-udev - userspace device management
-
-For more information see the files in the docs/ directory.
-
-Important Note:
- Integrating udev in the system is a whole lot of work, has complex dependencies
- and differs a lot from distro to distro. All reasonable distros use udev these
- days, the major ones make it mandatory and the system will not work without it.
-
- The upstream udev project does not support or recomend to replace a distro's udev
- installation with the upstream version. The installation of a unmodified upstream
- version may render your system unusable! There is no "default" setup or a set
- of "default" rules provided by the upstream udev version.
-
-udev requires:
- - 2.6 version of the Linux kernel
-
- - the kernel must have sysfs, netlink, and hotplug enabled
-
- - proc must be mounted on /proc
-
- - sysfs must be mounted at /sys, no other location is supported
-
- - udev creates and removes device nodes in /dev based on events
- the kernel sends out on device discovery or removal
-
- - during bootup /dev usually gets a tmpfs mounted which is populated scratch
- by udev (created nodes don't survive a reboot, it always starts from scratch)
-
- - udev replaces the hotplug event management invoked from /sbin/hotplug
- by the udevd daemon, which receives the kernel events over netlink
-
- - all kernel events are matched against a set of specified rules which
- make it posible to hook into the event processing
-
- - there is a copy of the rules files for all major distros in the etc/udev
- directory (you may look there how others distros are doing it)
-
-Setting which are used for building udev:
- prefix
- set this to the default root that you want to use
- Only override this if you really know what you are doing
- DESTDIR
- prefix for install target for package building
- USE_LOG
- if set to 'true', udev will emit messages to the syslog when
- it creates or removes device nodes. This is helpful to see
- what udev is doing. This is enabled by default.
- DEBUG
- if set to 'true', verbose debugging messages will be compiled into
- the udev binaries. Default value is 'false'.
- USE_SELINUX
- if set to 'true', udev will be built with SELinux support
- enabled. This is disabled by default.
- USE_KLIBC
- if set to 'true', udev is built and linked against klibc.
- Default value is 'false'. KLCC specifies the klibc compiler
- wrapper, usually in /usr/bin/klcc
- EXTRAS
- if set, will build the "extra" helper programs as specified
- as listed (see below for an example.)
-
-if you want to build the udev helper program cdrom_id and scsi_id:
- make EXTRAS="extras/cdrom_id extras/scsi_id"
-
-Please direct any comment/question/concern to the linux-hotplug-devel mailing list at:
- linux-hotplug-devel@lists.sourceforge.net
-
+udev - Linux userspace device management
+
+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. This is currently version 2.6.32.
+
+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 contents of
+the /run/udev directory are private to udev and do change whenever needed.
+
+Requirements:
+ - 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 needed:
+ CONFIG_HOTPLUG=y
+ CONFIG_UEVENT_HELPER_PATH=""
+ CONFIG_NET=y
+ CONFIG_UNIX=y
+ CONFIG_SYSFS=y
+ CONFIG_SYSFS_DEPRECATED*=n
+ CONFIG_PROC_FS=y
+ CONFIG_INOTIFY_USER=y
+ CONFIG_SIGNALFD=y
+
+ - These options might be needed:
+ CONFIG_TMPFS=y
+ CONFIG_TMPFS_POSIX_ACL=y (user ACLs for device nodes)
+ CONFIG_BLK_DEV_BSG=y (SCSI devices)
+
+ - Udev does 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, 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.
+
+ - Some udev extras have external dependencies like:
+ libacl, libglib2, usbutils, pciutils, and gperf.
+ All these extras can be disabled with configure options.
+
+Setup:
+ - At bootup, the /dev directory should get the 'devtmpfs' filesystem
+ mounted. Udev manages the permissions and ownership of the kernel-created
+ device nodes, and udev possibly creates additional symlinks. If needed, udev also
+ works on an empty 'tmpfs' filesystem, but some device nodes like
+ /dev/null, /dev/console, /dev/kmsg should be created before udevd is started.
+
+ - 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. This is usually done by:
+ /sbin/udevadm trigger --action=add --type=subsystems
+ /sbin/udevadm trigger --action=add --type=devices
+
+ - Restarting the daemon never applies any rules to existing devices.
+
+ - New/changed rule files are picked up automatically; there is no daemon
+ restart or signal needed.
+
+Operation:
+ - 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
~ianmdlvl / elogind.git / blobdiff