X-Git-Url: https://www.chiark.greenend.org.uk/ucgi/~ianmdlvl/git?p=elogind.git;a=blobdiff_plain;f=README;h=6e09b3a52da14540adf82a03a650a3df1bd0d750;hp=b4c0ee051faa4f014b6fb454ea7b999d2326b7db;hb=4c514e31734a704ca58e543fa4330616ba584b39;hpb=89eaa6a56eded9f76c452e20994aaf45c7c2dad9 diff --git a/README b/README index b4c0ee051..6e09b3a52 100644 --- a/README +++ b/README @@ -1,18 +1,22 @@ -udev - userspace device management +udev - Linux userspace device management -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. +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 to replace 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. 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. +by udevadm and libudev. Tools and rules in /lib/udev, and the entire content of +the /dev/.udev directory is private to udev and does change whenever needed. Requirements: - - Version 2.6.25 of the Linux kernel with sysfs, procfs, signalfd, inotify, + - 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="" @@ -22,18 +26,24 @@ Requirements: CONFIG_SYSFS_DEPRECATED*=n CONFIG_PROC_FS=y CONFIG_TMPFS=y - CONFIG_TMPFS_POSIX_ACL=y - CONFIG_INOTIFY=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 will not work with the CONFIG_SYSFS_DEPRECATED* option. - - For reliable operation, the kernel must not use the CONFIG_SYSFS_DEPRECATED* - option. + - Unix domain sockets (CONFIG_UNIX) as a loadable kernel module may work, + but it is not supported. - - Unix domain sockets (CONFIG_UNIX) as a loadable kernel module 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/, the sysfs filesystem must - be mounted at /sys/. No other locations are supported by udev. + - The proc filesystem must be mounted on /proc, 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, kmem. @@ -41,34 +51,51 @@ Requirements: these group names with only the rootfs mounted, and while no network is available. - - To build all udev extras, libacl, libglib2, libusb, usbutils, pciutils, + - 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. + --disable-extras configure option. - - 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. +Setup: + - At bootup, the /dev directory should get the 'devtmpfs' filesystem + mounted. Udev will manage permissions and ownership of the kernel-created + device nodes, and possibly create 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 content of /lib/udev/devices/ directory which contains the nodes, + - The content of /lib/udev/devices directory which contains static content like 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. + be copied over to the mounted /dev directory: + cp -axT --remove-destination /lib/udev/devices /dev - - 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. + - 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, to apply the configuration to these devices. This is usually done by: + /sbin/udevadm trigger --type=subsystems + /sbin/udevadm trigger --type=devices - - 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. + - Restarting the daemon does never apply any rules to existing devices. -Please direct any comment/question/concern to the linux-hotplug mailing list at: - linux-hotplug@vger.kernel.org + - 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 will create 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