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
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27 | is typically done via a <command>MAKEDEV</command> script, which contains a
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28 | number of calls to the <command>mknod</command> program with the relevant
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29 | major and minor device numbers for every possible device that might exist in
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30 | the world.</para>
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31 |
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32 | <para>Using the Udev method, only those devices which are detected by the
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33 | kernel get device nodes created for them. Because these device nodes will be
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34 | created each time the system boots, they will be stored on a <systemitem
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35 | class="filesystem">tmpfs</systemitem> file system (a virtual file system that
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36 | resides entirely in system memory). Device nodes do not require much space, so
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37 | the memory that is used is negligible.</para>
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38 |
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39 | <sect2>
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40 | <title>History</title>
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41 |
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42 | <para>In February 2000, a new filesystem called <systemitem
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43 | class="filesystem">devfs</systemitem> was merged into the 2.3.46 kernel
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44 | and was made available during the 2.4 series of stable kernels. Although
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45 | it was present in the kernel source itself, this method of creating devices
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46 | dynamically never received overwhelming support from the core kernel
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47 | developers.</para>
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48 |
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49 | <para>The main problem with the approach adopted by <systemitem
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50 | class="filesystem">devfs</systemitem> was the way it handled device
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51 | detection, creation, and naming. The latter issue, that of device node
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52 | naming, was perhaps the most critical. It is generally accepted that if
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53 | device names are allowed to be configurable, then the device naming policy
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54 | should be up to a system administrator, not imposed on them by any
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55 | particular developer(s). The <systemitem
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56 | class="filesystem">devfs</systemitem> file system also suffers from race
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57 | conditions that are inherent in its design and cannot be fixed without a
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58 | substantial revision to the kernel. It has also been marked as deprecated
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59 | due to a lack of recent maintenance.</para>
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60 |
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61 | <para>With the development of the unstable 2.5 kernel tree, later released
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62 | as the 2.6 series of stable kernels, a new virtual filesystem called
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63 | <systemitem class="filesystem">sysfs</systemitem> came to be. The job of
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64 | <systemitem class="filesystem">sysfs</systemitem> is to export a view of
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65 | the system's hardware configuration to userspace processes. With this
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66 | userspace-visible representation, the possibility of seeing a userspace
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67 | replacement for <systemitem class="filesystem">devfs</systemitem> became
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68 | much more realistic.</para>
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69 |
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70 | </sect2>
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71 |
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72 | <sect2>
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73 | <title>Udev Implementation</title>
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74 |
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75 | <sect3>
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76 | <title>Sysfs</title>
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77 |
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78 | <para>The <systemitem class="filesystem">sysfs</systemitem> filesystem was
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79 | mentioned briefly above. One may wonder how <systemitem
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80 | class="filesystem">sysfs</systemitem> knows about the devices present on
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81 | a system and what device numbers should be used for them. Drivers that
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82 | have been compiled into the kernel directly register their objects with
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83 | <systemitem class="filesystem">sysfs</systemitem> as they are detected by
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84 | the kernel. For drivers compiled as modules, this registration will happen
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85 | when the module is loaded. Once the <systemitem
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86 | class="filesystem">sysfs</systemitem> filesystem is mounted (on <filename
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87 | class="directory">/sys</filename>), data which the built-in drivers
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88 | registered with <systemitem class="filesystem">sysfs</systemitem> are
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89 | available to userspace processes and to <command>udev</command> for device
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90 | node creation.</para>
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91 | </sect3>
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92 |
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93 | <sect3>
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94 | <title>Udev Bootscript</title>
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95 |
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96 | <para>The <command>S10udev</command> initscript takes care of creating
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97 | device nodes when Linux is booted. The script starts by unsetting the
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98 | hotplug event handler from the default of <command>/sbin/hotplug</command>
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99 | This is done because, instead of the kernel calling out to an external
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100 | binary, <command>udev</command> will listen on a netlink socket for
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101 | hotplug events that the kernel raises. The bootscript copies any static
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102 | device nodes that exist in <filename
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103 | class="directory">/lib/udev/devices</filename> to <filename
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104 | class="directory">/dev</filename>. This is necessary because some devices
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105 | are needed before the dynamic device handling processes are available
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106 | during the early stages of booting a system. Creating static device nodes
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107 | in <filename class="directory">/lib/udev/devices</filename> also provides
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108 | an easy workaround for devices that are not supported by the dynamic
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109 | device handling infrastructure. The bootscript then starts the Udev
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110 | daemon, <command>udevd</command>, which will act on any hotplug events it
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111 | receives. Finally, the bootscript "coldplugs" any devices that
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112 | have already been registered with the kernel by forcing them to raise
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113 | hotplug events which <command>udevd</command> will then handle.</para>
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114 | </sect3>
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115 |
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116 | <sect3>
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117 | <title>Device Node Creation</title>
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118 |
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119 | <para>To obtain the right major and minor number for a device, Udev relies
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120 | on the information provided by <systemitem
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121 | class="filesystem">sysfs</systemitem> in <filename
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122 | class="directory">/sys</filename>. For example,
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123 | <filename>/sys/class/tty/vcs/dev</filename> contains the string
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124 | <quote>7:0</quote>. This string is used by <command>udevd</command>
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125 | to create a device node with major number <emphasis>7</emphasis> and minor
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126 | <emphasis>0</emphasis>. The names and permissions of the nodes created
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127 | under the <filename class="directory">/dev</filename> directory are
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128 | determined by rules specified in the files within the <filename
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129 | class="directory">/etc/udev/rules.d/</filename> directory. These are
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130 | numbered in a similar fashion to the LFS-Bootscripts package. If
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131 | <command>udevd</command> can't find a rule for the device it is creating,
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132 | it will default permissions to <emphasis>660</emphasis> and ownership to
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133 | <emphasis>root:root</emphasis>. Documentation on the syntax of the Udev
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134 | rules configuration files are available in
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135 | <filename>/usr/share/doc/udev-&udev-version;/index.html</filename></para>
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136 | </sect3>
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137 |
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138 | <sect3>
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139 | <title>Module Loading</title>
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140 |
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141 | <para>If a device driver has been compiled as a module, the rules that
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142 | LFS installs will cause <command>udevd</command> to call out to
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143 | <command>/sbin/modprobe</command> with the name of the corresponding
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144 | module, thereby loading the driver.</para>
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145 | </sect3>
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146 |
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147 | <sect3>
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148 | <title>Handling Hotpluggable/Dynamic Devices</title>
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149 |
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150 | <para>When you plug in a device, such as a Universal Serial Bus (USB) MP3
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151 | player, the kernel recognizes that the device is now connected and
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152 | generates a hotplug event. This hotplug event is then handled by
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153 | <command>udevd</command> as described above.</para>
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154 | </sect3>
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155 |
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156 | <!-- FIXME: These are questions Matt thought of while rewriting this page
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157 | to reflect the hotplug-less setup but didn't have time to investigate
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158 | straight away.
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159 | <sect3>
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160 | <title>Questions?</title>
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161 |
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162 | <para>7.4.2.3: Are default ownership/permissions still 0660 root:root? I
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163 | thought they'd changed, but can't be sure. Running without a config file
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164 | will prove this pretty quickly.</para>
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165 |
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166 | <para>7.4.2.4: How does <command>udevd</command> know which driver to
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167 | load, i.e. the correct module name? Is it in the hotplug event? I don't
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168 | think it can be in /sys as that won't be populated yet (it's the driver
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169 | itself that populates /sys, after all).</para>
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170 |
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171 | <para>Is the S05modules script still required? If so, what are the use
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172 | cases for it?</para>
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173 |
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174 | </sect3> -->
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175 |
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176 | </sect2>
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177 |
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178 | <sect2>
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179 | <title>Problems with Creating Devices</title>
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180 |
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181 | <para>There are a few known problems when it comes to automatically creating
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182 | device nodes:</para>
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183 |
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184 | <para>1) A kernel driver might not export its data to <systemitem
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185 | class="filesystem">sysfs</systemitem>.</para>
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186 |
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187 | <para>This is most common with third party drivers from outside the kernel
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188 | tree. Udev will be unable to automatically create device nodes for such
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189 | drivers. Create a static device node in
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190 | <filename>/lib/udev/devices</filename> with the appropriate major/minor
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191 | numbers (see the file <filename>devices.txt</filename> inside the kernel
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192 | documentation or the documentation provided by the third party driver
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193 | vendor). The static device node will be copied to
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194 | <filename class="directory">/dev</filename> by the
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195 | <command>S10udev</command> bootscript.</para>
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196 |
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197 | <para>2) A non-hardware device is required. This is most common with
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198 | the Advanced Linux Sound Architecture (ALSA) project's Open Sound
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199 | System (OSS) compatibility module. These types of devices can be
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200 | handled in one of two ways:</para>
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201 |
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202 | <itemizedlist>
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203 |
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204 | <listitem>
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205 | <para>Adding the module names to
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206 | <filename>/etc/sysconfig/modules</filename></para>
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207 | </listitem>
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208 |
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209 | <listitem>
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210 | <para>Using an <quote>install</quote> line in
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211 | <filename>/etc/modprobe.conf</filename>. This tells the
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212 | <command>modprobe</command> command <quote>when loading this module,
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213 | also load this other module, at the same time.</quote>
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214 | For example:</para>
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215 |
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216 | <screen role="nodump"><userinput>install snd-pcm modprobe -i snd-pcm ; modprobe \
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217 | snd-pcm-oss ; true</userinput></screen>
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218 |
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219 | <para>This will cause the system to load both the
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220 | <emphasis>snd-pcm</emphasis> and <emphasis>snd-pcm-oss</emphasis>
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221 | modules when any request is made to load the driver
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222 | <emphasis>snd-pcm</emphasis>.</para>
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223 | </listitem>
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224 |
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225 | </itemizedlist>
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226 |
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227 | </sect2>
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228 |
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229 | <sect2>
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230 | <title>Useful Reading</title>
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231 |
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232 | <para>Additional helpful documentation is available at the following
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233 | sites:</para>
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234 |
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235 | <itemizedlist>
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236 |
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237 | <listitem>
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238 | <para>A Userspace Implementation of <systemitem class="filesystem">devfs</systemitem>
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239 | <ulink url="http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf"/></para>
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240 | </listitem>
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241 |
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242 | <listitem>
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243 | <para>udev FAQ
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244 | <ulink url="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev-FAQ"/></para>
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245 | </listitem>
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246 |
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247 | <listitem>
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248 | <para>The Linux Kernel Driver Model
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249 | <ulink url="http://public.planetmirror.com/pub/lca/2003/proceedings/papers/Patrick_Mochel/Patrick_Mochel.pdf"/></para>
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250 | </listitem>
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251 |
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252 | </itemizedlist>
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253 |
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254 | </sect2>
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255 |
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256 | </sect1>
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