Rock64 devicetree ovelay

[Sébastien Lerique]

Hi Guix!

I'm starting a small embedded project using a Rock64, successfully
running Guix on it (and keeping track of config at
https://gitlab.com/wehlutyk/matrixbox/).

It seems that the rock64 has a few design mistakes (as always I guess,
but in my case they matter), 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:
- https://forum.pine64.org/showthread.php?tid=6282&pid=40056#pid40056
- https://forum.pine64.org/showthread.php?tid=5522

Trying to use the kernel's configfs to set up an overlay (as in the
first thread above) doesn't seem possible, as the device-tree seems to
be only made available on board-maintained versions of the kernel:
https://stackoverflow.com/questions/50877808/configfs-do-not-mount-device-tree-overlays

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

U-Boot's docs [0] 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.

Do guixers have experience that could help here? Any hints or help would
be very welcome!

Happy hacking,
Sébastien


[0] 
https://u-boot.readthedocs.io/en/stable/usage/fdt_overlays.html#ways-to-utilize-overlays-in-u-boot


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.