X-Git-Url: https://www.chiark.greenend.org.uk/ucgi/~ianmdlvl/git?p=elogind.git;a=blobdiff_plain;f=README;h=591e03a057a96a0d244ba5e3978c42b43f642c70;hp=1a46a3c1eebf6b0d96d7b21bee77326badecb83c;hb=b02140b6bcd080c5fd60306a493447ce7afdad64;hpb=d2ce1817117daa9523b1572523f1e7684bae779f

diff --git a/README b/README
index 1a46a3c1e..591e03a05 100644
--- a/README
+++ b/README
@@ -1,39 +1,75 @@
+udev - userspace device management
 
-udev - a userspace implementation of devfs
+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.
 
-For more information on the design, and structure of this project, see the
-files in the docs/ directory.
+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.
 
-To use:
+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)
 
-- Edit the udev.h file and replace the following variables with values
-  that make sense for your system:
-	#define SYSFS_ROOT      "/sys"
-	#define UDEV_ROOT       "/home/greg/linux/udev/"
-	#define DEV_FILE        "/dev"
-	#define MKNOD           "/bin/mknod"
-  The only value most people will have to change is the UDEV_ROOT
-  variable, as I doubt you really want device nodes to be created in my
-  home directory :)
+  - For reliable operation, the kernel must not use the CONFIG_SYSFS_DEPRECATED*
+    option.
 
-- Run make to build the project.
+  - Unix domain sockets (CONFIG_UNIX) as a loadable kernel module is not
+    supported.
 
-- Make sure sysfs is mounted.
+  - The proc filesystem must be mounted on /proc/, the sysfs filesystem must
+    be mounted at /sys/. No other locations are supported by udev.
 
-- Point /proc/sys/kernel/hotplug at the location of the udev binary that
-  is created.  Then plug some block devices in, or other types of
-  devices that create dev files in sysfs.  An easy way to do this,
-  without any hardware is to use the scsi_debug module to create virtual
-  scsi devices.
+  - 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.
 
-- Watch as the nodes get created and removed.
+  - 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.
 
-Yes this is a really rough first cut, I know.  It's mostly a proof of
-concept that this can actually work.  See the TODO file for a list of
-things left to be done.
+  - 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.
 
-Any comment/questions/concerns please let me know.
+  - 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
 
-greg k-h
-greg@kroah.com