X-Git-Url: http://www.chiark.greenend.org.uk/ucgi/~ianmdlvl/git?p=elogind.git;a=blobdiff_plain;f=README;h=38459c6b226141408e3b7d2d33c32324dd42c822;hp=664457de0f9b2f5cfe2dbfca2b07a15e22fa272c;hb=bd59d82322503b796ed6e80425df4bd24b5059ee;hpb=1da6c797fdbb94372c1a809acf1a0ca159b2d7b1 diff --git a/README b/README index 664457de0..38459c6b2 100644 --- a/README +++ b/README @@ -1,76 +1,101 @@ -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 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. 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 -the /dev/.udev directory is private to udev and does change whenever needed. +Never call any private tool in /usr/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 /usr/lib/udev and the entire contents +of the /run/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: + - 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 required: + CONFIG_DEVTMPFS=y CONFIG_HOTPLUG=y - CONFIG_UEVENT_HELPER_PATH="" + CONFIG_INOTIFY_USER=y 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_SYSFS=y + CONFIG_SYSFS_DEPRECATED*=n + CONFIG_UEVENT_HELPER_PATH="" + + - These options might be needed: CONFIG_BLK_DEV_BSG=y (SCSI devices) + CONFIG_TMPFS_POSIX_ACL=y (user ACLs for device nodes) - - For reliable operations, the kernel must not use the CONFIG_SYSFS_DEPRECATED* - option. + - The /dev directory needs the 'devtmpfs' filesystem mounted. + Udev only manages the permissions and ownership of the + kernel-provided device nodes, and possibly creates additional symlinks. - - Unix domain sockets (CONFIG_UNIX) as a loadable kernel module may work, - but it is not supported. + - Udev requires /run to be writable, which is usually done by mounting a + 'tmpfs' filesystem. - - The proc filesystem must be mounted on /proc, the sysfs filesystem must - be mounted at /sys. No other locations are supported by udev. + - This version of udev does not work properly with the CONFIG_SYSFS_DEPRECATED* + option enabled. - - 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 + - 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. - - To build all udev extras, libacl, libglib2, libusb, usbutils, pciutils, - gperf are needed. These dependencies can be disabled with the - --disable-extras option. + - Some udev extras have external dependencies like: + libglib2, usbutils, pciutils, and gperf. + All these extras can be disabled with configure options. -Operation: - Udev creates and removes device nodes in /dev, based on events the kernel - sends out on device discovery or removal. +Setup: + - The udev daemon should be started to handle device events sent by the kernel. + During bootup, the events for already existing devices can be replayed, so + that they are configured by udev. The systemd service files contain the + needed commands to start the udev daemon and the coldplug sequence. - - 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. + - Restarting the daemon never applies any rules to existing devices. - - 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. + - New/changed rule files are picked up automatically; there is usually no + daemon restart or signal needed. - - 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. +Operation: + - Based on events the kernel sends out on device creation/removal, udev + creates/removes device nodes and symlinks in the /dev directory. - - 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. + - 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 + device 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 -