How to Install Custom dts/dtbo?

[hftsai256]

I'm seeking a way to build a Raspberry Pi CM4 image with several customized dts/dtbo, e.g. mcp4728.dts. My first intuition is something like this:

hardware.deviceTree = {
  kernelPackage = pkgs.linux_rpi4;
  filter = "bcm2711-rpi-cm4.dtb";
  overlays = [
    { name = "mcp4728"; dtsFile = ./mcp4728.dts; }
    { name = "spi6-1cs"; dtboFile = "${pkgs.device-tree_rpi.overlays}/spi6-1cs.dtbo"; }
  ];
};

But then I find boot.loader.generic-extlinux-compatible.useGenerationDeviceTree = false, and I doubt the path I'm taking will be effective.

My another thought is to create a separate Nix package to handle the build/install part:

{ stdenvNoCC, dtc }:
stdenvNoCC.mkDerivation {
  pname = "dtoverlays";
  version = "0.0.1";
  src = ./.;
  buildInputs = [ dtc ];
  installPhase = ''
    install -d $out/boot/firmware/overlays
    mv build/*.dtbo $out/boot/firmware/overlays
  '';
}

However as far as I concerned, the install path /boot/firmware/overlays is in a different partition from rootfs, and therefore I also doubt that I can include this package in environment.systemPackages

Is there a way to build the dtbo from source and install them into /boot/firmware/overlays?

[tstat]

Hey @hftsai256. The short answer to your question is that overlays are taken from the package raspberrypifw, so you could override that to add an overlay. This is mentioned in the readme under this section.

Reading the firmware populating script might clarify how this all fits together in your mind. Specifically, the device tree overlays are populated here.

[hftsai256]

Thank you for your directions. I'm wondering if I can have a separate package that installs into the same path: in this case, $out/share/raspberrypi/boot/firmware/overlays, before migrate_firmware() is invoked.

I have set up the following structure that may be viewed as a standalone nix package:

dtoverlays
├── default.nix
├── flake.nix
├── Makefile
└── mcp4728.dts

On the other hand, I would try giving overriding raspberrypifw a shot. My initial thought would be something like this:

  outputs = { self, nixpkgs, nixos-hardware } @ inputs: let
    pkgs = import nixpkgs {
      system = "aarch64-linux";
      overlays = [ customDtOverlay ];
    };
    customDtOverlay = (final: prev: let
      newfw = prev.raspberrypifw.overrideAttrs (oldAttrs: {
        srcs = [ oldAttrs.src ./dtoverlays ];
        sourceRoot = ".";
        buildInputs = [ pkgs.dtc ];

        dontBuild = false;
        buildPhase = ''
          make -C ./dtoverlays
        '';

        installPhase = ''
          mkdir -p $out/share/raspberrypi
          mv ./source/boot $out/share/raspberrypi
          mkdir -p $out/share/raspberrypi/boot/overlays
          mv ./dtoverlays/build/*.dtbo $out/share/raspberrypi/boot/overlays
        '';
      });
  in {
  ...
  }

I have to override the entire installPhase as the paths are changed after modifying srcs

[tstat]

I'm not against the idea of having a list of packages that contain overlays.

An alternative to overriding the derivation with an additional source as you demonstrated here might to override raspberrypifw with the symlinkJoin of your package and the prev raspberrypifw.

[hftsai256]

Thanks for the help. I can sort of building my customized raspberypifw with the overlay above. However the overridden package would not apply to the SD image unless pin-inputs.enable = false (as I came across with #83 and realize this could be the key to my last puzzle.) The down side is to rebuild the entire Linux kernel under qemu, but I'm having a working image now.

I'm now on the journey of trimming the image down to ~500-800 MB instead of the current whopping 6 GB, but that's a different topic.