources are based on anyway), with
the dev board I am using defined as a target of that board. Should it be
Soc:AM33XX, Board:AM43XX, Target:varsomam43; or SoC:AM33XX,
Board:varsomam43, Target:varsomam43, or some combination thereof?
- Why do the AM43XX evm boards use the AM33XX SoC definition, even though
they are a part of the AM43XX soc series (on chip peripherals), and the
external differences (external peripherals, pin mux configurations) are
configured in the evm definitions?
- In the current ti-u-boot source tree in am43xx_evm_defconfig
CONFIG_AM43XX=y is defined, but not CONFIG_TARGET_AM43XX_EVM, and the
Kconfig options in the board source file are conditional upon that symbol
("if TARGET_AM43XX_EVM"). How are the Kconfig defaults configured, and
am43xx_evm.h selected for the AM43XX_EVM target?
- I have watched Marek Vasut's (thanks Marek!) ELC presentation on board
porting, and been reading README.kconfig, but I still feel like I am
feeling my way forward by trial and error. Are there any other documents or
sources that I should be reading?

All tips, pointers, and help warmly received!

Thanks,
Tim

[Tom Rini]

Post by Tim Godfrey
Hello!
I am working with a VAR-SOM-AM43 dev board from a company called Variscite.
The company doesn't support u-boot builds for this board after the
migration to Kconfig (maintenance happens on a 2014.07 branch), and I would
like to port the board to more recent u-boot sources.
The SoC on the dev board is a Texas Instruments AM4738 ARM processor. My
understanding of the abstraction layers is
- Architecture: ARM
- CPU: armv7
- SoC: AM33xx
- Board: varsomam43
- target build configurations

That sounds right, yes.

Post by Tim Godfrey
The vendor supplied Yocto layer includes the board support source files
(board.*, mux.c, etc), so my plan had been to build u-boot with the board
- CONFIG_VARSOMAM43 (board)
- CONFIG_TARGET_VARSOMAM43 (target)
Once I had these legacy configurations working with the new source tree I
was planning to migrate the code to the Kconfig way of doing things (which
I'm still learning about).

You might want to look at board/compulab/ as there's a number of am33xx
and am43xx boards there that aren't the TI reference platforms. There's
others too of course.

Post by Tim Godfrey
That is my plan, but there are a few things that are unclear to me or that
I am unsure of.
- Is my plan a sensible approach?

Yes.

Post by Tim Godfrey
- Do I understand the board/config layer division correctly? It is not
clear to me that I shouldn't build my board with the AM43XX board
definition (that is what the Variscite sources are based on anyway), with
the dev board I am using defined as a target of that board. Should it be
Soc:AM33XX, Board:AM43XX, Target:varsomam43; or SoC:AM33XX,
Board:varsomam43, Target:varsomam43, or some combination thereof?

That sounds about right.

Post by Tim Godfrey
- Why do the AM43XX evm boards use the AM33XX SoC definition, even though
they are a part of the AM43XX soc series (on chip peripherals), and the
external differences (external peripherals, pin mux configurations) are
configured in the evm definitions?

So, the am33xx line is really the 3rd generation platform from the
ti81xx line (that had gen 1/2). And am43xx is the 4th generation. For
historical reasons however, am33xx was the first one fully upstreamed
and so everyone else in the family keys off of that.

Post by Tim Godfrey
- In the current ti-u-boot source tree in am43xx_evm_defconfig
CONFIG_AM43XX=y is defined, but not CONFIG_TARGET_AM43XX_EVM, and the
Kconfig options in the board source file are conditional upon that symbol
("if TARGET_AM43XX_EVM"). How are the Kconfig defaults configured, and
am43xx_evm.h selected for the AM43XX_EVM target?

You should just look at the main u-boot tree itself. And note that the
defconfig files under configs/ contain only symbols that aren't deduced
when running "oldconfig" on top of it.