Devicetree overlay (on Rock64)

[Sébastien Lerique]

Hi Guix!

(Re-posting from help-guix@gnu.org as I didn't get any answers there.)

I'm starting a small embedded project using a Rock64, successfully
running Guix on it (and keeping track of config at [0]).

It seems that the Rock64 has a few design mistakes (as always I guess,
but in this case they matter to me), such that I would need to use a
tiny devicetree overlay to get access to the board's SPI device. The
pine64 forum has a couple threads on this: [1] [2].

Trying to use the kernel's configfs to set up an overlay (as in the
first thread above, [1]) doesn't seem possible, as the devicetree in
configfs seems to be only made available on board-maintained versions of
the kernel [3].

My question: is there a standard way to change a devicetree using Guix's
bootloader+kernel tooling?

U-Boot's docs [4] mention either manually loading an overlay, or putting it
in a Flat Image Tree, but I haven't figured out how to do that with
Guix's tooling. I'm also not understanding whether I would need to do
this for both u-boot and the kernel, or if the first would be enough.

This thread [5] from help-guix mentions it would be nice to be able to
parse custom devicetrees, but that wasn't enough to find next steps in
exploring this. Do guixers have experience that could help here? Any
hints or help would be very welcome!

Happy hacking -- and please let me know if this doesn't belong on this
list,
Sébastien


[0] https://gitlab.com/wehlutyk/matrixbox/
[1] https://forum.pine64.org/showthread.php?tid=6282&pid=40056#pid40056
[2] https://forum.pine64.org/showthread.php?tid=5522
[3] 
https://stackoverflow.com/questions/50877808/configfs-do-not-mount-device-tree-overlays
[4] 
https://u-boot.readthedocs.io/en/stable/usage/fdt_overlays.html#ways-to-utilize-overlays-in-u-boot
[5] https://mail.gnu.org/archive/html/help-guix/2020-04/msg00052.html


PS: in case anybody is interested, the project is to implement a Marble
Answering Machine <https://vimeo.com/183465991> on top of Matrix, with
added message-sending buttons, to enable young children (and grown-ups)
to message friends and relatives while still avoiding screen exposure.