X-Git-Url: https://www.chiark.greenend.org.uk/ucgi/~ianmdlvl/git?p=elogind.git;a=blobdiff_plain;f=README;h=7f0399e7110e9514e023e0d5fe91ed5c53f622de;hp=3f4f947f79255816665dd63777da2eee64ec374a;hb=cb936a3961f37ccb8eef0c505b8fffa6711c57be;hpb=f054627f500450a7aadeaf8a9930354fb268718e diff --git a/README b/README index 3f4f947f7..7f0399e71 100644 --- a/README +++ b/README @@ -1,68 +1,96 @@ -udev - userspace device management - -For more information see the files in the docs/ directory. - -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 reasonable distros use udev these - days, the major ones make it mandatory and the system will not work without it. - - 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! There is no "default" setup or a set - of "default" rules provided by the upstream udev version. - -udev requires: - - 2.6 version of the Linux kernel - - - the kernel must have sysfs, netlink, and hotplug enabled - - - proc must be mounted on /proc - - - sysfs must be mounted at /sys, no other location is supported - - - udev creates and removes device nodes in /dev based on events - the kernel sends out on device discovery or removal - - - during bootup /dev usually gets a tmpfs mounted which is populated scratch - by udev (created nodes don't survive a reboot, it always starts from scratch) - - - udev replaces the hotplug event management invoked from /sbin/hotplug - by the udevd daemon, which receives the kernel events over netlink - - - all kernel events are matched against a set of specified rules which - make it posible to hook into the event processing - - - there is a copy of the rules files for all major distros in the etc/udev - directory (you may look there how others distros are doing it) - -Setting which are used for building udev: - prefix - set this to the default root that you want to use - Only override this if you really know what you are doing - DESTDIR - prefix for install target for package building - USE_LOG - if set to 'true', udev will emit messages to the syslog when - it creates or removes device nodes. This is helpful to see - what udev is doing. This is enabled by default. - DEBUG - if set to 'true', verbose debugging messages will be compiled into - the udev binaries. Default value is 'false'. - 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 in /usr/bin/klcc - EXTRAS - if set, will build the "extra" helper programs as specified - as listed (see below for an example.) - -if you want to build the udev helper program cdrom_id and scsi_id: - make EXTRAS="extras/cdrom_id extras/scsi_id" - -Please direct any comment/question/concern to the linux-hotplug-devel mailing list at: - linux-hotplug-devel@lists.sourceforge.net - +udev - Linux userspace device management + +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. + +The upstream udev project's set of default rules may require a most recent +kernel release to work properly. This is currently version 2.6.31. + +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 2.6.27 of the Linux kernel with sysfs, procfs, signalfd, inotify, + unix domain sockets, networking and hotplug enabled: + 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_TMPFS=y + CONFIG_INOTIFY_USER=y + CONFIG_SIGNALFD=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 system must have 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. + + - The 'udev extras' has the following dependencies: + libacl, libglib2, libusb, usbutils, pciutils, and gperf. + These dependencies can be disabled with the --disable-extras configure option. + +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/removes device nodes in /dev, based on events the kernel + sends out on device creation/removal. + + - All kernel events are matched against a set of specified rules, which + possibly hook into the event processing and load required kernel + modules to setup 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(7) man page. + +Please direct any comment/question to the linux-hotplug mailing list at: + linux-hotplug@vger.kernel.org