Making modular my previous changes for armv5tel. I updated the way to use

grub. Its options are no more inside 'boot', but inside 'boot.loader.grub'.
I added a new bootloader configuration for nixos, generationsDir. It creates
/boot/default/{init,initrd,kernel,system} symlinks, and the same for the generations
in /boot/system-$gen/{init,initrd,kernel,system}.

I can program the u-boot loader to load /boot/default files always, and have
a minimal nixos boot loader installer functionality. Additionally, I can refer
to the other system generations easily, with a simple 'ls' in /boot.

svn path=/nixos/trunk/; revision=17460

modules/config/system-path.nix

@@ -60,7 +60,7 @@ let
       pkgs.usbutils
       pkgs.utillinux
       pkgs.wirelesstools
-    ] ++ optional (pkgs.stdenv.system != "armv5tel-linux") [ pkgs.grub ]
+    ]
     ++ config.environment.extraPackages;
 
 

modules/installer/generations-dir/generations-dir-builder.sh

@@ -0,0 +1,105 @@
+#! @bash@/bin/sh -e
+
+shopt -s nullglob
+
+export PATH=/empty
+for i in @path@; do PATH=$PATH:$i/bin; done
+
+default=$1
+if test -z "$1"; then
+    echo "Syntax: generations-dir-builder.sh <DEFAULT-CONFIG>"
+    exit 1
+fi
+
+echo "updating the boot generations directory..."
+
+mkdir -p /boot
+
+rm -Rf /boot/system* || true
+
+target=/boot/grub/menu.lst
+tmp=$target.tmp
+
+# 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 /boot/kernels.
+declare -A filesCopied
+
+copyToKernelsDir() {
+    local src="$1"
+    local dst="/boot/kernels/$(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 $src $dstTmp
+        mv $dstTmp $dst
+    fi
+    filesCopied[$dst]=1
+    result=$dst
+}
+
+
+# Add an entry for a configuration to the Grub menu, and if
+# appropriate, copy its kernel and initrd to /boot/kernels.
+addEntry() {
+    local path="$1"
+    local generation="$2"
+    local outdir=/boot/system-$generation
+
+    if ! test -e $path/kernel -a -e $path/initrd; then
+        return
+    fi
+
+    local kernel=$(readlink -f $path/kernel)
+    local initrd=$(readlink -f $path/initrd)
+
+    if test -n "@copyKernels@"; then
+        copyToKernelsDir $kernel; kernel=$result
+        copyToKernelsDir $initrd; initrd=$result
+    fi
+    
+    mkdir -p $outdir
+    ln -sf $(readlink -f $path) $outdir/system
+    ln -sf $(readlink -f $path/init) $outdir/init
+    ln -sf $(readlink -f $path/initrd) $outdir/initrd
+    ln -sf $(readlink -f $path/kernel) $outdir/kernel
+
+    if test $(readlink -f "$path") = "$default"; then
+      cp "$kernel" /boot/nixos-kernel
+      cp "$initrd" /boot/nixos-initrd
+      cp "$(readlink -f "$path/init")" /boot/nixos-init
+      mkdir -p /boot/default
+      ln -sf $(readlink -f $path) /boot/default/system
+      ln -sf $(readlink -f $path/init) /boot/default/init
+      ln -sf $(readlink -f $path/initrd) /boot/default/initrd
+      ln -sf $(readlink -f $path/kernel) /boot/default/kernel
+    fi
+}
+
+if test -n "@copyKernels@"; then
+    mkdir -p /boot/kernels
+fi
+
+# Add all 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); do
+    link=/nix/var/nix/profiles/system-$generation-link
+    addEntry $link $generation
+done
+
+# Remove obsolete files from /boot/kernels.
+for fn in /boot/kernels/*; do
+    if ! test "${filesCopied[$fn]}" = 1; then
+        rm -vf -- "$fn"
+    fi
+done

modules/installer/generations-dir/generations-dir.nix

@@ -0,0 +1,60 @@
+{pkgs, config, ...}:
+
+###### interface
+let
+  inherit (pkgs.lib) mkOption mkIf;
+
+  options = {
+    boot = {
+      loader = {
+        generationsDir = {
+
+          enable = mkOption {
+            default = false;
+            description = ''
+              Whether to enable the simple preparation of symlinks to the system
+              generations in /boot.
+            '';
+          };
+
+          copyKernels = mkOption {
+            default = false;
+            description = "
+              Whether copy the necessary boot files into /boot, so
+              /nix/store is not needed by the boot loadear.
+            ";
+          };
+        };
+      };
+    };
+  };
+
+in
+
+###### implementation
+let
+  generationsDirBuilder = pkgs.substituteAll {
+    src = ./generations-dir-builder.sh;
+    isExecutable = true;
+    inherit (pkgs) bash;
+    path = [pkgs.coreutils pkgs.gnused pkgs.gnugrep];
+    inherit (config.boot.loader.generationsDir) copyKernels;
+  };
+
+in
+{
+  require = [
+    options
+
+    # config.system.build
+    # ../system/system-options.nix
+  ];
+
+  system = mkIf config.boot.loader.generationsDir.enable {
+    build = {
+      menuBuilder = generationsDirBuilder;
+    };
+    boot.loader.id = "generationsDir";
+    boot.loader.kernelFile = "uImage";
+  };
+}

modules/installer/grub/grub.nix

@@ -2,10 +2,19 @@
 
 ###### interface
 let
-  inherit (pkgs.lib) mkOption;
+  inherit (pkgs.lib) mkOption mkIf;
 
   options = {
     boot = {
+      loader = {
+        grub = {
+
+          enable = mkOption {
+            default = true;
+            description = ''
+              Whether to enable the GRUB boot loader.
+            '';
+          };
 
           grubDevice = mkOption {
             default = "";
@@ -85,7 +94,8 @@ let
               different file system than /boot.
             ";
           };
-
+        };
+      };
     };
   };
 in
@@ -99,7 +109,7 @@ let
     isExecutable = true;
     inherit (pkgs) bash;
     path = [pkgs.coreutils pkgs.gnused pkgs.gnugrep];
-    inherit (config.boot) copyKernels extraGrubEntries extraGrubEntriesBeforeNixos
+    inherit (config.boot.loader.grub) copyKernels extraGrubEntries extraGrubEntriesBeforeNixos
       grubSplashImage bootMount configurationLimit;
   };
 in
@@ -112,11 +122,18 @@ in
     # ../system/system-options.nix
   ];
 
-  system = {
+  system = mkIf config.boot.loader.grub.enable {
     build = {
-      inherit grubMenuBuilder;
+      menuBuilder = grubMenuBuilder;
     };
+
+    # Common attribute for boot loaders so only one of them can be
+    # set at once
+    boot.loader.id = "grub";
+    boot.loader.kernelFile = "vmlinuz";
   };
 
+  environment.extraPackages = mkIf config.boot.loader.grub.enable [ pkgs.grub ];
+
   # and many other things that have to be moved inside this file.
 }

modules/module-list.nix

@@ -13,6 +13,7 @@
   ./hardware/network/rt73.nix 
   ./hardware/pcmcia.nix
   ./installer/grub/grub.nix
+  ./installer/generations-dir/generations-dir.nix
   ./installer/tools/nixos-checkout.nix
   ./installer/tools/tools.nix
   ./misc/assertions.nix

modules/system/activation/switch-to-configuration.sh

@@ -22,15 +22,21 @@ EOF
 fi
 
 if test "$action" = "switch" -o "$action" = "boot"; then
+    if [ "@bootLoader@" == "grub" ]; then
       if test -n "@grubDevice@"; then
           mkdir -m 0700 -p /boot/grub
-        @grubMenuBuilder@ @out@
+          @menuBuilder@ @out@
           if test "$NIXOS_INSTALL_GRUB" = 1; then
               @grub@/sbin/grub-install "@grubDevice@" --no-floppy --recheck
           fi
       else
           echo "Warning: don't know how to make this configuration bootable; please set \`boot.grubDevice'." 1>&2
       fi
+    elif [ "@bootLoader@" == "generationsDir" ]; then
+          @menuBuilder@ @out@
+    else
+          echo "Warning: don't know how to make this configuration bootable; please enable a boot loader." 1>&2
+    fi
 fi
 
 if test "$action" = "switch" -o "$action" = "test"; then

modules/system/activation/top-level.nix

@@ -18,6 +18,20 @@ let
       '';
     };
 
+    system.boot.loader.id = pkgs.lib.mkOption {
+      default = "";
+      description = ''
+        Id string of the used bootloader.
+      '';
+    };
+
+    system.boot.loader.kernelFile = pkgs.lib.mkOption {
+      default = "";
+      description = ''
+        Name of the kernel file to be passed to the bootloader.
+      '';
+    };
+    
   };
 
     
@@ -33,13 +47,17 @@ let
 
 
   systemBuilder = let
-      kernelfile = if (pkgs.stdenv.system == "armv5tel-linux")
-        then "${config.boot.kernelPackages.kernel}/uImage"
-        else "${config.boot.kernelPackages.kernel}/vmlinuz";
-    in ''
+      kernelPath = "${config.boot.kernelPackages.kernel}/" +
+        "${config.system.boot.loader.kernelFile}";
+    in 
+      ''
       ensureDir $out
 
-      ln -s ${kernelfile} $out/kernel
+      if [ ! -f ${kernelPath} ]; then
+        echo "The bootloader cannot find the proper kernel image."
+        echo "(Expecting ${kernelPath})"
+      fi
+      ln -s ${kernelPath} $out/kernel
       if [ -n "$grub" ]; then 
         ln -s $grub $out/grub
       fi
@@ -76,13 +94,9 @@ let
     name = "system";
     buildCommand = systemBuilder;
     inherit children;
-    grub = if (pkgs.stdenv.system != "armv5tel-linux") then pkgs.grub
-      else null;
-    grubDevice = config.boot.grubDevice;
     kernelParams =
       config.boot.kernelParams ++ config.boot.extraKernelParams;
-    grubMenuBuilder = config.system.build.grubMenuBuilder;
-    configurationName = config.boot.configurationName;
+    menuBuilder = config.system.build.menuBuilder;
     # Most of these are needed by grub-install.
     path = [
       pkgs.coreutils
@@ -92,6 +106,12 @@ let
       pkgs.diffutils
       pkgs.upstart # for initctl
     ];
+
+    # Boot loaders
+    bootLoader = config.system.boot.loader.id;
+    grub = if config.boot.loader.grub.enable then pkgs.grub else null;
+    grubDevice = config.boot.loader.grub.grubDevice;
+    configurationName = config.boot.loader.grub.configurationName;
   };