system: Add '%devtmpfs-file-system' for udev, and '%base-file-systems'.

Suggested by Adam Pribyl <pribyl@lowlevel.cz>.

* gnu/services/base.scm (udev-service)[requirement]: Add
  'file-system-/dev'.
* gnu/system/file-systems.scm (%devtmpfs-file-system,
  %base-file-systems): New variables.
* gnu/system/install.scm (installation-services)[file-systems]: Use
  %base-file-systems.
* build-aux/hydra/demo-os.scm (file-systems): Likewise.
* doc/guix.texi (System Installation): Show %BASE-FILE-SYSTEMS in the
  example.
  (Using the Configuration System): Likewise.
  (File Systems): Document %base-file-systems, %devtmpfs-file-system,
  %binary-format-file-system, and %fuse-control-file-system.

3 files changed, 27 insertions, 5 deletions

diff --git a/gnu/services/base.scm b/gnu/services/base.scm
index ae12c8e93d..42e232c9ac 100644
--- a/gnu/services/base.scm
+++ b/gnu/services/base.scm
@@ -473,8 +473,13 @@ passed to @command{guix-daemon}."
   (with-monad %store-monad
     (return (service
              (provision '(udev))
-             (requirement '(root-file-system))
-             (documentation "Populate the /dev directory.")
+
+             ;; Udev needs /dev to be a 'devtmpfs' mount so that new device
+             ;; nodes can be added: see
+             ;; <http://www.linuxfromscratch.org/lfs/view/development/chapter07/udev.html>.
+             (requirement '(root-file-system file-system-/dev))
+
+             (documentation "Populate the /dev directory, dynamically.")
              (start #~(lambda ()
                         (define udevd
                           (string-append #$udev "/libexec/udev/udevd"))
diff --git a/gnu/system/file-systems.scm b/gnu/system/file-systems.scm
index 7852a6ab26..0c2021d7b4 100644
--- a/gnu/system/file-systems.scm
+++ b/gnu/system/file-systems.scm
@@ -30,7 +30,10 @@
             file-system-options
 
             %fuse-control-file-system
-            %binary-format-file-system))
+            %binary-format-file-system
+            %devtmpfs-file-system
+
+            %base-file-systems))
 
 ;;; Commentary:
 ;;;
@@ -72,4 +75,17 @@
     (type "binfmt_misc")
     (check? #f)))
 
+(define %devtmpfs-file-system
+  ;; /dev as a 'devtmpfs' file system, needed for udev.
+  (file-system
+    (device "none")
+    (mount-point "/dev")
+    (type "devtmpfs")
+    (check? #f)))
+
+(define %base-file-systems
+  ;; List of basic file systems to be mounted.  Note that /proc and /sys are
+  ;; currently mounted by the initrd.
+  (list %devtmpfs-file-system))
+
 ;;; file-systems.scm ends here
diff --git a/gnu/system/install.scm b/gnu/system/install.scm
index 18fd587ead..d4a32609ba 100644
--- a/gnu/system/install.scm
+++ b/gnu/system/install.scm
@@ -117,10 +117,11 @@ Use Alt-F2 for documentation.
     (file-systems
      ;; Note: the disk image build code overrides this root file system with
      ;; the appropriate one.
-     (list (file-system
+     (cons (file-system
              (mount-point "/")
              (device "gnu-disk-image")
-             (type "ext4"))))
+             (type "ext4"))
+           %base-file-systems))
 
     (users (list (user-account
                   (name "guest")