X-Git-Url: https://www.chiark.greenend.org.uk/ucgi/~ianmdlvl/git?p=elogind.git;a=blobdiff_plain;f=README;h=148217d16a3e1b9f59e2612e5df053aa4669ab85;hp=7ac1d4509ccfbed7877e9efb2d3140d76da032d9;hb=617746e09795575c6258dd075ee7f0a44ce61e1e;hpb=cad4697469883b73cdd43e7e83db122951535ee8 diff --git a/README b/README index 7ac1d4509..148217d16 100644 --- a/README +++ b/README @@ -1,103 +1,104 @@ -udev - userspace device management +udev - Linux userspace device management -For more information see the files in the docs/ directory. +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. -Important Note: - Integrating udev in the system is a whole lot of work, has complex dependencies - and differs a lot from distro to distro. All major distros depend on udev these - days and the system may not work without a proper installed version. The upstream - udev project does not support or recomend to replace a distro's udev installation - with the upstream version. The installation of a unmodified upstream version may - render your system unusable. Until now, there is no "default" setup or a set of - "default" rules provided by the upstream udev version. +The upstream udev project's set of default rules may require a most recent +kernel release to work properly. This is currently version 2.6.32. -Requirements: - - 2.6.x version of the Linux kernel. See the RELEASE-NOTES file in the - udev tree and the Documentation/Changes in the kernel source tree for - the actual dependency. - - - The kernel must have sysfs and unix domain socket 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 - will be supported by udev. +Tools and rules shipped by udev are not public API and may change at any time. +Never call any private tool in /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 /lib/udev and the entire contents of +the /dev/.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 needed: + CONFIG_HOTPLUG=y + CONFIG_UEVENT_HELPER_PATH="" + CONFIG_NET=y + CONFIG_UNIX=y + CONFIG_SYSFS=y + CONFIG_SYSFS_DEPRECATED*=n + CONFIG_PROC_FS=y + CONFIG_INOTIFY_USER=y + CONFIG_SIGNALFD=y + + - These options might be needed: + CONFIG_TMPFS=y + CONFIG_TMPFS_POSIX_ACL=y (user ACLs for device nodes) + CONFIG_BLK_DEV_BSG=y (SCSI devices) + + - Udev does not work with the CONFIG_SYSFS_DEPRECATED* option. + + - Unix domain sockets (CONFIG_UNIX) as a loadable kernel module may work, + but it is not supported. + + - 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: + libacl, libglib2, libusb, usbutils, pciutils, and gperf. + All these extras can be disabled with configure options. + +Setup: + - At bootup, the /dev directory should get the 'devtmpfs' filesystem + mounted. Udev manages the permissions and ownership of the kernel-created + device nodes, and udev possibly creates additional symlinks. If needed, udev also + works on an empty 'tmpfs' filesystem, but some static device nodes like + /dev/null, /dev/console, /dev/kmsg are needed to be able to start udev itself. + + - The udev daemon should be started to handle device events sent by the kernel. + During bootup, the kernel can be asked to send events for all already existing + devices so that they too can be configured by udev. This is usually done by: + /sbin/udevadm trigger --type=subsystems + /sbin/udevadm trigger --type=devices + + - Restarting the daemon never applies any rules to existing devices. + + - New/changed rule files are picked up automatically; there is no daemon + restart or signal needed. Operation: - Udev creates and removes device nodes in /dev, based on events the kernel - sends out on device discovery or removal. - - - 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 udevd daemon must be started by an init script to receive netlink - uevents from the kernel driver core. - - - From kernel version 2.6.15 on, the hotplug helper /sbin/hotplug should - be disabled with an init script before actions like loading kernel - modules are taken, which may cause a lot of events. - - - 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. - USE_KLIBC - If set to 'true', udev is built and linked against klibc. - Default value is 'false'. KLCC specifies the klibc compiler - wrapper, usually located at /usr/bin/klcc. - 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-devel@lists.sourceforge.net - + - Based on events the kernel sends out on device creation/removal, udev + creates/removes device nodes 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 + 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