release 176

[elogind.git] / README

diff --git a/README b/README
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@@ -1,74 +1,101 @@
-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 replacing 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.
 
 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
-just go away in the next release. Access to udev information is only offered
-by udevadm and libudev. Tools and rules in /lib/udev and the entire content of
-/dev/.udev/ is private to udev.
+Never call any private tool in /usr/lib/udev from any external application; it
+might just go away in the next release. Access to udev information is only offered
+by udevadm and libudev. Tools and rules in /usr/lib/udev and the entire contents
+of the /run/udev directory are private to udev and do change whenever needed.

 
 Requirements:
 
 Requirements:

-  - Version 2.6.25 of the Linux kernel with sysfs, procfs, signalfd, inotify,
-    unix domain sockets, networking and hotplug enabled:
+  - Version 2.6.34 of the Linux kernel with sysfs, procfs, signalfd, inotify,
+    unix domain sockets, networking and hotplug enabled
+
+  - Some architectures might need a later kernel, that supports accept4(),
+    or need to backport the accept4() syscall wiring in the kernel.
+
+  - These options are required:
+      CONFIG_DEVTMPFS=y

       CONFIG_HOTPLUG=y
       CONFIG_HOTPLUG=y

-      CONFIG_UEVENT_HELPER_PATH=""
+      CONFIG_INOTIFY_USER=y

       CONFIG_NET=y
       CONFIG_NET=y

-      CONFIG_UNIX=y
-      CONFIG_SYSFS=y
-      CONFIG_SYSFS_DEPRECATED*=n

       CONFIG_PROC_FS=y
       CONFIG_PROC_FS=y

-      CONFIG_TMPFS=y
-      CONFIG_TMPFS_POSIX_ACL=y
-      CONFIG_INOTIFY=y

       CONFIG_SIGNALFD=y
       CONFIG_SIGNALFD=y

+      CONFIG_SYSFS=y
+      CONFIG_SYSFS_DEPRECATED*=n
+      CONFIG_UEVENT_HELPER_PATH=""
+
+  - These options might be needed:
+      CONFIG_BLK_DEV_BSG=y (SCSI devices)
+      CONFIG_TMPFS_POSIX_ACL=y (user ACLs for device nodes)

 
 

-  - For reliable operation, the kernel must not use the CONFIG_SYSFS_DEPRECATED*
-    option.
+  - The /dev directory needs the 'devtmpfs' filesystem mounted.
+    Udev only manages the permissions and ownership of the
+    kernel-provided device nodes, and possibly creates additional symlinks.

 
 

-  - Unix domain sockets (CONFIG_UNIX) as a loadable kernel module is not
-    supported.
+  - Udev requires /run to be writable, which is usually done by mounting a
+    'tmpfs' filesystem.

 
 

-  - The proc filesystem must be mounted on /proc/, the sysfs filesystem must
-    be mounted at /sys/. No other locations are supported by udev.
+  - This version of udev does not work properly with the CONFIG_SYSFS_DEPRECATED*
+    option enabled.

 
 

-  - The system must have the following group names resolvable at udev startup:
-      disk, cdrom, floppy, tape, audio, video, lp, tty, dialout, kmem.
-    Especially in LDAP setups, it is required, that getgrnam() is able to resolve
-    these group names with only the rootfs mounted, and while no network is
+  - 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, and the sysfs filesystem must
+    be mounted at /sys. No other locations are supported by a standard
+    udev installation.
+
+  - The default rule sset requires the following group names resolvable at udev startup:
+      disk, cdrom, floppy, tape, audio, video, lp, tty, dialout, and kmem.
+    Especially in LDAP setups, it is required that getgrnam() be able to resolve
+    these group names with only the rootfs mounted and while no network is

     available.
 
     available.
 

-  - To build all udev extras, libacl, libglib2, libusb, usbutils, pciutils,
-    gperf are needed. These dependencies can be disabled with the
-    --disable-extras option.
+  - Some udev extras have external dependencies like:
+      libglib2, usbutils, pciutils, and gperf.
+    All these extras can be disabled with configure options.
+
+Setup:
+  - The udev daemon should be started to handle device events sent by the kernel.
+    During bootup, the events for already existing devices can be replayed, so
+    that they are configured by udev. The systemd service files contain the
+    needed commands to start the udev daemon and the coldplug sequence.
+
+  - Restarting the daemon never applies any rules to existing devices.
+
+  - New/changed rule files are picked up automatically; there is usually no
+    daemon restart or signal needed.

 
 Operation:
 
 Operation:

-  Udev creates and removes device nodes in /dev/, based on events the kernel
-  sends out on device discovery or removal.
-
-  - 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.
-
-  - The content of /lib/udev/devices/ directory which contains the nodes,
-    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.
-
-  - The old hotplug helper /sbin/hotplug should be disabled in the kernel
-    configuration, it is not needed, and may render the system unusable
-    because of a fork-bombing behavior.
-
-  - 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.
-
-Please direct any comment/question/concern to the linux-hotplug mailing list at:
-  linux-hotplug@vger.kernel.org
+  - Based on events the kernel sends out on device creation/removal, udev
+    creates/removes device nodes and symlinks in the /dev directory.
+
+  - All kernel events are matched against a set of specified rules, which
+    possibly hook into the event processing and load required kernel
+    modules to set up devices. For all devices, the kernel exports a major/minor
+    number; if needed, udev creates a device node with the default kernel
+    device 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 man pages:
+      http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev/
+
+Please direct any comment/question to the linux-hotplug mailing list at:
+  linux-hotplug@vger.kernel.org