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100 lines
4.3 KiB
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100 lines
4.3 KiB
Plaintext
udev - userspace device management
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For more information see the files in the docs/ directory.
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Important Note:
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Integrating udev in the system is a whole lot of work, has complex dependencies
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and differs a lot from distro to distro. All major distros depend on udev these
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days and the system may not work without a proper installed version. The upstream
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udev project does not support or recomend to replace a distro's udev installation
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with the upstream version. The installation of a unmodified upstream version may
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render your system unusable. Until now, there is no "default" setup or a set of
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"default" rules provided by the upstream udev version.
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Requirements:
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- 2.6.x version of the Linux kernel. See the RELEASE-NOTES file in the
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udev tree and the Documentation/Changes in the kernel source tree for
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the actual dependency.
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- The kernel must have sysfs and unix domain socket enabled.
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(unix domain sockets (CONFIG_UNIX) as a loadable kernel module may work,
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but it is completely silly - don't complain if anything goes wrong.)
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- The proc filesystem must be mounted on /proc.
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- The sysfs filesystem must be mounted at /sys. No other location
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will be supported by udev.
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Operation:
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Udev creates and removes device nodes in /dev, based on events the kernel
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sends out on device discovery or removal.
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- Early in the boot process, the /dev directory should get a tmpfs
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filesystem mounted, which is populated from scratch by udev. Created nodes
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or changed permissions will not survive a reboot, which is intentional.
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- The content of /lib/udev/devices directory which contains the nodes,
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symlinks and directories, which are always expected to be in /dev, should
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be copied over to the tmpfs mounted /dev, to provide the required nodes
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to initialize udev and continue booting.
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- The udevd daemon must be started by an init script to receive netlink
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uevents from the kernel driver core.
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- From kernel version 2.6.15 on, the hotplug helper /sbin/hotplug should
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be disabled with an init script before actions like loading kernel
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modules are taken, which may cause a lot of events.
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- All kernel events are matched against a set of specified rules in
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/etc/udev/rules.d/ which make it possible to hook into the event
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processing to load required kernel modules and setup devices. For all
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devices the kernel exports a major/minor number, udev will create a
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device node with the default kernel name or the one specified by a
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matching udev rule.
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Compile Options:
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DESTDIR
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Prefix of install target, used for package building.
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USE_LOG
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If set to 'true', udev is able to pass errors or debug information
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to syslog. This is very useful to see what udev is doing or not doing.
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It is enabled by default, don't expect any useful answer, if you
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need to hunt a bug, but you can't enable syslog.
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DEBUG
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If set to 'true', very verbose debugging messages will be compiled
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into the udev binaries. The actual level of debugging is specified
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in the udev config file.
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USE_SELINUX
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If set to 'true', udev will be built with SELinux support
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enabled. This is disabled by default.
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EXTRAS
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list of helper programs in extras/ to build.
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make EXTRAS="extras/cdrom_id extras/scsi_id extras/volume_id"
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Installation:
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- The install target intalls the udev binaries in the default locations,
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All at boot time reqired binaries will be installed in /lib/udev or /sbin.
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- The default location for scripts and binaries that are called from
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rules is /lib/udev. Other packages who install udev rules, should use
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that directory too.
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- It is recommended to use the /lib/udev/devices directory to place
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device nodes and symlinks in, which are copied to /dev at every boot.
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That way, nodes for broken subsystems or devices which can't be
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detected automatically by the kernel, will always be available.
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- Copies of the rules files for the major distros are provided as examples
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in the etc/udev directory.
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- The persistent device naming links in /dev/disk/ are required by other
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software that depends on the data udev has collected from the devices
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and should be installed by default with every udev installation.
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Please direct any comment/question/concern to the linux-hotplug-devel mailing list at:
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linux-hotplug-devel@lists.sourceforge.net
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