source: BOOK/bootscripts/common/symlinks.xml@ 3aeeba02

clfs-1.2 clfs-2.1 clfs-3.0.0-systemd clfs-3.0.0-sysvinit systemd sysvinit
Last change on this file since 3aeeba02 was bc88d7a, checked in by Joe Ciccone <jciccone@…>, 16 years ago

use udevadm for udevinfo and udevtest.

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1<?xml version="1.0" encoding="ISO-8859-1"?>
2<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
3 "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
4 <!ENTITY % general-entities SYSTEM "../../general.ent">
5 %general-entities;
6]>
7
8<sect1 id="ch-scripts-symlinks">
9 <?dbhtml filename="symlinks.html"?>
10
11 <title>Creating custom symlinks to devices</title>
12
13 <sect2>
14
15 <title>CD-ROM symlinks</title>
16
17 <para>Some software that you may want to install later (e.g., various
18 media players) expect the /dev/cdrom and /dev/dvd symlinks to exist.
19 Also, it may be convenient to put references to those symlinks into
20 <filename>/etc/fstab</filename>. For each of your CD-ROM devices,
21 find the corresponding directory under
22 <filename class="directory">/sys</filename> (e.g., this can be
23 <filename class="directory">/sys/block/hdd</filename>) and
24 run a command similar to the following:</para>
25
26<screen role="nodump"><userinput>udevadm test /sys/block/hdd</userinput></screen>
27
28 <para>Look at the lines containing the output of various *_id programs.</para>
29
30 <para>There are two approaches to creating symlinks. The first one is to
31 use the model name and the serial number, the second one is based on the
32 location of the device on the bus. If you are going to use the first
33 approach, create a file similar to the following:</para>
34
35<screen role="nodump"><userinput>cat &gt;/etc/udev/rules.d/82-cdrom.rules &lt;&lt; EOF
36<literal>
37# Custom CD-ROM symlinks
38SUBSYSTEM=="block", ENV{ID_MODEL}=="SAMSUNG_CD-ROM_SC-148F", \
39 ENV{ID_REVISION}=="PS05", SYMLINK+="cdrom"
40SUBSYSTEM=="block", ENV{ID_MODEL}=="PHILIPS_CDD5301", \
41 ENV{ID_SERIAL}=="5VO1306DM00190", SYMLINK+="cdrom1 dvd"
42</literal>
43EOF</userinput></screen>
44
45 <note>
46 <para>Although the examples in this book work properly, be aware
47 that Udev does not recognize the backslash for line continuation.
48 If modifying Udev rules with an editor, be sure to leave each rule
49 on one physical line.</para>
50 </note>
51
52 <para>This way, the symlinks will stay correct even if you move the drives
53 to different positions on the IDE bus, but the
54 <filename>/dev/cdrom</filename> symlink won't be created if you replace
55 the old SAMSUNG CD-ROM with a new drive.</para>
56<!-- The symlinks in the first approach survive even the transition
57 to libata for IDE drives, but that is not for the book. -->
58
59 <para>The SUBSYSTEM==&quot;block&quot; key is needed in order to avoid
60 matching SCSI generic devices. Without it, in the case with SCSI
61 CD-ROMs, the symlinks will sometimes point to the correct
62 <filename>/dev/srX</filename> devices, and sometimes to
63 <filename>/dev/sgX</filename>, which is wrong.</para>
64
65 <para>The second approach yields:</para>
66
67<screen role="nodump"><userinput>cat &gt;/etc/udev/rules.d/82-cdrom.rules &lt;&lt; EOF
68<literal>
69# Custom CD-ROM symlinks
70SUBSYSTEM=="block", ENV{ID_TYPE}=="cd", \
71 ENV{ID_PATH}=="pci-0000:00:07.1-ide-0:1", SYMLINK+="cdrom"
72SUBSYSTEM=="block", ENV{ID_TYPE}=="cd", \
73 ENV{ID_PATH}=="pci-0000:00:07.1-ide-1:1", SYMLINK+="cdrom1 dvd"
74</literal>
75EOF</userinput></screen>
76
77 <para>This way, the symlinks will stay correct even if you replace drives
78 with different models, but place them to the old positions on the IDE
79 bus. The ENV{ID_TYPE}==&quot;cd&quot; key makes sure that the symlink
80 disappears if you put something other than a CD-ROM in that position on
81 the bus.</para>
82
83 <para>Of course, it is possible to mix the two approaches.</para>
84
85 </sect2>
86
87 <sect2>
88
89 <title>Dealing with duplicate devices</title>
90
91 <para>As explained in <xref linkend="ch-scripts-udev"/>, the order in
92 which devices with the same function appear in
93 <filename class="directory">/dev</filename> is essentially random.
94 E.g., if you have a USB web camera and a TV tuner, sometimes
95 <filename>/dev/video0</filename> refers to the camera and
96 <filename>/dev/video1</filename> refers to the tuner, and sometimes
97 after a reboot the order changes to the opposite one.
98 For all classes of hardware except sound cards and network cards, this is
99 fixable by creating udev rules for custom persistent symlinks.
100 The case of network cards is covered separately in
101 <xref linkend="ch-scripts-network"/>, and sound card configuration can
102 be found in <ulink url="&cblfs-root;">CBLFS</ulink>.</para>
103
104 <para>For each of your devices that is likely to have this problem
105 (even if the problem doesn't exist in your current Linux distribution),
106 find the corresponding directory under
107 <filename class="directory">/sys/class</filename> or
108 <filename class="directory">/sys/block</filename>.
109 For video devices, this may be
110 <filename
111 class="directory">/sys/class/video4linux/video<replaceable>X</replaceable></filename>.
112 Figure out the attributes that identify the device uniquely (usually,
113 vendor and product IDs and/or serial numbers work):</para>
114
115<screen role="nodump"><userinput>udevadm info -a -p /sys/class/video4linux/video0</userinput></screen>
116
117 <para>Then write rules that create the symlinks, e.g.:</para>
118
119<screen role="nodump"><userinput>cat &gt;/etc/udev/rules.d/83-duplicate_devs.rules &lt;&lt; EOF
120<literal>
121# Persistent symlinks for webcam and tuner
122KERNEL=="video*", SYSFS{idProduct}=="1910", SYSFS{idVendor}=="0d81", \
123 SYMLINK+="webcam"
124KERNEL=="video*", SYSFS{device}=="0x036f", SYSFS{vendor}=="0x109e", \
125 SYMLINK+="tvtuner"
126</literal>
127EOF</userinput></screen>
128
129 <para>The result is that <filename>/dev/video0</filename> and
130 <filename>/dev/video1</filename> devices still refer randomly to the tuner
131 and the web camera (and thus should never be used directly), but there are
132 symlinks <filename>/dev/tvtuner</filename> and
133 <filename>/dev/webcam</filename> that always point to the correct
134 device.</para>
135
136 <para>More information on writing Udev rules can be found in
137 <filename>/usr/share/doc/udev-&udev-version;/index.html</filename>.</para>
138
139 </sect2>
140
141</sect1>
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