-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. This is currently version 2.6.31.
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.
+by udevadm and libudev. Tools and rules in /lib/udev, and the entire content of
+the /dev/.udev directory is private to udev and does change whenever needed.
Requirements:
- - Version 2.6.25 of the Linux kernel with sysfs, procfs, signalfd, inotify,
+ - Version 2.6.27 of the Linux kernel with sysfs, procfs, signalfd, inotify,
unix domain sockets, networking and hotplug enabled:
CONFIG_HOTPLUG=y
CONFIG_UEVENT_HELPER_PATH=""
CONFIG_SYSFS_DEPRECATED*=n
CONFIG_PROC_FS=y
CONFIG_TMPFS=y
- CONFIG_INOTIFY=y
+ CONFIG_INOTIFY_USER=y
CONFIG_SIGNALFD=y
CONFIG_TMPFS_POSIX_ACL=y (user ACLs for device nodes)
- CONFIG_BLK_DEV_BSG=y (SCSI tape devices)
+ CONFIG_BLK_DEV_BSG=y (SCSI devices)
+
+ - Udev will not work with the CONFIG_SYSFS_DEPRECATED* option.
- - For reliable operation, the kernel must not use the CONFIG_SYSFS_DEPRECATED*
- option.
+ - Unix domain sockets (CONFIG_UNIX) as a loadable kernel module may work,
+ but it is not supported.
- - Unix domain sockets (CONFIG_UNIX) as a loadable kernel module 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.
+ - The proc filesystem must be mounted on /proc, the sysfs filesystem must
+ 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.
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.
+ --disable-extras configure option.
+
+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 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.
+
+ - New/changed rule files are picked up automatically, there is no daemon
+ restart or signal needed.
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
+ - 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
~ianmdlvl / elogind.git / blobdiff