add kernel cross build support for faster builds on x86_64-linux

This adds the option `raspberry-pi-nix.kernel-build-system`, which can be used to drastically decrease the build times.

Description of the option:

The build system to compile the kernel on.

Only the linux kernel will be cross compiled, while most of the derivations are still pulled from cache.nixos.org.

Use this in any of these cases:
  - you are building on an x86_64 system using binfmt_misc for aarch64-linux.
  - or if your x86_64 builder has a better CPU than your aarch64 builder.

Author: DavHau

overlays/default.nix

@@ -26,13 +26,13 @@ let
   boards = [ "bcm2711" "bcm2712" ];
 
   # Helpers for building the `pkgs.rpi-kernels' map.
-  rpi-kernel = { version, board }:
+  rpi-kernel = { version, board, pkgs ? final }:
     let
       kernel = builtins.getAttr version versions;
       version-slug = builtins.replaceStrings [ "v" "_" ] [ "" "." ] version;
     in
     {
-      "${version}"."${board}" = (final.buildLinux {
+      "${version}"."${board}" = (pkgs.buildLinux {
         modDirVersion = version-slug;
         version = version-slug;
         pname = "linux-rpi";
@@ -70,9 +70,21 @@ let
           '';
         });
     };
+
   rpi-kernels = builtins.foldl'
     (b: a: final.lib.recursiveUpdate b (rpi-kernel a))
     { };
+
+  rip-kernels-cross = buildSystem: builtins.foldl'
+    (b: a: final.lib.recursiveUpdate b (rpi-kernel (
+      a // {
+        pkgs = import final.pkgs.path {
+          system = buildSystem;
+          crossSystem = "aarch64-linux";
+        };
+      }
+    )))
+    { };
 in
 {
   # disable firmware compression so that brcm firmware can be found at
@@ -114,11 +126,16 @@ in
 
 } // {
   # rpi kernels and firmware are available at
-  # `pkgs.rpi-kernels.<VERSION>.<BOARD>'. 
+  # `pkgs.rpi-kernels.<VERSION>.<BOARD>'.
   #
   # For example: `pkgs.rpi-kernels.v6_6_54.bcm2712'
   rpi-kernels = rpi-kernels (
     final.lib.cartesianProduct
       { board = boards; version = (builtins.attrNames versions); }
   );
+
+  rpi-kernels-cross = buildSystem: rip-kernels-cross buildSystem (
+    final.lib.cartesianProduct
+      { board = boards; version = (builtins.attrNames versions); }
+  );
 }

rpi/default.nix

@@ -18,6 +18,19 @@ in
         type = types.str;
         description = "Kernel version to build.";
       };
+      kernel-build-system = mkOption {
+        type = types.nullOr (types.enum [ "x86_64-linux" ]);
+        description = ''
+          The build system to compile the kernel on.
+
+          Only the linux kernel will be cross compiled, while most of the derivations are still pulled from cache.nixos.org.
+
+          Use this in any of these cases:
+            - you are building on an x86_64 system using binfmt_misc for aarch64-linux.
+            - or if your x86_64 builder has a better CPU than your aarch64 builder.
+        '';
+        example = "x86_64-linux";
+      };
       board = mkOption {
         type = types.enum [ "bcm2711" "bcm2712" ];
         description = ''
@@ -334,7 +347,11 @@ in
           "reset-raspberrypi" # required for vl805 firmware to load
         ];
       };
-      kernelPackages = pkgs.linuxPackagesFor pkgs.rpi-kernels."${version}"."${board}";
+      kernelPackages =
+        if cfg.kernel-build-system == null then
+          pkgs.linuxPackagesFor pkgs.rpi-kernels."${version}"."${board}"
+        else
+          pkgs.linuxPackagesFor (pkgs.rpi-kernels-cross cfg.kernel-build-system)."${version}"."${board}";
       loader = {
         grub.enable = lib.mkDefault false;
         initScript.enable = !cfg.uboot.enable;