README

   1 udev - userspace device management
   2
   3 For more information see the files in the docs/ directory.
   4
   5 Important Note:
   6   Integrating udev in the system has complex dependencies and differs from distro
   7   to distro. All major distros depend on udev these days and the system may not
   8   work without a properly installed version. The upstream udev project does not
   9   recommend to replace a distro's udev installation with the upstream version.
  10
  11 Requirements:
  12   - Version 2.6.20 of the Linux kernel for reliable operation of this release of
  13     udev. The kernel must not use the CONFIG_SYSFS_DEPRECATED* option.
  14
  15   - The kernel must have sysfs, unix domain sockets and networking enabled.
  16     Unix domain sockets (CONFIG_UNIX) as a loadable kernel module is not
  17     supported.
  18
  19   - The proc filesystem must be mounted on /proc/, the sysfs filesystem must
  20     be mounted at /sys/. No other locations are supported by udev.
  21
  22
  23 Operation:
  24   Udev creates and removes device nodes in /dev/, based on events the kernel
  25   sends out on device discovery or removal.
  26
  27   - Very early in the boot process, the /dev/ directory should get a 'tmpfs'
  28     filesystem mounted, which is populated from scratch by udev. Created nodes
  29     or changed permissions will not survive a reboot, which is intentional.
  30
  31   - The content of /lib/udev/devices/ directory which contains the nodes,
  32     symlinks and directories, which are always expected to be in /dev, should
  33     be copied over to the tmpfs mounted /dev, to provide the required nodes
  34     to initialize udev and continue booting.
  35
  36   - The old hotplug helper /sbin/hotplug should be disabled on bootup, before
  37     actions like loading kernel modules are taken, which may cause a lot of
  38     events.
  39
  40   - The udevd daemon must be started on bootup to receive netlink uevents
  41     from the kernel driver core.
  42
  43   - All kernel events are matched against a set of specified rules in
  44     /lib/udev/rules.d/ which make it possible to hook into the event
  45     processing to load required kernel modules and setup devices. For all
  46     devices the kernel exports a major/minor number, udev will create a
  47     device node with the default kernel name, or the one specified by a
  48     matching udev rule.
  49
  50 Please direct any comment/question/concern to the linux-hotplug mailing list at:
  51   linux-hotplug@vger.kernel.org
  52