raspberry-pi: add config.txt generation module

Two new files in raspberry-pi/common/:

config-txt.nix lets you declare config.txt contents through
hardware.raspberry-pi.configtxt.settings (RFC 42 style — plain
values, null to remove, no enable/value wrappers).

config-txt-defaults.nix has the same defaults as Raspberry Pi OS:
vc4-kms-v3d, audio on, arm_boost, CM4 otg_mode, CM5 dwc2, etc.
All mkDefault so they are easy to override.

Not wired into board profiles yet — that comes with the RPi 5
sub-module work. Based on nvmd/nixos-raspberrypi.

raspberry-pi/common/config-txt-defaults.nix

@@ -0,0 +1,29 @@
+# Default config.txt settings matching the official Raspberry Pi OS defaults.
+#
+# Source: https://github.com/RPi-Distro/pi-gen/blob/master/stage1/00-boot-files/files/config.txt
+# All values use mkDefault so they can be easily overridden.
+#
+# Reference: https://www.raspberrypi.com/documentation/computers/config_txt.html
+
+{ lib, ... }:
+
+{
+  hardware.raspberry-pi.configtxt.settings = {
+    all = {
+      camera_auto_detect = lib.mkDefault true;
+      display_auto_detect = lib.mkDefault true;
+      max_framebuffers = lib.mkDefault 2;
+      disable_fw_kms_setup = lib.mkDefault true;
+      disable_overscan = lib.mkDefault true;
+      arm_boost = lib.mkDefault true;
+      dtparam = lib.mkDefault [ "audio=on" ];
+      dtoverlay = lib.mkDefault [ "vc4-kms-v3d" ];
+    };
+    cm4 = {
+      otg_mode = lib.mkDefault true;
+    };
+    cm5 = {
+      dtoverlay = lib.mkDefault [ "dwc2,dr_mode=host" ];
+    };
+  };
+}

raspberry-pi/common/config-txt.nix

@@ -0,0 +1,130 @@
+# config.txt generation module for Raspberry Pi
+#
+# Generates config.txt from structured Nix options. The type system models
+# config.txt as nested attrs: each nesting level adds a conditional filter,
+# and leaves are config values. Repeated keys (dtparam, dtoverlay, gpio)
+# are supported via lists.
+#
+# Based on work from nvmd/nixos-raspberrypi (MIT License) and rendering
+# approach suggested by @quentinmit. Follows RFC 42.
+#
+# Reference: https://www.raspberrypi.com/documentation/computers/config_txt.html
+
+{ lib, config, ... }:
+
+let
+  cfg = config.hardware.raspberry-pi.configtxt;
+
+  mkValueString =
+    v:
+    if builtins.isInt v then
+      toString v
+    else if builtins.isString v then
+      v
+    else if true == v then
+      "1"
+    else if false == v then
+      "0"
+    else
+      builtins.abort "config.txt: unsupported value type: ${builtins.typeOf v}";
+
+  mkKeyValue = lib.generators.mkKeyValueDefault { inherit mkValueString; } "=";
+
+  # Recursively flatten nested attrs into a list of { conditionals, name, value } records.
+  # Each nesting level (except the leaf) is a conditional filter.
+  # "all" filters are omitted from the conditionals list since they reset state.
+  recurse =
+    path: value:
+    if builtins.isAttrs value && !(builtins.isList value) && !(value ? _type) then
+      lib.flatten (lib.mapAttrsToList (name: recurse ([ name ] ++ path)) value)
+    else
+      {
+        conditionals = lib.sort builtins.lessThan (lib.filter (k: k != "all") (lib.tail path));
+        name = lib.head path;
+        inherit value;
+      };
+
+  # Group flattened items by their conditional filter set, then render.
+  groupItems =
+    items:
+    lib.mapAttrsToList (groupJSON: groupItems: {
+      conditionals = builtins.fromJSON groupJSON;
+      items = builtins.listToAttrs groupItems;
+    }) (builtins.groupBy (x: builtins.toJSON x.conditionals) items);
+
+  mkGroup =
+    group:
+    lib.concatMapStrings (k: "[${k}]\n") group.conditionals
+    + lib.generators.toKeyValue {
+      inherit mkKeyValue;
+      listsAsDuplicateKeys = true;
+    } group.items;
+
+  toConfigTxt =
+    attrs:
+    let
+      groups = lib.sort (a: b: a.conditionals < b.conditionals) (
+        groupItems (lib.flatten (recurse [ ] attrs))
+      );
+    in
+    lib.concatMapStringsSep "\n[all]\n" mkGroup groups;
+
+in
+{
+  options.hardware.raspberry-pi.configtxt = {
+    settings = lib.mkOption {
+      type =
+        with lib.types;
+        let
+          atom = oneOf [
+            str
+            int
+            bool
+          ];
+          molecule = oneOf [
+            atom
+            (listOf atom)
+            (attrsOf molecule)
+          ];
+        in
+        attrsOf molecule;
+      default = { };
+      description = ''
+        Structured configuration for the Raspberry Pi `config.txt` file.
+
+        Top-level keys are conditional filter sections (`all`, `pi4`, `pi5`,
+        `cm4`, etc.). Nesting adds stacked conditional filters. Leaves are
+        config values rendered as `key=value`. Lists produce repeated keys.
+
+        Booleans render as `0`/`1`. Use `null` with `mkForce` to remove a
+        default.
+
+        See <https://www.raspberrypi.com/documentation/computers/config_txt.html>
+      '';
+      example = lib.literalExpression ''
+        {
+          all = {
+            arm_boost = true;
+            disable_overscan = true;
+            dtparam = [ "audio=on" ];
+            dtoverlay = [ "vc4-kms-v3d" ];
+          };
+          pi5.arm_freq = 2400;
+          cm4.otg_mode = true;
+        }
+      '';
+    };
+
+    file = lib.mkOption {
+      type = lib.types.str;
+      description = ''
+        The generated config.txt content. Defaults to the rendered output of
+        `settings`, but can be overridden to supply a custom config.txt.
+      '';
+    };
+  };
+
+  config = {
+    hardware.raspberry-pi.configtxt.file = lib.mkDefault (toConfigTxt cfg.settings);
+  };
+}