update README and NEWS

1 files changed, 56 insertions, 31 deletions

diff --git a/README b/README
index 664457de0f..6e09b3a52d 100644
--- a/README
+++ b/README
@@ -1,9 +1,13 @@
-udev - userspace device management
+udev - Linux userspace device management
 
-Integrating udev in the system has complex dependencies and differs from distro
-to distro. All major distros depend on udev these days and the system may not
-work without a properly installed version. The upstream udev project does not
-recommend to replace a distro's udev installation with the upstream version.
+Integrating udev in the system has complex dependencies and may differ from
+distribution to distribution. A system may not be able to boot up or work
+reliably without a properly installed udev version. The upstream udev project
+does not recommend to replace a distro's udev installation with the upstream
+version.
+
+The upstream udev project's set of default rules may require a most recent
+kernel release to work properly.
 
 Tools and rules shipped by udev are not public API and may change at any time.
 Never call any private tool in /lib/udev from any external application, it might
@@ -27,14 +31,19 @@ Requirements:
       CONFIG_TMPFS_POSIX_ACL=y (user ACLs for device nodes)
       CONFIG_BLK_DEV_BSG=y (SCSI devices)
 
-  - For reliable operations, the kernel must not use the CONFIG_SYSFS_DEPRECATED*
-    option.
+  - Udev will not work with the CONFIG_SYSFS_DEPRECATED* option.
 
   - Unix domain sockets (CONFIG_UNIX) as a loadable kernel module may work,
     but it is not supported.
 
+  - The deprecated hotplug helper /sbin/hotplug should be disabled in the
+    kernel configuration, it is not needed today, and may render the system
+    unusable because the kernel may create too many processes in parallel
+    so that the system runs out-of-memory.
+
   - The proc filesystem must be mounted on /proc, the sysfs filesystem must
-    be mounted at /sys. No other locations are supported by udev.
+    be mounted at /sys. No other locations are supported by a standard
+    udev installation.
 
   - The system must have the following group names resolvable at udev startup:
       disk, cdrom, floppy, tape, audio, video, lp, tty, dialout, kmem.
@@ -42,35 +51,51 @@ Requirements:
     these group names with only the rootfs mounted, and while no network is
     available.
 
-  - To build all udev extras, libacl, libglib2, libusb, usbutils, pciutils,
+  - To build all 'udev extras', libacl, libglib2, libusb, usbutils, pciutils,
     gperf are needed. These dependencies can be disabled with the
-    --disable-extras option.
-
-Operation:
-  Udev creates and removes device nodes in /dev, based on events the kernel
-  sends out on device discovery or removal.
+    --disable-extras configure option.
 
-  - Early in the boot process, the /dev directory should get a 'tmpfs'
-    filesystem mounted, which is maintained by udev. Created nodes or changed
-    permissions will not survive a reboot, which is intentional.
+Setup:
+  - At bootup, the /dev directory should get the 'devtmpfs' filesystem
+    mounted. Udev will manage permissions and ownership of the kernel-created
+    device nodes, and possibly create additional symlinks. If needed, udev also
+    works on an empty 'tmpfs' filesystem, but some static device nodes like
+    /dev/null, /dev/console, /dev/kmsg are needed to be able to start udev itself.
 
-  - The content of /lib/udev/devices directory which contains the nodes,
+  - The content of /lib/udev/devices directory which contains static content like
     symlinks and directories, which are always expected to be in /dev, should
-    be copied over to the tmpfs mounted /dev, to provide the required nodes
-    to initialize udev and continue booting.
+    be copied over to the mounted /dev directory:
+      cp -axT --remove-destination /lib/udev/devices /dev
 
-  - The deprecated hotplug helper /sbin/hotplug should be disabled in the
-    kernel configuration, it is not needed today, and may render the system
-    unusable because the kernel may create too many processes in parallel
-    so that the system runs out-of-memory.
+  - The udev daemon should be started to handle device events sent by the kernel.
+    During bootup, the kernel can be asked to send events for all already existing
+    devices, to apply the configuration to these devices. This is usually done by:
+      /sbin/udevadm trigger --type=subsystems
+      /sbin/udevadm trigger --type=devices
+
+  - Restarting the daemon does never apply any rules to existing devices.
 
-  - All kernel events are matched against a set of specified rules in
-    /lib/udev/rules.d which make it possible to hook into the event
-    processing to load required kernel modules and setup devices. For all
-    devices the kernel exports a major/minor number, udev will create a
-    device node with the default kernel name, or the one specified by a
-    matching udev rule.
+  - New/changed rule files are picked up automatically, there is no daemon
+    restart or signal needed.
+
+Operation:
+  - Udev creates/removes device nodes in /dev, based on events the kernel
+    sends out on device creation/removal.
+
+  - All kernel events are matched against a set of specified rules, which
+    possibly hook into the event processing and load required kernel
+    modules to setup devices. For all devices the kernel exports a major/minor
+    number, if needed, udev will create a device node with the default kernel
+    name. If specified, udev applies permissions/ownership to the device
+    node, creates additional symlinks pointing to the node, and executes
+    programs to handle the device.
+
+  - The events udev handles, and the information udev merges into its device
+    database, can be accessed with libudev:
+      http://www.kernel.org/pub/linux/utils/kernel/hotplug/libudev/
+      http://www.kernel.org/pub/linux/utils/kernel/hotplug/gudev/
+
+For more details about udev and udev rules see the udev(7) man page.
 
 Please direct any comment/question to the linux-hotplug mailing list at:
   linux-hotplug@vger.kernel.org
-