sd-image.nix

# This module was lifted from nixpkgs installer code. It is modified
# so as to not import all-hardware. The goal here is to write the
# nixos image for a raspberry pi to an sd-card in a way so that we can
# pop it in and go. We don't need to support many possible hardware
# targets since we know we are targeting raspberry pi products.

# This module creates a bootable SD card image containing the given NixOS
# configuration. The generated image is MBR partitioned, with a FAT
# /boot/firmware partition, and ext4 root partition. The generated image
# is sized to fit its contents, and a boot script automatically resizes
# the root partition to fit the device on the first boot.
#
# The firmware partition is built with expectation to hold the Raspberry
# Pi firmware and bootloader, and be removed and replaced with a firmware
# build for the target SoC for other board families.
#
# The derivation for the SD image will be placed in
# config.system.build.sdImage

{ modulesPath, config, lib, pkgs, ... }:

with lib;

let
  rootfsImage = pkgs.callPackage "${modulesPath}/../lib/make-ext4-fs.nix" ({
    inherit (config.sdImage) storePaths;
    compressImage = true;
    populateImageCommands = config.sdImage.populateRootCommands;
    volumeLabel = "NIXOS_SD";
  } // optionalAttrs (config.sdImage.rootPartitionUUID != null) {
    uuid = config.sdImage.rootPartitionUUID;
  });
in
{
  imports = [ ];

  options.sdImage = {
    imageName = mkOption {
      default =
        "${config.sdImage.imageBaseName}-${config.system.nixos.label}-${pkgs.stdenv.hostPlatform.system}.img";
      description = ''
        Name of the generated image file.
      '';
    };

    imageBaseName = mkOption {
      default = "nixos-sd-image";
      description = ''
        Prefix of the name of the generated image file.
      '';
    };

    storePaths = mkOption {
      type = with types; listOf package;
      example = literalExpression "[ pkgs.stdenv ]";
      description = ''
        Derivations to be included in the Nix store in the generated SD image.
      '';
    };

    firmwarePartitionOffset = mkOption {
      type = types.int;
      default = 8;
      description = ''
        Gap in front of the /boot/firmware partition, in mebibytes (1024×1024
        bytes).
        Can be increased to make more space for boards requiring to dd u-boot
        SPL before actual partitions.

        Unless you are building your own images pre-configured with an
        installed U-Boot, you can instead opt to delete the existing `FIRMWARE`
        partition, which is used **only** for the Raspberry Pi family of
        hardware.
      '';
    };

    firmwarePartitionID = mkOption {
      type = types.str;
      default = "0x2178694e";
      description = ''
        Volume ID for the /boot/firmware partition on the SD card. This value
        must be a 32-bit hexadecimal number.
      '';
    };

    rootPartitionUUID = mkOption {
      type = types.nullOr types.str;
      default = null;
      example = "14e19a7b-0ae0-484d-9d54-43bd6fdc20c7";
      description = ''
        UUID for the filesystem on the main NixOS partition on the SD card.
      '';
    };

    firmwareSize = mkOption {
      type = types.int;
      # As of 2019-08-18 the Raspberry pi firmware + u-boot takes ~18MiB
      default = 128;
      description = ''
        Size of the /boot/firmware partition, in megabytes.
      '';
    };

    populateFirmwareCommands = mkOption {
      example =
        literalExpression "'' cp \${pkgs.myBootLoader}/u-boot.bin firmware/ ''";
      description = ''
        Shell commands to populate the ./firmware directory.
        All files in that directory are copied to the
        /boot/firmware partition on the SD image.
      '';
    };

    populateRootCommands = mkOption {
      example = literalExpression
        "''\${config.boot.loader.generic-extlinux-compatible.populateCmd} -c \${config.system.build.toplevel} -d ./files/boot''";
      description = ''
        Shell commands to populate the ./files directory.
        All files in that directory are copied to the
        root (/) partition on the SD image. Use this to
        populate the ./files/boot (/boot) directory.
      '';
    };

    postBuildCommands = mkOption {
      example = literalExpression
        "'' dd if=\${pkgs.myBootLoader}/SPL of=$img bs=1024 seek=1 conv=notrunc ''";
      default = "";
      description = ''
        Shell commands to run after the image is built.
        Can be used for boards requiring to dd u-boot SPL before actual partitions.
      '';
    };

    compressImage = mkOption {
      type = types.bool;
      default = true;
      description = ''
        Whether the SD image should be compressed using
        <command>zstd</command>.
      '';
    };

    expandOnBoot = mkOption {
      type = types.bool;
      default = true;
      description = ''
        Whether to configure the sd image to expand it's partition on boot.
      '';
    };
  };

  config = {
    fileSystems = {
      "/boot/firmware" = {
        device = "/dev/disk/by-label/${config.raspberry-pi-nix.firmware-partition-label}";
        fsType = "vfat";
      };
      "/" = {
        device = "/dev/disk/by-label/NIXOS_SD";
        fsType = "ext4";
      };
    };

    sdImage.storePaths = [ config.system.build.toplevel ];

    system.build.sdImage = pkgs.callPackage
      ({ stdenv, dosfstools, e2fsprogs, mtools, libfaketime, util-linux, zstd }:
        stdenv.mkDerivation {
          name = config.sdImage.imageName;

          nativeBuildInputs =
            [ dosfstools e2fsprogs mtools libfaketime util-linux zstd ];

          inherit (config.sdImage) compressImage;

          buildCommand = ''
            mkdir -p $out/nix-support $out/sd-image
            export img=$out/sd-image/${config.sdImage.imageName}

            echo "${pkgs.stdenv.buildPlatform.system}" > $out/nix-support/system
            if test -n "$compressImage"; then
              echo "file sd-image $img.zst" >> $out/nix-support/hydra-build-products
            else
              echo "file sd-image $img" >> $out/nix-support/hydra-build-products
            fi

            echo "Decompressing rootfs image"
            zstd -d --no-progress "${rootfsImage}" -o ./root-fs.img

            # Gap in front of the first partition, in MiB
            gap=${toString config.sdImage.firmwarePartitionOffset}

            # Create the image file sized to fit /boot/firmware and /, plus slack for the gap.
            rootSizeBlocks=$(du -B 512 --apparent-size ./root-fs.img | awk '{ print $1 }')
            firmwareSizeBlocks=$((${
              toString config.sdImage.firmwareSize
            } * 1024 * 1024 / 512))
            imageSize=$((rootSizeBlocks * 512 + firmwareSizeBlocks * 512 + gap * 1024 * 1024))
            truncate -s $imageSize $img

            # type=b is 'W95 FAT32', type=83 is 'Linux'.
            # The "bootable" partition is where u-boot will look file for the bootloader
            # information (dtbs, extlinux.conf file).
            sfdisk $img <<EOF
                label: dos
                label-id: ${config.sdImage.firmwarePartitionID}

                start=''${gap}M, size=$firmwareSizeBlocks, type=b
                start=$((gap + ${
                  toString config.sdImage.firmwareSize
                }))M, type=83, bootable
            EOF

            # Copy the rootfs into the SD image
            eval $(partx $img -o START,SECTORS --nr 2 --pairs)
            dd conv=notrunc if=./root-fs.img of=$img seek=$START count=$SECTORS

            # Create a FAT32 /boot/firmware partition of suitable size into firmware_part.img
            eval $(partx $img -o START,SECTORS --nr 1 --pairs)
            truncate -s $((SECTORS * 512)) firmware_part.img
            faketime "1970-01-01 00:00:00" mkfs.vfat -i ${config.sdImage.firmwarePartitionID} -n ${config.raspberry-pi-nix.firmware-partition-label} firmware_part.img

            # Populate the files intended for /boot/firmware
            mkdir firmware
            ${config.sdImage.populateFirmwareCommands}

            # Copy the populated /boot/firmware into the SD image
            (cd firmware; mcopy -psvm -i ../firmware_part.img ./* ::)
            # Verify the FAT partition before copying it.
            fsck.vfat -vn firmware_part.img
            dd conv=notrunc if=firmware_part.img of=$img seek=$START count=$SECTORS

            ${config.sdImage.postBuildCommands}

            if test -n "$compressImage"; then
                zstd -T$NIX_BUILD_CORES --rm $img
            fi
          '';
        })
      { };

    boot.postBootCommands = lib.mkIf config.sdImage.expandOnBoot ''
      # On the first boot do some maintenance tasks
      if [ -f /nix-path-registration ]; then
        set -euo pipefail
        set -x
        # Figure out device names for the boot device and root filesystem.
        rootPart=$(${pkgs.util-linux}/bin/findmnt -n -o SOURCE /)
        bootDevice=$(lsblk -npo PKNAME $rootPart)
        partNum=$(lsblk -npo PARTN $rootPart)

        # Resize the root partition and the filesystem to fit the disk
        echo ",+," | sfdisk -N$partNum --no-reread $bootDevice
        ${pkgs.parted}/bin/partprobe
        ${pkgs.e2fsprogs}/bin/resize2fs $rootPart

        # Register the contents of the initial Nix store
        ${config.nix.package.out}/bin/nix-store --load-db < /nix-path-registration

        # nixos-rebuild also requires a "system" profile and an /etc/NIXOS tag.
        touch /etc/NIXOS
        ${config.nix.package.out}/bin/nix-env -p /nix/var/nix/profiles/system --set /run/current-system

        # Prevents this from running on later boots.
        rm -f /nix-path-registration
      fi
    '';
  };
}