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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-config-udev">
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9 | <?dbhtml filename="udev.html"?>
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10 |
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11 | <title>Device and Module Handling on a CLFS System</title>
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12 |
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13 | <indexterm zone="ch-config-udev">
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14 | <primary sortas="a-Udev">Udev</primary>
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15 | <secondary>usage</secondary>
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16 | </indexterm>
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17 |
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18 | <para>In <xref linkend="chapter-building-system"/>, we installed Udev,
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19 | as one of the components of systemd. Before we go into the details regarding
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20 | how this works, a brief history of previous methods of handling devices is in
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21 | order.</para>
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22 |
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23 | <sect2>
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24 | <title>History</title>
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25 |
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26 | <sect3>
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27 | <title>Static Device Nodes</title>
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28 |
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29 | <para>Linux systems in general traditionally use a static device creation
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30 | method, whereby a great many device nodes are created under <filename
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31 | class="directory">/dev</filename> (sometimes literally thousands of
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32 | nodes), regardless of whether the corresponding hardware devices actually
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33 | exist. This is typically done via a <command>MAKEDEV</command> script,
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34 | which contains a number of calls to the <command>mknod</command> program
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35 | with the relevant major and minor device numbers for every possible
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36 | device that might exist in the world.</para>
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37 |
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38 | </sect3>
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39 |
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40 | <sect3>
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41 | <title>Devfs</title>
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42 |
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43 | <para>In February 2000, a new filesystem called <systemitem
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44 | class="filesystem">devfs</systemitem>, which dynamically created device
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45 | nodes as devices were found by the kernel, was merged into the
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46 | 2.3.46 kernel and was made available during the 2.4 series of stable
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47 | kernels. Although it was present in the kernel source itself, this method
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48 | of creating devices dynamically never received overwhelming support from
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49 | the core kernel developers.</para>
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50 |
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51 | <para>The main problem with the approach adopted by <systemitem
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52 | class="filesystem">devfs</systemitem> was the way it handled device
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53 | detection, creation, and naming. The latter issue, that of device node
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54 | naming, was perhaps the most critical. It is generally accepted that if
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55 | device names are allowed to be configurable, then the device naming policy
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56 | should be up to a system administrator, not imposed on them by any
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57 | particular developer(s). The <systemitem
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58 | class="filesystem">devfs</systemitem> file system also suffered from race
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59 | conditions that were inherent in its design and could not be fixed without
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60 | a substantial revision to the kernel. It was marked deprecated with the
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61 | release of the 2.6 kernel series, and was removed entirely as of version
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62 | 2.6.18.</para>
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63 |
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64 | </sect3>
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65 |
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66 | <sect3>
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67 | <title>Sysfs</title>
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68 |
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69 | <para>With the development of the unstable 2.5 kernel tree, later released
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70 | as the 2.6 series of stable kernels, a new virtual filesystem called
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71 | <systemitem class="filesystem">sysfs</systemitem> came to be. The job of
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72 | <systemitem class="filesystem">sysfs</systemitem> is to export a view of
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73 | the system's hardware configuration to userspace processes. Drivers that
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74 | have been compiled into the kernel directly register their objects with
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75 | <systemitem class="filesystem">sysfs</systemitem> as they are detected by
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76 | the kernel. For drivers compiled as modules, this registration will happen
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77 | when the module is loaded. Once the <systemitem
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78 | class="filesystem">sysfs</systemitem> filesystem is mounted (on <filename
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79 | class="directory">/sys</filename>), data which the built-in drivers
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80 | registered with <systemitem class="filesystem">sysfs</systemitem> are
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81 | available to userspace processes. With this userspace-visible
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82 | representation, the possibility of seeing a userspace replacement for
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83 | <systemitem class="filesystem">devfs</systemitem> became much more
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84 | realistic.</para>
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85 |
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86 | </sect3>
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87 |
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88 | <sect3>
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89 | <title>Udev Implementation</title>
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90 |
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91 | <para>Shortly after the introduction of
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92 | <systemitem class="filesystem">sysfs</systemitem>, work began on a
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93 | program called Udev to advantage of it. The <command>udev</command>
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94 | daemon made calls to <function>mknod()</function> to create device nodes
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95 | in <filename class="directory">/dev</filename> dynamically, based on the
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96 | information from <systemitem class="filesystem">sysfs</systemitem>, in
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97 | <filename class="directory">/sys</filename>. For example,
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98 | <filename>/sys/class/tty/vcs/dev</filename> contains the string
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99 | <quote>7:0</quote>. This string was used by <command>udev</command>
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100 | to create a device node with major number <emphasis>7</emphasis> and
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101 | minor number <emphasis>0</emphasis>.</para>
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102 |
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103 | <para>Linux kernel version 2.6.32 introduced a new virtual file system
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104 | called <systemitem class="filesystem">devtmpfs</systemitem>, an improved
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105 | replacement for <systemitem class="filesystem">devfs</systemitem>. This
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106 | allows device nodes to once again be dynamically created by the kernel,
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107 | without many of the problems of
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108 | <systemitem class="filesystem">devfs</systemitem>. As of version 176,
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109 | Udev no longer creates device nodes itself, instead relying on
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110 | <systemitem class="filesystem">devtmpfs</systemitem> to do so.</para>
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111 |
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112 | </sect3>
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113 |
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114 | <sect3>
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115 | <title>Systemd and Eudev</title>
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116 |
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117 | <para>In 2010, development began on systemd, an alternate
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118 | <command>init</command> implementation. Starting with Udev 183, Udev's
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119 | source tree was merged with systemd. Several Gentoo
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120 | developers who disagreed with this merge announced a project fork
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121 | called Eudev in December 2012, created by extracting the
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122 | Udev code from systemd. One of the goals of Eudev is to allow for
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123 | easier installation and usage of <command>udevd</command> without
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124 | the need for the rest of systemd.</para>
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125 | </sect3>
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126 |
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127 | </sect2>
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128 |
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129 | <sect2>
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130 | <title>Device Node Creation</title>
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131 |
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132 | <para>By default, device nodes created by the kernel in a
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133 | <systemitem class="filesystem">devtmpfs</systemitem> are owned by
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134 | <emphasis>root:root</emphasis> and have <emphasis>600</emphasis>
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135 | permissions. <command>udevd</command> can modify ownership and permissions
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136 | of the nodes under the <filename class="directory">/dev</filename>
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137 | directory, and can also create additional symlinks, based on rules
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138 | specified in the files within the
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139 | <filename class="directory">/etc/udev/rules.d</filename>,
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140 | <filename class="directory">/lib/udev/rules.d</filename>,
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141 | and <filename class="directory">/run/udev/rules.d</filename> directories.
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142 | The names for these files start with a number, to indicate the order in
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143 | which they are run, and they have a <filename>.rules</filename>
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144 | extension (<command>udevd</command> will ignore files with any other
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145 | extension). All of the rules files from these directories are combined into
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146 | a single list, sorted by filename, and run in that order. In the event of
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147 | a conflict, where a rules file with the same name exists in two or more of
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148 | these directories, the rules in <filename class="directory">/etc</filename>
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149 | take the highest priority, followed by rules files in
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150 | <filename class="directory">/run</filename>, and finally
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151 | <filename class="directory">/lib</filename>. Any device for which a rule
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152 | cannot be found will just be ignored by <command>udevd</command>
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153 | and be left at the defaults defined by the kernel, as described above. For
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154 | more details about writing Udev rules, see
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155 | <filename>/usr/share/doc/systemd-&systemd-version;/udev.html</filename>.</para>
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156 |
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157 | </sect2>
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158 |
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159 | <sect2>
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160 | <title>Module Loading</title>
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161 |
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162 | <para>Device drivers compiled as modules may have aliases built into them.
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163 | Aliases are visible in the output of the <command>modinfo</command>
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164 | program and are usually related to the bus-specific identifiers of devices
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165 | supported by a module. For example, the <emphasis>snd-fm801</emphasis>
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166 | driver supports PCI devices with vendor ID 0x1319 and device ID 0x0801,
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167 | and has an alias of <quote>pci:v00001319d00000801sv*sd*bc04sc01i*</quote>.
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168 | For most devices, the bus driver exports the alias of the driver that
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169 | would handle the device via <systemitem
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170 | class="filesystem">sysfs</systemitem>. E.g., the
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171 | <filename>/sys/bus/pci/devices/0000:00:0d.0/modalias</filename> file
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172 | might contain the string
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173 | <quote>pci:v00001319d00000801sv00001319sd00001319bc04sc01i00</quote>.
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174 | The default rules provided by Udev will cause <command>udevd</command>
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175 | to call out to <command>/sbin/modprobe</command> with the contents of the
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176 | <envar>MODALIAS</envar> uevent environment variable (that should be the
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177 | same as the contents of the <filename>modalias</filename> file in sysfs),
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178 | thus loading all modules whose aliases match this string after wildcard
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179 | expansion.</para>
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180 |
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181 | <para>In this example, this means that, in addition to
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182 | <emphasis>snd-fm801</emphasis>, the obsolete (and unwanted)
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183 | <emphasis>forte</emphasis> driver will be loaded if it is
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184 | available. See below for ways in which the loading of unwanted drivers can
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185 | be prevented.</para>
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186 |
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187 | <para>The kernel itself is also able to load modules for network
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188 | protocols, filesystems and NLS support on demand.</para>
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189 |
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190 | </sect2>
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191 |
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192 | <sect2>
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193 | <title>Problems with Loading Modules and Creating Devices</title>
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194 |
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195 | <para>There are a few possible problems when it comes to automatically
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196 | creating device nodes.</para>
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197 |
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198 | <sect3>
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199 | <title>A kernel module is not loaded automatically</title>
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200 |
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201 | <para>Udev will only load a module if it has a bus-specific alias and the
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202 | bus driver properly exports the necessary aliases to <systemitem
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203 | class="filesystem">sysfs</systemitem>. In other cases, one should
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204 | arrange module loading by other means. With Linux-&linux-version;, Udev is
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205 | known to load properly-written drivers for INPUT, IDE, PCI, USB, SCSI,
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206 | SERIO and FireWire devices.</para>
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207 |
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208 | <para>To determine if the device driver you require has the necessary
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209 | support for Udev, run <command>modinfo</command> with the module name as
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210 | the argument. Now try locating the device directory under
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211 | <filename class="directory">/sys/bus</filename> and check whether there is
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212 | a <filename>modalias</filename> file there.</para>
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213 |
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214 | <para>If the <filename>modalias</filename> file exists in <systemitem
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215 | class="filesystem">sysfs</systemitem>, the driver supports the device and
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216 | can talk to it directly, but doesn't have the alias, it is a bug in the
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217 | driver. Load the driver without the help from Udev and expect the issue
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218 | to be fixed later.</para>
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219 |
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220 | <para>If there is no <filename>modalias</filename> file in the relevant
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221 | directory under <filename class="directory">/sys/bus</filename>, this
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222 | means that the kernel developers have not yet added modalias support to
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223 | this bus type. With Linux-&linux-version;, this is the case with ISA
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224 | busses. Expect this issue to be fixed in later kernel versions.</para>
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225 |
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226 | <para>Udev is not intended to load <quote>wrapper</quote> drivers such as
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227 | <emphasis>snd-pcm-oss</emphasis> and non-hardware drivers such as
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228 | <emphasis>loop</emphasis> at all.</para>
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229 |
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230 | </sect3>
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231 |
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232 | <sect3>
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233 | <title>A kernel module is not loaded automatically, and Udev is not
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234 | intended to load it</title>
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235 |
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236 | <para>If the <quote>wrapper</quote> module only enhances the functionality
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237 | provided by some other module (e.g., <emphasis>snd-pcm-oss</emphasis>
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238 | enhances the functionality of <emphasis>snd-pcm</emphasis> by making the
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239 | sound cards available to OSS applications), configure
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240 | <command>modprobe</command> to load the wrapper after Udev loads the
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241 | wrapped module. To do this, add an <quote>install</quote> line to a file
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242 | in <filename>/etc/modprobe.d</filename>. For example:</para>
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243 |
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244 | <screen role="nodump"><literal>install snd-pcm /sbin/modprobe -i snd-pcm ; \
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245 | /sbin/modprobe snd-pcm-oss ; true</literal></screen>
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246 |
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247 | <para>If the module in question is not a wrapper and is useful by itself,
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248 | configure the <command>S05modules</command> bootscript to load this
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249 | module on system boot. To do this, add the module name to the
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250 | <filename>/etc/sysconfig/modules</filename> file on a separate line.
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251 | This works for wrapper modules too, but is suboptimal in that case.</para>
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252 |
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253 | </sect3>
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254 |
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255 | <sect3>
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256 | <title>Udev loads some unwanted module</title>
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257 |
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258 | <para>Either don't build the module, or blacklist it in
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259 | <filename>/etc/modprobe.d</filename> file as done with the
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260 | <emphasis>forte</emphasis> module in the example below:</para>
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261 |
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262 | <screen role="nodump"><literal>blacklist forte</literal></screen>
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263 |
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264 | <para>Blacklisted modules can still be loaded manually with the
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265 | explicit <command>modprobe</command> command.</para>
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266 |
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267 | </sect3>
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268 |
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269 | <sect3>
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270 | <title>Udev makes a wrong symlink</title>
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271 |
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272 | <para>This usually happens if a rule unexpectedly matches a device. For
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273 | example, a poorly-written rule can match both a SCSI disk (as desired)
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274 | and the corresponding SCSI generic device (incorrectly) by vendor.
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275 | Find the offending rule and make it more specific, with the help of
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276 | <command>udevadm info</command>.</para>
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277 |
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278 | </sect3>
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279 |
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280 | <sect3>
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281 | <title>Udev rule works unreliably</title>
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282 |
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283 | <para>This may be another manifestation of the previous problem. If not,
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284 | and your rule uses <systemitem class="filesystem">sysfs</systemitem>
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285 | attributes, it may be a kernel timing issue, to be fixed in later kernels.
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286 | For now, you can work around it by creating a rule that waits for the used
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287 | <systemitem class="filesystem">sysfs</systemitem> attribute and appending
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288 | it to the <filename>/etc/udev/rules.d/10-wait_for_sysfs.rules</filename>
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289 | file. Please notify the CLFS Development list if you do so and it
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290 | helps.</para>
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291 |
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292 | </sect3>
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293 |
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294 | <sect3>
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295 | <title>Device naming order changes randomly after rebooting</title>
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296 |
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297 | <para>This is due to the fact that Udev, by design, handles uevents and
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298 | loads modules in parallel, and thus in an unpredictable order. This will
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299 | never be <quote>fixed</quote>. You should not rely upon the kernel device
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300 | names being stable. Instead, create your own rules that make symlinks with
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301 | stable names based on some stable attributes of the device, such as a
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302 | serial number or the output of various *_id utilities installed by Udev.
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303 | See <xref linkend="ch-config-symlinks"/> and
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304 | <xref linkend="chapter-network"/> for examples.</para>
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305 |
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306 | </sect3>
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307 |
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308 | </sect2>
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309 |
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310 | <sect2>
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311 | <title>Useful Reading</title>
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312 |
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313 | <para>Additional helpful documentation is available at the following
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314 | sites:</para>
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315 |
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316 | <itemizedlist>
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317 |
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318 | <listitem>
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319 | <para remap="verbatim">A Userspace Implementation of <systemitem class="filesystem">devfs</systemitem>
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320 | <ulink url="http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf"/></para>
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321 | </listitem>
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322 |
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323 | <listitem>
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324 | <para remap="verbatim">The <systemitem class="filesystem">sysfs</systemitem> Filesystem
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325 | <ulink url="http://www.kernel.org/pub/linux/kernel/people/mochel/doc/papers/ols-2005/mochel.pdf"/></para>
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326 | </listitem>
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327 |
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328 | </itemizedlist>
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329 |
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330 | </sect2>
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331 |
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332 | </sect1>
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