nixpkgs/nixos/modules/installer/cd-dvd/sd-image.nix

# 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

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

with lib;

let
  rootfsImage = pkgs.callPackage ../../../lib/make-ext4-fs.nix ({
    inherit (config.sdImage) storePaths;
    populateImageCommands = config.sdImage.populateRootCommands;
    volumeLabel = "NIXOS_SD";
  } // optionalAttrs (config.sdImage.rootPartitionUUID != null) {
    uuid = config.sdImage.rootPartitionUUID;
  });
in
{
  imports = [
    (mkRemovedOptionModule [ "sdImage" "bootPartitionID" ] "The FAT partition for SD image now only holds the Raspberry Pi firmware files. Use firmwarePartitionID to configure that partition's ID.")
    (mkRemovedOptionModule [ "sdImage" "bootSize" ] "The boot files for SD image have been moved to the main ext4 partition. The FAT partition now only holds the Raspberry Pi firmware files. Changing its size may not be required.")
  ];

  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 = literalExample "[ pkgs.stdenv ]";
      description = ''
        Derivations to be included in the Nix store in the generated SD image.
      '';
    };

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

    firmwareSize = mkOption {
      type = types.int;
      # As of 2019-05-31 the Raspberry pi firmware + u-bot takes ~13MiB
      default = 20;
      description = ''
        Size of the /boot/firmware partition, in megabytes.
      '';
    };

    populateFirmwareCommands = mkOption {
      example = literalExample "'' 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 = literalExample "''\${extlinux-conf-builder} -t 3 -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.
      '';
    };
  };

  config = {
    fileSystems = {
      "/boot/firmware" = {
        device = "/dev/disk/by-label/FIRMWARE";
        fsType = "vfat";
        # Alternatively, this could be removed from the configuration.
        # The filesystem is not needed at runtime, it could be treated
        # as an opaque blob instead of a discrete FAT32 filesystem.
        options = [ "nofail" "noauto" ];
      };
      "/" = {
        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, utillinux }: stdenv.mkDerivation {
      name = config.sdImage.imageName;

      nativeBuildInputs = [ dosfstools e2fsprogs mtools libfaketime utillinux ];

      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
        echo "file sd-image $img" >> $out/nix-support/hydra-build-products

        # Gap in front of the first partition, in MiB
        gap=8

        # Create the image file sized to fit /boot/firmware and /, plus slack for the gap.
        rootSizeBlocks=$(du -B 512 --apparent-size ${rootfsImage} | 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 $img -o START,SECTORS --nr 2 --pairs)
        dd conv=notrunc if=${rootfsImage} 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 FIRMWARE 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
      '';
    }) {};

    boot.postBootCommands = ''
      # On the first boot do some maintenance tasks
      if [ -f /nix-path-registration ]; then
        # Figure out device names for the boot device and root filesystem.
        rootPart=$(readlink -f /dev/disk/by-label/NIXOS_SD)
        bootDevice=$(lsblk -npo PKNAME $rootPart)

        # Resize the root partition and the filesystem to fit the disk
        echo ",+," | sfdisk -N2 --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
    '';
  };
}