mirror of
https://github.com/systemd/systemd.git
synced 2024-11-02 19:21:53 +03:00
102 lines
4.4 KiB
Plaintext
102 lines
4.4 KiB
Plaintext
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.
|
|
|
|
Tools and rules shipped by udev are not public API and may change at any time.
|
|
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:
|
|
- 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_INOTIFY_USER=y
|
|
CONFIG_NET=y
|
|
CONFIG_PROC_FS=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)
|
|
|
|
- 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.
|
|
|
|
- Udev requires /run to be writable, which is usually done by mounting a
|
|
'tmpfs' filesystem.
|
|
|
|
- This version of udev does not work properly with the CONFIG_SYSFS_DEPRECATED*
|
|
option enabled.
|
|
|
|
- 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:
|
|
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:
|
|
- 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
|