generic-extlinux-compatible: Add new bootloader for ARM

This module generates a /boot/extlinux/extlinux.conf bootloader
configuration file that is supported by e.g. U-Boot:

http://git.denx.de/?p=u-boot.git;a=blob;f=doc/README.distro;hb=refs/heads/master

With this, all ARM boards supported by U-Boot can be booted in a common
way (a single boot file generator, all boards booting via initrd like
x86) and with same boot menu functionality as GRUB has.

-- sample extlinux.conf file --
# Generated file, all changes will be lost on nixos-rebuild!

# Change this to e.g. nixos-42 to temporarily boot to an older configuration.
DEFAULT nixos-default

TIMEOUT 50

LABEL nixos-default
  MENU LABEL NixOS - Default
  LINUX ../nixos/n7vxfk60nb5h0mcbhkwwxhcz2q2nvxzv-linux-4.1.0-rc3-cpufreq-zImage
  INITRD ../nixos/0ss2zs8sb6d1qn4gblxpwlxkfjsgs5f0-initrd-initrd
  FDTDIR ../nixos/n7vxfk60nb5h0mcbhkwwxhcz2q2nvxzv-linux-4.1.0-rc3-cpufreq-dtbs
  APPEND systemConfig=/nix/store/469qvr43ln8bfsnk5lzcz6m6jfcgdd4r-nixos-15.06.git.0b7a7a6M init=/nix/store/469qvr43ln8bfsnk5lzcz6m6jfcgdd4r-nixos-15.06.git.0b7a7a6M/init loglevel=8 console=ttyS0,115200n8 drm.debug=0xf

LABEL nixos-71
  MENU LABEL NixOS - Configuration 71 (2015-05-17 21:32 - 15.06.git.0b7a7a6M)
  LINUX ../nixos/n7vxfk60nb5h0mcbhkwwxhcz2q2nvxzv-linux-4.1.0-rc3-cpufreq-zImage
  INITRD ../nixos/0ss2zs8sb6d1qn4gblxpwlxkfjsgs5f0-initrd-initrd
  FDTDIR ../nixos/n7vxfk60nb5h0mcbhkwwxhcz2q2nvxzv-linux-4.1.0-rc3-cpufreq-dtbs
  APPEND systemConfig=/nix/store/469qvr43ln8bfsnk5lzcz6m6jfcgdd4r-nixos-15.06.git.0b7a7a6M init=/nix/store/469qvr43ln8bfsnk5lzcz6m6jfcgdd4r-nixos-15.06.git.0b7a7a6M/init loglevel=8 console=ttyS0,115200n8 drm.debug=0xf

nixos/modules/module-list.nix

@@ -411,6 +411,7 @@
   ./system/boot/loader/efi.nix
   ./system/boot/loader/loader.nix
   ./system/boot/loader/generations-dir/generations-dir.nix
+  ./system/boot/loader/generic-extlinux-compatible
   ./system/boot/loader/grub/grub.nix
   ./system/boot/loader/grub/ipxe.nix
   ./system/boot/loader/grub/memtest.nix

nixos/modules/system/boot/loader/generic-extlinux-compatible/default.nix

@@ -0,0 +1,44 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  blCfg = config.boot.loader;
+  cfg = blCfg.generic-extlinux-compatible;
+
+  timeoutStr = if blCfg.timeout == null then "-1" else toString blCfg.timeout;
+
+  builder = import ./extlinux-conf-builder.nix { inherit pkgs; };
+in
+{
+  options = {
+    boot.loader.generic-extlinux-compatible = {
+      enable = mkOption {
+        default = false;
+        type = types.bool;
+        description = ''
+          Whether to generate an extlinux-compatible configuration file
+          under <literal>/boot/extlinux.conf</literal>.  For instance,
+          U-Boot's generic distro boot support uses this file format.
+
+          See <link xlink:href="http://git.denx.de/?p=u-boot.git;a=blob;f=doc/README.distro;hb=refs/heads/master">U-boot's documentation</link>
+          for more information.
+        '';
+      };
+
+      configurationLimit = mkOption {
+        default = 20;
+        example = 10;
+        type = types.int;
+        description = ''
+          Maximum number of configurations in the boot menu.
+        '';
+      };
+    };
+  };
+
+  config = mkIf cfg.enable {
+    system.build.installBootLoader = "${builder} -g ${toString cfg.configurationLimit} -t ${timeoutStr} -c";
+    system.boot.loader.id = "generic-extlinux-compatible";
+  };
+}

nixos/modules/system/boot/loader/generic-extlinux-compatible/extlinux-conf-builder.nix

@@ -0,0 +1,8 @@
+{ pkgs }:
+
+pkgs.substituteAll {
+  src = ./extlinux-conf-builder.sh;
+  isExecutable = true;
+  inherit (pkgs) bash;
+  path = [pkgs.coreutils pkgs.gnused pkgs.gnugrep];
+}

nixos/modules/system/boot/loader/generic-extlinux-compatible/extlinux-conf-builder.sh

@@ -0,0 +1,132 @@
+#! @bash@/bin/sh -e
+
+shopt -s nullglob
+
+export PATH=/empty
+for i in @path@; do PATH=$PATH:$i/bin; done
+
+usage() {
+    echo "usage: $0 -t <timeout> -c <path-to-default-configuration> [-d <boot-dir>] [-g <num-generations>]" >&2
+    exit 1
+}
+
+timeout=                # Timeout in centiseconds
+default=                # Default configuration
+target=/boot            # Target directory
+numGenerations=0        # Number of other generations to include in the menu
+
+while getopts "t:c:d:g:" opt; do
+    case "$opt" in
+        t) # U-Boot interprets '0' as infinite and negative as instant boot
+            if [ "$OPTARG" -lt 0 ]; then
+                timeout=0
+            elif [ "$OPTARG" = 0 ]; then
+                timeout=-10
+            else
+                timeout=$((OPTARG * 10))
+            fi
+            ;;
+        c) default="$OPTARG" ;;
+        d) target="$OPTARG" ;;
+        g) numGenerations="$OPTARG" ;;
+        \?) usage ;;
+    esac
+done
+
+[ "$timeout" = "" -o "$default" = "" ] && usage
+
+mkdir -p $target/nixos
+mkdir -p $target/extlinux
+
+# Convert a path to a file in the Nix store such as
+# /nix/store/<hash>-<name>/file to <hash>-<name>-<file>.
+cleanName() {
+    local path="$1"
+    echo "$path" | sed 's|^/nix/store/||' | sed 's|/|-|g'
+}
+
+# Copy a file from the Nix store to $target/nixos.
+declare -A filesCopied
+
+copyToKernelsDir() {
+    local src=$(readlink -f "$1")
+    local dst="$target/nixos/$(cleanName $src)"
+    # Don't copy the file if $dst already exists.  This means that we
+    # have to create $dst atomically to prevent partially copied
+    # kernels or initrd if this script is ever interrupted.
+    if ! test -e $dst; then
+        local dstTmp=$dst.tmp.$$
+        cp -r $src $dstTmp
+        mv $dstTmp $dst
+    fi
+    filesCopied[$dst]=1
+    result=$dst
+}
+
+# Copy its kernel, initrd and dtbs to $target/nixos, and echo out an
+# extlinux menu entry
+addEntry() {
+    local path=$(readlink -f "$1")
+    local tag="$2" # Generation number or 'default'
+
+    if ! test -e $path/kernel -a -e $path/initrd; then
+        return
+    fi
+
+    copyToKernelsDir "$path/kernel"; kernel=$result
+    copyToKernelsDir "$path/initrd"; initrd=$result
+    # XXX UGLY: maybe the system config should have a top-level "dtbs" entry?
+    copyToKernelsDir $(readlink -m "$path/kernel/../dtbs"); dtbs=$result
+
+    timestampEpoch=$(stat -L -c '%Z' $path)
+
+    timestamp=$(date "+%Y-%m-%d %H:%M" -d @$timestampEpoch)
+    nixosVersion="$(cat $path/nixos-version)"
+    extraParams="$(cat $path/kernel-params)"
+
+    echo
+    echo "LABEL nixos-$tag"
+    if [ "$tag" = "default" ]; then
+        echo "  MENU LABEL NixOS - Default"
+    else
+        echo "  MENU LABEL NixOS - Configuration $tag ($timestamp - $nixosVersion)"
+    fi
+    echo "  LINUX ../nixos/$(basename $kernel)"
+    echo "  INITRD ../nixos/$(basename $initrd)"
+    echo "  FDTDIR ../nixos/$(basename $dtbs)"
+    echo "  APPEND systemConfig=$path init=$path/init $extraParams"
+}
+
+tmpFile="$target/extlinux/extlinux.conf.tmp.$$"
+
+cat > $tmpFile <<EOF
+# Generated file, all changes will be lost on nixos-rebuild!
+
+# Change this to e.g. nixos-42 to temporarily boot to an older configuration.
+DEFAULT nixos-default
+
+TIMEOUT $timeout
+$(addEntry $default default)
+EOF
+
+# Add up to $numGenerations generations of the system profile to the menu,
+# in reverse (most recent to least recent) order.
+for generation in $(
+        (cd /nix/var/nix/profiles && ls -d system-*-link) \
+        | sed 's/system-\([0-9]\+\)-link/\1/' \
+        | sort -n -r \
+        | head -n $numGenerations); do
+    link=/nix/var/nix/profiles/system-$generation-link
+    addEntry $link $generation
+done >> $tmpFile
+
+mv -f $tmpFile $target/extlinux/extlinux.conf
+
+# Remove obsolete files from $target/nixos.
+for fn in $target/nixos/*; do
+    if ! test "${filesCopied[$fn]}" = 1; then
+        echo "Removing no longer needed boot file: $fn"
+        chmod +w -- "$fn"
+        rm -rf -- "$fn"
+    fi
+done