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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></indexterm> |
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16 | |
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17 | <para>In <xref linkend="chapter-building-system"/>, we installed the Udev |
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18 | package. Before we go into the details regarding how this works, a brief |
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19 | history of previous methods of handling devices is in order.</para> |
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20 | |
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21 | <para>Linux systems in general traditionally use a static device creation |
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22 | method, whereby a great many device nodes are created under <filename |
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23 | class="directory">/dev</filename> (sometimes literally thousands of nodes), |
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24 | regardless of whether the corresponding hardware devices actually exist. |
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25 | This is typically done via a <command>MAKEDEV</command> script, which |
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26 | contains a number of calls to the <command>mknod</command> program with |
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27 | the relevant major and minor device numbers for every possible device that |
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28 | might exist in the world. Using the Udev method, only those devices which |
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29 | are detected by the kernel get device nodes created for them. Because |
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30 | these device nodes will be created each time the system boots, they will |
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31 | be stored on a <systemitem class="filesystem">tmpfs</systemitem> (a virtual |
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32 | file system that resides entirely in system memory). Device nodes do not |
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33 | require much space, so the memory that is used is negligible.</para> |
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34 | |
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35 | <sect2> |
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36 | <title>History</title> |
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37 | |
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38 | <para>In February 2000, a new filesystem called <systemitem |
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39 | class="filesystem">devfs</systemitem> was merged into the 2.3.46 kernel |
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40 | and was made available during the 2.4 series of stable kernels. Although |
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41 | it was present in the kernel source itself, this method of creating |
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42 | devices dynamically never received overwhelming support from the core |
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43 | kernel developers.</para> |
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44 | |
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45 | <para>The main problem with the approach adopted by <systemitem |
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46 | class="filesystem">devfs</systemitem> was the way it handled |
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47 | device detection, creation, and naming. The latter issue, that of |
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48 | device node naming, was perhaps the most critical. It is generally |
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49 | accepted that if device names are allowed to be configurable, then |
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50 | the device naming policy should be up to a system administrator, not |
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51 | imposed on them by any particular developer(s). The <systemitem |
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52 | class="filesystem">devfs</systemitem> file system also suffers from race |
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53 | conditions that are inherent in its design and cannot be fixed |
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54 | without a substantial revision to the kernel. It has also been marked |
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55 | as deprecated due to a lack of recent maintenance.</para> |
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56 | |
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57 | <para>With the development of the unstable 2.5 kernel tree, later |
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58 | released as the 2.6 series of stable kernels, a new virtual filesystem |
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59 | called <systemitem class="filesystem">sysfs</systemitem> came to be. The |
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60 | job of <systemitem class="filesystem">sysfs</systemitem> is to export a |
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61 | view of the system's hardware configuration to userspace processes. With |
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62 | this userspace-visible representation, the possibility of seeing a |
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63 | userspace replacement for <systemitem class="filesystem">devfs</systemitem> |
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64 | became much more realistic.</para> |
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65 | |
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66 | </sect2> |
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67 | |
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68 | <sect2> |
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69 | <title>Udev Implementation</title> |
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70 | |
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71 | <para>The <systemitem class="filesystem">sysfs</systemitem> filesystem |
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72 | was mentioned briefly above. One may wonder how <systemitem |
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73 | class="filesystem">sysfs</systemitem> knows about the devices present |
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74 | on a system and what device numbers should be used for them. Drivers |
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75 | that have been compiled into the kernel directly register their objects |
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76 | with <systemitem class="filesystem">sysfs</systemitem> as they are |
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77 | detected by the kernel. For drivers compiled as modules, this |
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78 | registration will happen when the module is loaded. Once the |
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79 | <systemitem class="filesystem">sysfs</systemitem> filesystem is mounted |
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80 | (on <filename class="directory">/sys</filename>), data which the built-in |
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81 | drivers registered with <systemitem class="filesystem">sysfs</systemitem> |
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82 | are available to userspace processes and to <command>udev</command> for |
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83 | device node creation.</para> |
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84 | |
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85 | <para>The <command>S10udev</command> initscript takes care of creating |
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86 | these device nodes when Linux is booted. This script starts by registering |
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87 | <command>/sbin/udevsend</command> as a hotplug event handler. Hotplug |
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88 | events (discussed below) are not usually generated during this stage, |
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89 | but <command>udev</command> is registered just in case they do occur. |
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90 | The <command>udevstart</command> program then walks through the |
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91 | <systemitem class="filesystem">/sys</systemitem> filesystem and creates |
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92 | devices under <filename class="directory">/dev</filename> that match the |
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93 | descriptions. For example, <filename>/sys/class/tty/vcs/dev</filename> |
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94 | contains the string <quote>7:0</quote> This string is used by |
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95 | <command>udevstart</command> to create <filename>/dev/vcs</filename> |
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96 | with major number <emphasis>7</emphasis> and minor <emphasis>0</emphasis>. |
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97 | The names and permissions of the nodes created under the <filename |
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98 | class="directory">/dev</filename> directory are configured according to |
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99 | the rules specified in the files within the <filename |
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100 | class="directory">/etc/udev/rules.d/</filename> directory. These are |
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101 | numbered in a similar fashion to the CLFS-Bootscripts package. If |
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102 | <command>udev</command> can't find a rule for the device it is creating, |
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103 | it will default permissions to <emphasis>660</emphasis> and ownership to |
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104 | <emphasis>root:root</emphasis>.</para> |
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105 | |
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106 | <para>Once the above stage is complete, all devices that were already |
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107 | present and have compiled-in drivers will be available for use. This |
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108 | leads us to the devices that have modular drivers.</para> |
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109 | |
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110 | <para>Earlier, we mentioned the concept of a <quote>hotplug event |
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111 | handler.</quote> When a new device connection is detected by the kernel, |
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112 | the kernel will generate a hotplug event and look at the file |
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113 | <filename>/proc/sys/kernel/hotplug</filename> to determine the userspace |
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114 | program that handles the device's connection. The <command>udev</command> |
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115 | bootscript registered <command>udevsend</command> as this handler. When |
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116 | these hotplug events are generated, the kernel will tell |
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117 | <command>udev</command> to check the <filename |
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118 | class="directory">/sys</filename> filesystem for the information |
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119 | pertaining to this new device and create the <filename |
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120 | class="directory">/dev</filename> entry for it.</para> |
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121 | |
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122 | <para>This brings us to one problem that exists with |
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123 | <command>udev</command>, and likewise with <systemitem |
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124 | class="filesystem">devfs</systemitem> before it. It is commonly |
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125 | 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 |
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128 | the appropriate kernel module to load. With <command>udev</command>, |
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129 | this method will not work because the device node does not exist until |
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130 | the module is loaded. To solve this, the <command>S05modules</command> |
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131 | bootscript was added to the CLFS-Bootscripts package, along with the |
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132 | <filename>/etc/sysconfig/modules</filename> file. By adding module |
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133 | names to the <filename>modules</filename> file, these modules will be |
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134 | loaded when the computer starts up. This allows <command>udev</command> |
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135 | to detect the devices 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 |
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138 | of device nodes, the process of creating devices may take a few |
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139 | seconds to complete. This means that some device nodes may not be |
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140 | immediately accessible.</para> |
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141 | |
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142 | </sect2> |
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143 | |
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144 | <sect2> |
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145 | <title>Handling Hotpluggable/Dynamic Devices</title> |
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146 | |
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147 | <para>When you plug in a device, such as a Universal Serial Bus (USB) |
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148 | MP3 player, the kernel recognizes that the device is now connected and |
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149 | generates a hotplug event. If the driver is already loaded (either |
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150 | because it was compiled into the kernel or because it was loaded via |
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151 | the <command>S05modules</command> bootscript), <command>udev</command> |
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152 | will be called upon to create the relevant device node(s) according to |
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153 | the <systemitem class="filesystem">sysfs</systemitem> data available in |
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154 | <filename class="directory">/sys</filename>.</para> |
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155 | |
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156 | <para>If the driver for the just plugged in device is available as a |
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157 | module but currently unloaded, the Hotplug package will load the |
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158 | appropriate module and make this device available by creating the |
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159 | device node(s) for it.</para> |
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160 | |
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161 | </sect2> |
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162 | |
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163 | <sect2> |
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164 | <title>Problems with Creating Devices</title> |
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165 | |
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166 | <para>There are a few known problems when it comes to automatically |
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167 | creating device nodes:</para> |
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168 | |
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169 | <para>1) A kernel driver may not export its data to <systemitem |
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170 | class="filesystem">sysfs</systemitem>.</para> |
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171 | |
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172 | <para>This is most common with third party drivers from outside the |
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173 | kernel tree. Udev will be unable to automatically create device nodes |
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174 | for such drivers. Use the <filename>/etc/sysconfig/createfiles</filename> |
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175 | configuration file to manually create the devices. Consult the |
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176 | <filename>devices.txt</filename> file inside the kernel documentation |
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177 | or the documentation for that driver to find the proper major/minor |
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178 | numbers.</para> |
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179 | |
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180 | <para>2) A non-hardware device is required. This is most common with |
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181 | the Advanced Linux Sound Architecture (ALSA) project's Open Sound |
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182 | System (OSS) compatibility module. These types of devices can be |
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183 | handled in one of two ways:</para> |
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184 | |
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185 | <itemizedlist> |
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186 | <listitem> |
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187 | <para>Adding the module names to |
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188 | <filename>/etc/sysconfig/modules</filename></para> |
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189 | </listitem> |
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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 |
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194 | module, 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><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 | </itemizedlist> |
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206 | |
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207 | </sect2> |
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208 | |
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209 | <sect2> |
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210 | <title>Useful Reading</title> |
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211 | |
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212 | <para>Additional helpful documentation is available at the following |
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213 | sites:</para> |
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214 | |
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215 | <itemizedlist> |
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216 | <listitem> |
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217 | <para remap="verbatim">A Userspace Implementation of <systemitem class="filesystem">devfs</systemitem> |
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218 | <ulink url="http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf"/></para> |
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219 | </listitem> |
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220 | <listitem> |
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221 | <para remap="verbatim">udev FAQ |
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222 | <ulink url="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev-FAQ"/></para> |
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223 | </listitem> |
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224 | <listitem> |
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225 | <para remap="verbatim">The Linux Kernel Driver Model |
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226 | <ulink url="http://public.planetmirror.com/pub/lca/2003/proceedings/papers/Patrick_Mochel/Patrick_Mochel.pdf"/></para> |
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227 | </listitem> |
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228 | </itemizedlist> |
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229 | |
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230 | </sect2> |
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231 | |
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232 | </sect1> |
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