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udev - userspace device management
For more information see the files in the docs/ directory.
Important Note:
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.
Requirements:
- Version 2.6.15 of the Linux kernel for reliable operation of this release of
udev. The kernel may have a requirement on udev too, see Documentation/Changes
in the kernel source tree for the actual dependency.
- The kernel must have sysfs, unix domain sockets and networking enabled.
(unix domain sockets (CONFIG_UNIX) as a loadable kernel module may work,
but it is completely silly - don't complain if anything goes wrong.)
- The proc filesystem must be mounted on /proc, the sysfs filesystem must
be mounted at /sys. No other location is supported by udev.
Operation:
Udev creates and removes device nodes in /dev, based on events the kernel
sends out on device discovery or removal.
- Very early in the boot process, the /dev directory should get a 'tmpfs'
filesystem mounted, which is populated from scratch 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 on bootup, before
actions like loading kernel modules are taken, which may cause a lot of
events.
- The udevd daemon must be started on bootup to receive netlink uevents
from the kernel driver core.
- All kernel events are matched against a set of specified rules in
/etc/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.
Compile Options:
DESTDIR
Prefix of install target, used for package building.
USE_LOG
If set to 'true', udev is able to pass errors or debug information
to syslog. This is very useful to see what udev is doing or not doing.
It is enabled by default, don't expect any useful answer, if you
need to hunt a bug, but you can't enable syslog.
DEBUG
If set to 'true', very verbose debugging messages will be compiled
into the udev binaries. The actual level of debugging is specified
in the udev config file.
USE_SELINUX
If set to 'true', udev will be built with SELinux support
enabled. This is disabled by default.
EXTRAS
list of helper programs in extras/ to build.
make EXTRAS="extras/cdrom_id extras/scsi_id extras/volume_id"
Installation:
- The install target intalls the udev binaries in the default locations,
All at boot time reqired binaries will be installed in /lib/udev or /sbin.
- The default location for scripts and binaries that are called from
rules is /lib/udev. Other packages who install udev rules, should use
that directory too.
- It is recommended to use the /lib/udev/devices directory to place
device nodes and symlinks in, which are copied to /dev at every boot.
That way, nodes for broken subsystems or devices which can't be
detected automatically by the kernel, will always be available.
- Copies of the rules files for the major distros are provided as examples
in the etc/udev directory.
- The persistent device naming links in /dev/disk/ are required by other
software that depends on the data udev has collected from the devices
and should be installed by default with every udev installation.
Please direct any comment/question/concern to the linux-hotplug-devel mailing list at:
linux-hotplug@vger.kernel.org
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