1 | <?xml version="1.0" encoding="ISO-8859-1"?>
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2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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3 | "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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4 | <!ENTITY % general-entities SYSTEM "../general.ent">
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5 | %general-entities;
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6 | ]>
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7 |
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8 | <sect1 id="ch-scripts-symlinks">
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9 | <?dbhtml filename="symlinks.html"?>
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10 |
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11 | <title>Creating Custom Symlinks to Devices</title>
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12 |
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13 | <sect2>
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14 |
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15 | <title>CD-ROM symlinks</title>
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16 |
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17 | <para>Some software that you may want to install later (e.g., various
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18 | media players) expect the <filename class="symlink">/dev/cdrom</filename>
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19 | and <filename class="symlink">/dev/dvd</filename> symlinks to exist, and
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20 | to point to a CD-ROM or DVD-ROM device. Also, it may be convenient to put
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21 | references to those symlinks into <filename>/etc/fstab</filename>. Udev
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22 | comes with a script that will generate rules files to create these symlinks
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23 | for you, depending on the capabilities of each device, but you need to
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24 | decide which of two modes of operation you wish to have the script use.</para>
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25 |
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26 | <para>First, the script can operate in <quote>by-path</quote> mode (used by
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27 | default for USB and FireWire devices), where the rules it creates depend on
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28 | the physical path to the CD or DVD device. Second, it can operate in
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29 | <quote>by-id</quote> mode (default for IDE and SCSI devices), where the
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30 | rules it creates depend on identification strings stored in the CD or DVD
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31 | device itself. The path is determined by Udev's <command>path_id</command>
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32 | script, and the identification strings are read from the hardware by its
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33 | <command>ata_id</command> or <command>scsi_id</command> programs, depending
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34 | on which type of device you have.</para>
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35 |
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36 | <para>There are advantages to each approach; the correct approach to use
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37 | will depend on what kinds of device changes may happen. If you expect the
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38 | physical path to the device (that is, the ports and/or slots that it plugs
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39 | into) to change, for example because you plan on moving the drive to a
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40 | different IDE port or a different USB connector, then you should use the
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41 | <quote>by-id</quote> mode. On the other hand, if you expect the device's
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42 | identification to change, for example because it may die, and you would
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43 | replace it with a different device with the same capabilities and which
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44 | is plugged into the same connectors, then you should use the
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45 | <quote>by-path</quote> mode.</para>
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46 |
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47 | <para>If either type of change is possible with your drive, then choose a
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48 | mode based on the type of change you expect to happen more often.</para>
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49 |
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50 | <!-- If you use by-id mode, the symlinks will survive even the transition
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51 | to libata for IDE drives, but that is not for the book. -->
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52 |
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53 | <important><para>External devices (for example, a USB-connected CD drive)
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54 | should not use by-path persistence, because each time the device is plugged
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55 | into a new external port, its physical path will change. All
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56 | externally-connected devices will have this problem if you write Udev rules
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57 | to recognize them by their physical path; the problem is not limited to CD
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58 | and DVD drives.</para></important>
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59 |
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60 | <para>If you wish to see the values that the Udev scripts will use, then
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61 | for the appropriate CD-ROM device, find the corresponding directory under
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62 | <filename class="directory">/sys</filename> (e.g., this can be
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63 | <filename class="directory">/sys/block/hdd</filename>) and
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64 | run a command similar to the following:</para>
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65 |
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66 | <screen role="nodump"><userinput>udevadm test /sys/block/hdd</userinput></screen>
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67 |
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68 | <para>Look at the lines containing the output of various *_id programs.
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69 | The <quote>by-id</quote> mode will use the ID_SERIAL value if it exists and
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70 | is not empty, otherwise it will use a combination of ID_MODEL and
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71 | ID_REVISION. The <quote>by-path</quote> mode will use the ID_PATH value.</para>
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72 |
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73 | <para>If the default mode is not suitable for your situation, then the
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74 | following modification can be made to the
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75 | <filename>/etc/udev/rules.d/75-cd-aliases-generator.rules</filename> file,
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76 | as follows (where <replaceable>mode</replaceable> is one of
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77 | <quote>by-id</quote> or <quote>by-path</quote>):</para>
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78 |
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79 | <screen role="nodump"><userinput>sed -i -e 's/write_cd_rules/& <replaceable>mode</replaceable>/' \
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80 | /etc/udev/rules.d/75-cd-aliases-generator.rules</userinput></screen>
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81 |
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82 | <para>Note that it is not necessary to create the rules files or symlinks
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83 | at this time, because you have bind-mounted the host's
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84 | <filename class="directory">/dev</filename> directory into the LFS system,
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85 | and we assume the symlinks exist on the host. The rules and symlinks will
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86 | be created the first time you boot your LFS system.</para>
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87 |
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88 | <para>However, if you have multiple CD-ROM devices, then the symlinks
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89 | generated at that time may point to different devices than they point to on
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90 | your host, because devices are not discovered in a predictable order. The
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91 | assignments created when you first boot the LFS system will be stable, so
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92 | this is only an issue if you need the symlinks on both systems to point to
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93 | the same device. If you need that, then inspect (and possibly edit) the
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94 | generated <filename>/etc/udev/rules.d/70-persistent-cd.rules</filename>
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95 | file after booting, to make sure the assigned symlinks match what you need.</para>
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96 |
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97 | </sect2>
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98 |
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99 | <sect2>
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100 |
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101 | <title>Dealing with duplicate devices</title>
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102 |
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103 | <para>As explained in <xref linkend="ch-scripts-udev"/>, the order in
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104 | which devices with the same function appear in
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105 | <filename class="directory">/dev</filename> is essentially random.
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106 | E.g., if you have a USB web camera and a TV tuner, sometimes
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107 | <filename>/dev/video0</filename> refers to the camera and
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108 | <filename>/dev/video1</filename> refers to the tuner, and sometimes
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109 | after a reboot the order changes to the opposite one.
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110 | For all classes of hardware except sound cards and network cards, this is
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111 | fixable by creating udev rules for custom persistent symlinks.
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112 | The case of network cards is covered separately in
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113 | <xref linkend="ch-scripts-network"/>, and sound card configuration can
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114 | be found in <ulink url="&blfs-root;">BLFS</ulink>.</para>
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115 |
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116 | <para>For each of your devices that is likely to have this problem
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117 | (even if the problem doesn't exist in your current Linux distribution),
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118 | find the corresponding directory under
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119 | <filename class="directory">/sys/class</filename> or
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120 | <filename class="directory">/sys/block</filename>.
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121 | For video devices, this may be
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122 | <filename
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123 | class="directory">/sys/class/video4linux/video<replaceable>X</replaceable></filename>.
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124 | Figure out the attributes that identify the device uniquely (usually,
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125 | vendor and product IDs and/or serial numbers work):</para>
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126 |
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127 | <screen role="nodump"><userinput>udevadm info -a -p /sys/class/video4linux/video0</userinput></screen>
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128 |
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129 | <para>Then write rules that create the symlinks, e.g.:</para>
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130 |
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131 | <screen role="nodump"><userinput>cat > /etc/udev/rules.d/83-duplicate_devs.rules << "EOF"
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132 | <literal>
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133 | # Persistent symlinks for webcam and tuner
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134 | KERNEL=="video*", ATTRS{idProduct}=="1910", ATTRS{idVendor}=="0d81", \
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135 | SYMLINK+="webcam"
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136 | KERNEL=="video*", ATTRS{device}=="0x036f", ATTRS{vendor}=="0x109e", \
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137 | SYMLINK+="tvtuner"
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138 | </literal>
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139 | EOF</userinput></screen>
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140 |
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141 | <para>The result is that <filename>/dev/video0</filename> and
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142 | <filename>/dev/video1</filename> devices still refer randomly to the tuner
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143 | and the web camera (and thus should never be used directly), but there are
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144 | symlinks <filename>/dev/tvtuner</filename> and
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145 | <filename>/dev/webcam</filename> that always point to the correct
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146 | device.</para>
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147 |
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148 | <para>More information on writing Udev rules can be found in
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149 | <filename>/usr/share/doc/udev-&udev-version;/index.html</filename>.</para>
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150 |
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151 | </sect2>
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152 |
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153 | </sect1>
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