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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-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-scripts-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-scripts-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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