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 2.6.15 of the Linux kernel for reliable operation of this release of
  13     udev. The kernel may have a requirement on udev too, see Documentation/Changes
  14     in the kernel source tree for the actual dependency.
  15
  16   - The kernel must have sysfs, unix domain sockets and networking enabled.
  17     (unix domain sockets (CONFIG_UNIX) as a loadable kernel module may work,
  18     but it is completely silly - don't complain if anything goes wrong.)
  19
  20   - The proc filesystem must be mounted on /proc, the sysfs filesystem must
  21     be mounted at /sys. No other location is supported by udev.
  22
  23
  24 Operation:
  25   Udev creates and removes device nodes in /dev, based on events the kernel
  26   sends out on device discovery or removal.
  27
  28   - Very early in the boot process, the /dev directory should get a 'tmpfs'
  29     filesystem mounted, which is populated from scratch by udev. Created nodes
  30     or changed permissions will not survive a reboot, which is intentional.
  31
  32   - The content of /lib/udev/devices directory which contains the nodes,
  33     symlinks and directories, which are always expected to be in /dev, should
  34     be copied over to the tmpfs mounted /dev, to provide the required nodes
  35     to initialize udev and continue booting.
  36
  37   - The old hotplug helper /sbin/hotplug should be disabled on bootup, before
  38     actions like loading kernel modules are taken, which may cause a lot of
  39     events.
  40
  41   - The udevd daemon must be started on bootup to receive netlink uevents
  42     from the kernel driver core.
  43
  44   - All kernel events are matched against a set of specified rules in
  45     /etc/udev/rules.d/ which make it possible to hook into the event
  46     processing to load required kernel modules and setup devices. For all
  47     devices the kernel exports a major/minor number, udev will create a
  48     device node with the default kernel name or the one specified by a
  49     matching udev rule.
  50
  51
  52 Compile Options:
  53   DESTDIR
  54         Prefix of install target, used for package building.
  55   USE_LOG
  56         If set to 'true', udev is able to pass errors or debug information
  57         to syslog. This is very useful to see what udev is doing or not doing.
  58         It is enabled by default, don't expect any useful answer, if you
  59         need to hunt a bug, but you can't enable syslog.
  60   DEBUG
  61         If set to 'true', very verbose debugging messages will be compiled
  62         into the udev binaries. The actual level of debugging is specified
  63         in the udev config file.
  64   USE_SELINUX
  65         If set to 'true', udev will be built with SELinux support
  66         enabled.  This is disabled by default.
  67   EXTRAS
  68         list of helper programs in extras/ to build.
  69         make EXTRAS="extras/cdrom_id extras/scsi_id extras/volume_id"
  70
  71
  72 Installation:
  73   - The install target intalls the udev binaries in the default locations,
  74     All at boot time reqired binaries will be installed in /lib/udev or /sbin.
  75
  76   - The default location for scripts and binaries that are called from
  77     rules is /lib/udev. Other packages who install udev rules, should use
  78     that directory too.
  79
  80   - It is recommended to use the /lib/udev/devices directory to place
  81     device nodes and symlinks in, which are copied to /dev at every boot.
  82     That way, nodes for broken subsystems or devices which can't be
  83     detected automatically by the kernel, will always be available.
  84
  85   - Copies of the rules files for the major distros are provided as examples
  86     in the etc/udev directory.
  87
  88   - The persistent device naming links in /dev/disk/ are required by other
  89     software that depends on the data udev has collected from the devices
  90     and should be installed by default with every udev installation.
  91
  92 Please direct any comment/question/concern to the linux-hotplug-devel mailing list at:
  93   linux-hotplug@vger.kernel.org
  94