1 | <?xml version="1.0" encoding="ISO-8859-1"?>
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2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
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3 | "http://www.oasis-open.org/docbook/xml/4.4/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-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 an LFS 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 the Udev
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19 | package. Before we go into the details regarding how this works,
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20 | 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 | <para>Linux systems in general traditionally use a static device creation
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24 | method, whereby a great many device nodes are created under <filename
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25 | class="directory">/dev</filename> (sometimes literally thousands of nodes),
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26 | regardless of whether the corresponding hardware devices actually exist. This is
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27 | typically done via a <command>MAKEDEV</command> script, which contains a number
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28 | of calls to the <command>mknod</command> program with the relevant major and
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29 | minor device numbers for every possible device that might exist in the world.
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30 | Using the Udev method, only those devices which are detected by the kernel get
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31 | device nodes created for them. Because these device nodes will be created each
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32 | time the system boots, they will be stored on a <systemitem
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33 | class="filesystem">tmpfs</systemitem> file system (a virtual file system that
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34 | resides entirely in system memory). Device nodes do not require much space, so
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35 | the memory that is used is negligible.</para>
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36 |
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37 | <sect2>
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38 | <title>History</title>
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39 |
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40 | <para>In February 2000, a new filesystem called <systemitem
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41 | class="filesystem">devfs</systemitem> was merged into the 2.3.46 kernel
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42 | and was made available during the 2.4 series of stable kernels. Although
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43 | it was present in the kernel source itself, this method of creating devices
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44 | dynamically never received overwhelming support from the core kernel
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45 | developers.</para>
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46 |
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47 | <para>The main problem with the approach adopted by <systemitem
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48 | class="filesystem">devfs</systemitem> was the way it handled device
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49 | detection, creation, and naming. The latter issue, that of device node
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50 | naming, was perhaps the most critical. It is generally accepted that if
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51 | device names are allowed to be configurable, then the device naming policy
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52 | should be up to a system administrator, not imposed on them by any
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53 | particular developer(s). The <systemitem class="filesystem">devfs</systemitem>
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54 | file system also suffers from race conditions that are inherent in its design
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55 | and cannot be fixed without a substantial revision to the kernel. It has also
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56 | been marked as deprecated due to a lack of recent maintenance.</para>
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57 |
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58 | <para>With the development of the unstable 2.5 kernel tree, later released as
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59 | the 2.6 series of stable kernels, a new virtual filesystem called <systemitem
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60 | class="filesystem">sysfs</systemitem> came to be. The job of <systemitem
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61 | class="filesystem">sysfs</systemitem> is to export a view of the system's
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62 | hardrware configuration to userspace processes. With this userspace-visible
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63 | representation, the possibility of seeing a userspace replacement for
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64 | <systemitem class="filesystem">devfs</systemitem> became much more
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65 | realistic.</para>
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66 |
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67 | </sect2>
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68 |
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69 | <sect2>
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70 | <title>Udev Implementation</title>
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71 |
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72 | <para>The <systemitem class="filesystem">sysfs</systemitem> filesystem was
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73 | mentioned briefly above. One may wonder how <systemitem
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74 | class="filesystem">sysfs</systemitem> knows about the devices present on
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75 | a system and what device numbers should be used for them. Drivers that have
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76 | been compiled into the kernel directly register their objects with
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77 | <systemitem class="filesystem">sysfs</systemitem> as they are detected by
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78 | the kernel. For drivers compiled as modules, this registration will happen
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79 | when the module is loaded. Once the <systemitem
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80 | class="filesystem">sysfs</systemitem> filesystem is mounted (on <filename
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81 | class="directory">/sys</filename>), data which the built-in drivers
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82 | registered with <systemitem class="filesystem">sysfs</systemitem> are
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83 | available to userspace processes and to <command>udev</command> for device
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84 | node creation.</para>
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85 |
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86 | <para>The <command>S10udev</command> initscript takes care of creating
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87 | these device nodes when Linux is booted. This script starts by registering
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88 | <command>/sbin/udevsend</command> as a hotplug event handler. Hotplug events
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89 | (discussed below) are not usually generated during this stage, but
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90 | <command>udev</command> is registered just in case they do occur. The
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91 | <command>udevstart</command> program then walks through the <systemitem
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92 | class="filesystem">/sys</systemitem> filesystem and creates devices under
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93 | <filename class="directory">/dev</filename> that match the descriptions.
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94 | For example, <filename>/sys/class/tty/vcs/dev</filename> contains the
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95 | string <quote>7:0</quote> This string is used by <command>udevstart</command>
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96 | to create <filename>/dev/vcs</filename> with major number
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97 | <emphasis>7</emphasis> and minor <emphasis>0</emphasis>. The names and
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98 | permissions of the nodes created under the <filename
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99 | class="directory">/dev</filename> directory are configured according to the
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100 | rules specified in the files within the <filename
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101 | class="directory">/etc/udev/rules.d/</filename> directory. These are
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102 | numbered in a similar fashion to the LFS-Bootscripts package. If
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103 | <command>udev</command> can't find a rule for the device it is creating,
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104 | it will default permissions to <emphasis>660</emphasis> and ownership to
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105 | <emphasis>root:root</emphasis>.</para>
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106 |
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107 | <para>Once the above stage is complete, all devices that were already present
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108 | and have compiled-in drivers will be available for use. This leads us to the
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109 | devices that have modular drivers.</para>
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110 |
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111 | <para>Earlier, we mentioned the concept of a <quote>hotplug event
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112 | handler.</quote> When a new device connection is detected by the kernel,
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113 | the kernel will generate a hotplug event and look at the file
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114 | <filename>/proc/sys/kernel/hotplug</filename> to determine the userspace
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115 | program that handles the device's connection. The <command>udev</command>
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116 | bootscript registered <command>udevsend</command> as this handler. When
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117 | these hotplug events are generated, the kernel will tell
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118 | <command>udev</command> to check the <filename
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119 | class="directory">/sys</filename> filesystem for the information pertaining
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120 | to this new device and create the <filename class="directory">/dev</filename>
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121 | entry for it.</para>
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122 |
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123 | <para>This brings us to one problem that exists with <command>udev</command>,
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124 | and likewise with <systemitem class="filesystem">devfs</systemitem> before it.
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125 | It is commonly referred to as the <quote>chicken and egg</quote> problem. Most
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126 | Linux distributions handle loading modules via entries in
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127 | <filename>/etc/modules.conf</filename>. Access to a device node causes the
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128 | appropriate kernel module to load. With <command>udev</command>, this method
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129 | will not work because the device node does not exist until the module is loaded.
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130 | To solve this, the <command>S05modules</command> bootscript was added to the
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131 | LFS-Bootscripts package, along with the
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132 | <filename>/etc/sysconfig/modules</filename> file. By adding module names to the
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133 | <filename>modules</filename> file, these modules will be loaded when the
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134 | computer starts up. This allows <command>udev</command> to detect the devices
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135 | and create the appropriate device nodes.</para>
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136 |
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137 | <para>Note that on slower machines or for drivers that create a lot of device
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138 | nodes, the process of creating devices may take a few seconds to complete.
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139 | This means that some device nodes may not be immediately accessible.</para>
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140 |
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141 | </sect2>
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142 |
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143 | <sect2>
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144 | <title>Handling Hotpluggable/Dynamic Devices</title>
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145 |
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146 | <para>When you plug in a device, such as a Universal Serial Bus (USB) MP3
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147 | player, the kernel recognizes that the device is now connected and generates
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148 | a hotplug event. If the driver is already loaded (either because it was
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149 | compiled into the kernel or because it was loaded via the
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150 | <command>S05modules</command> bootscript), <command>udev</command> will be
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151 | called upon to create the relevant device node(s) according to the
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152 | <systemitem class="filesystem">sysfs</systemitem> data available in
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153 | <filename class="directory">/sys</filename>.</para>
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154 |
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155 | <para>If the driver for the just plugged in device is available as a module but
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156 | currently unloaded, the Hotplug package will load the appropriate module
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157 | and make this device available by creating the device node(s) for it.</para>
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158 |
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159 | </sect2>
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160 |
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161 | <sect2>
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162 | <title>Problems with Creating Devices</title>
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163 |
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164 | <para>There are a few known problems when it comes to automatically creating
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165 | device nodes:</para>
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166 |
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167 | <para>1) A kernel driver may not export its data to <systemitem
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168 | class="filesystem">sysfs</systemitem>.</para>
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169 |
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170 | <para>This is most common with third party drivers from outside the kernel
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171 | tree. Udev will be unable to automatically create device nodes for such
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172 | drivers. Use the <filename>/etc/sysconfig/createfiles</filename>
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173 | configuration file to manually create the devices. Consult the
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174 | <filename>devices.txt</filename> file inside the kernel documentation or
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175 | the documentation for that driver to find the proper major/minor
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176 | numbers.</para>
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177 |
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178 | <para>2) A non-hardware device is required. This is most common with
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179 | the Advanced Linux Sound Architecture (ALSA) project's Open Sound
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180 | System (OSS) compatibility module. These types of devices can be
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181 | handled in one of two ways:</para>
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182 |
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183 | <itemizedlist>
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184 |
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185 | <listitem>
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186 | <para>Adding the module names to
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187 | <filename>/etc/sysconfig/modules</filename></para>
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188 | </listitem>
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189 |
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190 | <listitem>
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191 | <para>Using an <quote>install</quote> line in
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192 | <filename>/etc/modprobe.conf</filename>. This tells the
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193 | <command>modprobe</command> command <quote>when loading this module,
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194 | also load this other module, at the same time.</quote>
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195 | For example:</para>
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196 |
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197 | <screen role="nodump"><userinput>install snd-pcm modprobe -i snd-pcm ; modprobe \
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198 | snd-pcm-oss ; true</userinput></screen>
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199 |
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200 | <para>This will cause the system to load both the
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201 | <emphasis>snd-pcm</emphasis> and <emphasis>snd-pcm-oss</emphasis>
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202 | modules when any request is made to load the driver
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203 | <emphasis>snd-pcm</emphasis>.</para>
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204 | </listitem>
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205 |
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206 | </itemizedlist>
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207 |
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208 | </sect2>
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209 |
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210 | <sect2>
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211 | <title>Useful Reading</title>
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212 |
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213 | <para>Additional helpful documentation is available at the following
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214 | sites:</para>
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215 |
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216 | <itemizedlist>
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217 |
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218 | <listitem>
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219 | <para>A Userspace Implementation of <systemitem class="filesystem">devfs</systemitem>
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220 | <ulink url="http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf"/></para>
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221 | </listitem>
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222 |
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223 | <listitem>
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224 | <para>udev FAQ
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225 | <ulink url="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev-FAQ"/></para>
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226 | </listitem>
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227 |
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228 | <listitem>
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229 | <para>The Linux Kernel Driver Model
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230 | <ulink url="http://public.planetmirror.com/pub/lca/2003/proceedings/papers/Patrick_Mochel/Patrick_Mochel.pdf"/></para>
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231 | </listitem>
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232 |
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233 | </itemizedlist>
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234 |
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235 | </sect2>
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236 |
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237 | </sect1>
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