source: BOOK/bootscripts/common/udev.xml @ 3f8be484

Last change on this file since 3f8be484 was 3f8be484, checked in by Jim Gifford <clfs@…>, 13 years ago

r627@server (orig r625): jim | 2005-10-31 12:59:34 -0800
Import of Cross-LFS Book

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1<?xml version="1.0" encoding="ISO-8859-1"?>
2<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
3  "" [
4  <!ENTITY % general-entities SYSTEM "../../general.ent">
5  %general-entities;
8<sect1 id="ch-scripts-udev">
9  <?dbhtml filename="udev.html"?>
11  <title>Device and Module Handling on an LFS System</title>
13  <indexterm zone="ch-scripts-udev">
14    <primary sortas="a-Udev">Udev</primary>
15  <secondary>usage</secondary></indexterm>
17  <para>In <xref linkend="chapter-building-system"/>, we installed the Udev
18  package. Before we go into the details regarding how this works, a brief
19  history of previous methods of handling devices is in order.</para>
21  <para>Linux systems in general traditionally use a static device creation
22  method, whereby a great many device nodes are created under <filename
23  class="directory">/dev</filename> (sometimes literally thousands of nodes),
24  regardless of whether the corresponding hardware devices actually exist.
25  This is typically done via a <command>MAKEDEV</command> script, which
26  contains a number of calls to the <command>mknod</command> program with
27  the relevant major and minor device numbers for every possible device that
28  might exist in the world. Using the Udev method, only those devices which
29  are detected by the kernel get device nodes created for them. Because
30  these device nodes will be created each time the system boots, they will
31  be stored on a <systemitem class="filesystem">tmpfs</systemitem> (a virtual
32  file system that resides entirely in system memory). Device nodes do not
33  require much space, so the memory that is used is negligible.</para>
35  <sect2>
36    <title>History</title>
38    <para>In February 2000, a new filesystem called <systemitem
39    class="filesystem">devfs</systemitem> was merged into the 2.3.46 kernel
40    and was made available during the 2.4 series of stable kernels. Although
41    it was present in the kernel source itself, this method of creating
42    devices dynamically never received overwhelming support from the core
43    kernel developers.</para>
45    <para>The main problem with the approach adopted by <systemitem
46    class="filesystem">devfs</systemitem> was the way it handled
47    device detection, creation, and naming. The latter issue, that of
48    device node naming, was perhaps the most critical. It is generally
49    accepted that if device names are allowed to be configurable, then
50    the device naming policy should be up to a system administrator, not
51    imposed on them by any particular developer(s). The <systemitem
52    class="filesystem">devfs</systemitem> file system also suffers from race
53    conditions that are inherent in its design and cannot be fixed
54    without a substantial revision to the kernel. It has also been marked
55    as deprecated due to a lack of recent maintenance.</para>
57    <para>With the development of the unstable 2.5 kernel tree, later
58    released as the 2.6 series of stable kernels, a new virtual filesystem
59    called <systemitem class="filesystem">sysfs</systemitem> came to be. The
60    job of <systemitem class="filesystem">sysfs</systemitem> is to export a
61    view of the system's hardware configuration to userspace processes. With
62    this userspace-visible representation, the possibility of seeing a
63    userspace replacement for <systemitem class="filesystem">devfs</systemitem>
64    became much more realistic.</para>
66  </sect2>
68  <sect2>
69    <title>Udev Implementation</title>
71    <para>The <systemitem class="filesystem">sysfs</systemitem> filesystem
72    was mentioned briefly above. One may wonder how <systemitem
73    class="filesystem">sysfs</systemitem> knows about the devices present
74    on a system and what device numbers should be used for them. Drivers
75    that have been compiled into the kernel directly register their objects
76    with <systemitem class="filesystem">sysfs</systemitem> as they are
77    detected by the kernel. For drivers compiled as modules, this
78    registration will happen when the module is loaded. Once the
79    <systemitem class="filesystem">sysfs</systemitem> filesystem is mounted
80    (on <filename class="directory">/sys</filename>), data which the built-in
81    drivers registered with <systemitem class="filesystem">sysfs</systemitem>
82    are available to userspace processes and to <command>udev</command> for
83    device node creation.</para>
85    <para>The <command>S10udev</command> initscript takes care of creating
86    these device nodes when Linux is booted. This script starts by registering
87    <command>/sbin/udevsend</command> as a hotplug event handler. Hotplug
88    events (discussed below) are not usually generated during this stage,
89    but <command>udev</command> is registered just in case they do occur.
90    The <command>udevstart</command> program then walks through the
91    <systemitem class="filesystem">/sys</systemitem> filesystem and creates
92    devices under <filename class="directory">/dev</filename> that match the
93    descriptions. For example, <filename>/sys/class/tty/vcs/dev</filename>
94    contains the string <quote>7:0</quote> This string is used by
95    <command>udevstart</command> to create <filename>/dev/vcs</filename>
96    with major number <emphasis>7</emphasis> and minor <emphasis>0</emphasis>.
97    The names and permissions of the nodes created under the <filename
98    class="directory">/dev</filename> directory are configured according to
99    the rules specified in the files within the <filename
100    class="directory">/etc/udev/rules.d/</filename> directory. These are
101    numbered in a similar fashion to the LFS-Bootscripts package. If
102    <command>udev</command> can't find a rule for the device it is creating,
103    it will default permissions to <emphasis>660</emphasis> and ownership to
104    <emphasis>root:root</emphasis>.</para>
106    <para>Once the above stage is complete, all devices that were already
107    present and have compiled-in drivers will be available for use. This
108    leads us to the devices that have modular drivers.</para>
110    <para>Earlier, we mentioned the concept of a <quote>hotplug event
111    handler.</quote> When a new device connection is detected by the kernel,
112    the kernel will generate a hotplug event and look at the file
113    <filename>/proc/sys/kernel/hotplug</filename> to determine the userspace
114    program that handles the device's connection. The <command>udev</command>
115    bootscript registered <command>udevsend</command> as this handler. When
116    these hotplug events are generated, the kernel will tell
117    <command>udev</command> to check the <filename
118    class="directory">/sys</filename> filesystem for the information
119    pertaining to this new device and create the <filename
120    class="directory">/dev</filename> entry for it.</para>
122    <para>This brings us to one problem that exists with
123    <command>udev</command>, and likewise with <systemitem
124    class="filesystem">devfs</systemitem> before it. It is commonly
125    referred to as the <quote>chicken and egg</quote> problem. Most
126    Linux distributions handle loading modules via entries in
127    <filename>/etc/modules.conf</filename>. Access to a device node causes
128    the appropriate kernel module to load. With <command>udev</command>,
129    this method will not work because the device node does not exist until
130    the module is loaded. To solve this, the <command>S05modules</command>
131    bootscript was added to the LFS-Bootscripts package, along with the
132    <filename>/etc/sysconfig/modules</filename> file. By adding module
133    names to the <filename>modules</filename> file, these modules will be
134    loaded when the computer starts up. This allows <command>udev</command>
135    to detect the devices and create the appropriate device nodes.</para>
137    <para>Note that on slower machines or for drivers that create a lot
138    of device nodes, the process of creating devices may take a few
139    seconds to complete. This means that some device nodes may not be
140    immediately accessible.</para>
142  </sect2>
144  <sect2>
145    <title>Handling Hotpluggable/Dynamic Devices</title>
147    <para>When you plug in a device, such as a Universal Serial Bus (USB)
148    MP3 player, the kernel recognizes that the device is now connected and
149    generates a hotplug event. If the driver is already loaded (either
150    because it was compiled into the kernel or because it was loaded via
151    the <command>S05modules</command> bootscript), <command>udev</command>
152    will be called upon to create the relevant device node(s) according to
153    the <systemitem class="filesystem">sysfs</systemitem> data available in
154    <filename class="directory">/sys</filename>.</para>
156    <para>If the driver for the just plugged in device is available as a
157    module but currently unloaded, the Hotplug package will load the
158    appropriate module and make this device available by creating the
159    device node(s) for it.</para>
161  </sect2>
163  <sect2>
164    <title>Problems with Creating Devices</title>
166    <para>There are a few known problems when it comes to automatically
167    creating device nodes:</para>
169    <para>1) A kernel driver may not export its data to <systemitem
170    class="filesystem">sysfs</systemitem>.</para>
172    <para>This is most common with third party drivers from outside the
173    kernel tree. Udev will be unable to automatically create device nodes
174    for such drivers. Use the <filename>/etc/sysconfig/createfiles</filename>
175    configuration file to manually create the devices. Consult the
176    <filename>devices.txt</filename> file inside the kernel documentation
177    or the documentation for that driver to find the proper major/minor
178    numbers.</para>
180    <para>2) A non-hardware device is required. This is most common with
181    the Advanced Linux Sound Architecture (ALSA) project's Open Sound
182    System (OSS) compatibility module. These types of devices can be
183    handled in one of two ways:</para>
185    <itemizedlist>
186      <listitem>
187        <para>Adding the module names to
188        <filename>/etc/sysconfig/modules</filename></para>
189      </listitem>
190      <listitem>
191        <para>Using an <quote>install</quote> line in
192        <filename>/etc/modprobe.conf</filename>. This tells the
193        <command>modprobe</command> command <quote>when loading this
194        module, also load this other module, at the same time.</quote>
195        For example:</para>
197<screen><userinput>install snd-pcm modprobe -i snd-pcm ; modprobe \
198    snd-pcm-oss ; true</userinput></screen>
200        <para>This will cause the system to load both the
201        <emphasis>snd-pcm</emphasis> and <emphasis>snd-pcm-oss</emphasis>
202        modules when any request is made to load the driver
203        <emphasis>snd-pcm</emphasis>.</para>
204      </listitem>
205    </itemizedlist>
207  </sect2>
209  <sect2>
210    <title>Useful Reading</title>
212    <para>Additional helpful documentation is available at the following
213    sites:</para>
215    <itemizedlist>
216      <listitem>
217        <para remap="verbatim">A Userspace Implementation of <systemitem class="filesystem">devfs</systemitem>
218        <ulink url=""/></para>
219      </listitem>
220      <listitem>
221        <para remap="verbatim">udev FAQ
222        <ulink url=""/></para>
223      </listitem>
224      <listitem>
225        <para remap="verbatim">The Linux Kernel Driver Model
226        <ulink url=""/></para>
227      </listitem>
228    </itemizedlist>
230  </sect2>
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