+udev - Linux userspace device management
-udev - a userspace implementation of devfs
+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.
-For more information on the design, and structure of this project, see the
-files in the docs/ directory.
+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.
-To use:
+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
+the /dev/.udev directory is private to udev and does change whenever needed.
-- Edit the udev.h file and replace the following variables with values
- that make sense for your system:
- #define UDEV_ROOT "/udev/"
- #define MKNOD "/bin/mknod"
- The only value most people will have to change is the UDEV_ROOT
- variable, as I doubt you really want device nodes to be created in my
- home directory :)
+Requirements:
+ - 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_NET=y
+ CONFIG_UNIX=y
+ CONFIG_SYSFS=y
+ CONFIG_SYSFS_DEPRECATED*=n
+ CONFIG_PROC_FS=y
+ CONFIG_TMPFS=y
+ CONFIG_INOTIFY_USER=y
+ CONFIG_SIGNALFD=y
+ CONFIG_TMPFS_POSIX_ACL=y (user ACLs for device nodes)
+ CONFIG_BLK_DEV_BSG=y (SCSI devices)
-- Run make to build the project.
+ - Udev will not work with the CONFIG_SYSFS_DEPRECATED* option.
-- Make sure sysfs is mounted.
+ - Unix domain sockets (CONFIG_UNIX) as a loadable kernel module may work,
+ but it is not supported.
-- Point /proc/sys/kernel/hotplug at the location of the udev binary that
- is created. Then plug some block devices in, or other types of
- devices that create dev files in sysfs. An easy way to do this,
- without any hardware is to use the scsi_debug module to create virtual
- scsi devices.
+ - 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.
-- Watch as the nodes get created and removed.
+ - 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.
+ 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
+ available.
-Yes this is a really rough first cut, I know. It's mostly a proof of
-concept that this can actually work. See the TODO file for a list of
-things left to be done.
+ - To build all 'udev extras', libacl, libglib2, libusb, usbutils, pciutils,
+ gperf are needed. These dependencies can be disabled with the
+ --disable-extras configure option.
-Any comment/questions/concerns please let me know.
+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.
-greg k-h
-greg@kroah.com
+ - 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 mounted /dev directory:
+ cp -axT --remove-destination /lib/udev/devices /dev
+
+ - 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/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