X-Git-Url: https://www.chiark.greenend.org.uk/ucgi/~ianmdlvl/git?p=elogind.git;a=blobdiff_plain;f=README;h=591e03a057a96a0d244ba5e3978c42b43f642c70;hp=3f4f947f79255816665dd63777da2eee64ec374a;hb=504a553e78eaf3f10a41f72c85a626d3942b13db;hpb=f054627f500450a7aadeaf8a9930354fb268718e diff --git a/README b/README index 3f4f947f7..591e03a05 100644 --- a/README +++ b/README @@ -1,68 +1,75 @@ 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 +Integrating udev in the system has complex dependencies and differs from distro +to distro. All major distros depend on udev these days and the system may not +work without a properly installed version. The upstream udev project does not +recommend to replace a distro's udev installation with the upstream version. + +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 content of +/dev/.udev/ is private to udev. + +Requirements: + - Version 2.6.25 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=y + CONFIG_SIGNALFD=y + CONFIG_TMPFS_POSIX_ACL=y (user ACLs for device nodes) + CONFIG_BLK_DEV_BSG=y (SCSI tape devices) + + - For reliable operation, the kernel must not use the CONFIG_SYSFS_DEPRECATED* + option. + + - Unix domain sockets (CONFIG_UNIX) as a loadable kernel module is not + supported. + + - The proc filesystem must be mounted on /proc/, the sysfs filesystem must + be mounted at /sys/. No other locations are supported by udev. + + - The system must have the following group names resolvable at udev startup: + disk, cdrom, floppy, tape, audio, video, lp, tty, dialout, kmem. + Especially in LDAP setups, it is required, that getgrnam() is able to resolve + these group names with only the rootfs mounted, and while no network is + available. + + - To build all udev extras, libacl, libglib2, libusb, usbutils, pciutils, + gperf are needed. These dependencies can be disabled with the + --disable-extras option. + +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 maintained 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 old hotplug helper /sbin/hotplug should be disabled in the kernel + configuration, it is not needed, and may render the system unusable + because of a fork-bombing behavior. + + - All kernel events are matched against a set of specified rules in + /lib/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. + +Please direct any comment/question/concern to the linux-hotplug mailing list at: + linux-hotplug@vger.kernel.org