[5ba3d1d] | 1 | <?xml version="1.0" encoding="ISO-8859-1"?>
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| 2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN" "http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd" [
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| 3 | <!ENTITY % general-entities SYSTEM "../general.ent">
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| 4 | %general-entities;
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| 5 | ]>
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| 6 | <sect1 id="ch-scripts-udev">
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[0cb7f8b] | 7 | <title>Device and Module handling on an LFS system</title>
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[5ba3d1d] | 8 | <?dbhtml filename="udev.html"?>
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| 9 |
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| 10 | <indexterm zone="ch-scripts-udev">
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| 11 | <primary sortas="a-Udev">Udev</primary>
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| 12 | <secondary>usage</secondary></indexterm>
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| 13 |
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| 14 | <para>In <xref linkend="chapter-building-system"/> we installed the udev
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| 15 | package. Before we go into the details regarding how this does its
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| 16 | job, a brief history of previous methods of handling devices is in
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| 17 | order.</para>
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| 18 |
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[0cb7f8b] | 19 | <para>Linux systems in general traditionally use a static device
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| 20 | creation method, whereby a great many device nodes are created under
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| 21 | <filename class="directory">/dev</filename> (sometimes literally
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| 22 | thousands of nodes), regardless of whether the corresponding hardware
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| 23 | devices actually exist. This is typically done via a
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| 24 | <command>MAKEDEV</command> script, which simply contains a number of
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| 25 | calls to the 'mknod' program with the relevant major and minor device
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| 26 | numbers for every possible device that might exist in the world. Using
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| 27 | the udev method, only those devices which are detected by the kernel
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| 28 | get device nodes created for them. As these device nodes will be
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| 29 | created each time the system boots, they will be stored on a
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| 30 | <systemitem class="filesystem">ramfs</systemitem> (a file system that
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[5ba3d1d] | 31 | resides entirely in memory and does not take up any disk space).
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| 32 | Device nodes do not require much disk space, so the memory that is
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| 33 | used in negligable.</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 Febraury of 2000, a new filesystem called <systemitem
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| 39 | class="filesystem">devfs</systemitem> was merged into the 2.3.46
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| 40 | kernel and was made generally available during the 2.4 series of
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| 41 | stable kernels. Although it was present in the kernel source itself,
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[0cb7f8b] | 42 | this method of creating devices dynamically never received
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[0aa61a48] | 43 | overwhelming support from the core kernel developers.</para>
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[0cb7f8b] | 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 that 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 you are to allow device names 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 upon them by any particular developer(s). <systemitem
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| 52 | class="filesystem">devfs</systemitem> 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, came a new virtual
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| 59 | filesystem called <systemitem class="filesystem">sysfs</systemitem>.
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| 60 | The job of <systemitem class="filesystem">sysfs</systemitem> is to
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| 61 | export a view of the system's structure 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
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| 64 | class="filesystem">devfs</systemitem> became much more
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[0aa61a48] | 65 | realistic.</para>
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| 66 | </sect2>
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[5ba3d1d] | 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. Drivers that
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[0cb7f8b] | 75 | have been compiled into the kernel directly register their objects
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[5ba3d1d] | 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 will
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| 78 | happen when the module is loaded. Once the <systemitem
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| 79 | class="filesystem">sysfs</systemitem> filesystem is mounted (on
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| 80 | <filename class="directory">/sys</filename>), the data which the
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| 81 | built-in drivers registered with <systemitem
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| 82 | class="filesystem">sysfs</systemitem> is available to userspace
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[0cb7f8b] | 83 | processes and to <command>udev</command> for device node creation.</para>
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[5ba3d1d] | 84 |
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| 85 | <para>The <command>S10udev</command> initscript takes care of creating
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[0cb7f8b] | 86 | these device nodes when Linux is booted. This script starts with
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| 87 | registering <command>/sbin/udev</command> as a hotplug event handler.
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| 88 | Hotplug events (discussed below) should not be generated during this
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| 89 | stage, but <command>udev</command> is registered just in case they do
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| 90 | occur. The <command>udevstart</command> program then walks through
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| 91 | the <systemitem class="filesystem">/sys</systemitem> filesystem and
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[5ba3d1d] | 92 | creates devices under <filename class="directory">/dev</filename> that
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| 93 | match the descriptions. For example,
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| 94 | <filename>/sys/class/tty/vcs/dev</filename> contains the string
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| 95 | <quote>7:0</quote>. <command>udevstart</command> uses this to create
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| 96 | <filename>/dev/vcs</filename> with major number <emphasis>7</emphasis>
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| 97 | and minor <emphasis>0</emphasis>. The permissions of each and every
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| 98 | device that <command>udevstart</command> creates are set using files
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| 99 | from the <filename
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| 100 | class="directory">/etc/udev.d/permissions.d/</filename> directory.
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[0cb7f8b] | 101 | These are numbered in a similar fashion to the LFS bootscripts. If
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| 102 | <command>udev</command> cannot find a permissions file for the device
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| 103 | it is creating, it will default permissions to
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| 104 | <emphasis>600</emphasis> and ownership to
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[5ba3d1d] | 105 | <emphasis>root:root</emphasis>. The names of the nodes created under
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| 106 | the <filename class="directory">/dev</filename> directory are
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| 107 | configured according to the rules specified in the files within the
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| 108 | <filename class="directory">/etc/udev/rules.d/</filename>
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| 109 | directory.</para>
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| 110 |
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| 111 | <para>Once the above stage is complete, all devices that were already
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| 112 | present and have compiled-in drivers will be available for use. How
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| 113 | about those devices that have modular drivers?</para>
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| 114 |
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| 115 | <para>We mentioned earlier the concept of a <quote>hotplug event
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| 116 | handler</quote>. When a new device connection is detected by the
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[0cb7f8b] | 117 | kernel, the kernel will generate a hotplug event and look at the file
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[5ba3d1d] | 118 | <filename>/proc/sys/kernel/hotplug</filename> to find out the
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[0cb7f8b] | 119 | userspace program to handle that device's connection. The
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[5ba3d1d] | 120 | <command>udev</command> initscript registered <command>udev</command>
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| 121 | as this handler. When these hotplug events are generated, the kernel
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| 122 | will tell <command>udev</command> to check the <filename
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| 123 | class="directory">/sys</filename> filesystem for the information
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| 124 | pertaining to this new device, and create the <filename
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| 125 | class="directory">/dev</filename> entry for it.</para>
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| 126 |
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| 127 | <para>This brings us to one problem that exists with
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| 128 | <command>udev</command>, and likewise with <systemitem
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| 129 | class="filesystem">devfs</systemitem> before it. It is commonly
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| 130 | referred to as the <quote>chicken and egg</quote> problem. Most Linux
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| 131 | distrubtions handle loading modules via entries in
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| 132 | <filename>/etc/modules.conf</filename>. Access to a device node causes
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| 133 | the appropriate kernel module to load. With <command>udev</command>,
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| 134 | this method will not work because the device node does not exist until
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[0cb7f8b] | 135 | the module is loaded. To solve this, the
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| 136 | <command>S05modules</command> bootscript was added to the
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| 137 | lfs-bootscripts package along with the
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| 138 | <filename>/etc/sysconfig/modules</filename> file. By adding module
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| 139 | names to the <filename>modules</filename> file, these modules will be
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| 140 | loaded when the computer is starting up. This allows
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| 141 | <command>udev</command> to detect the devices and create the
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| 142 | appropriate device nodes.</para>
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| 143 |
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| 144 | <para>Note that on slower machines, or for drivers that create a lot
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| 145 | of device nodes, the process of creating devices may take a few
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| 146 | seconds to complete. This means that some device nodes may not be
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[5ba3d1d] | 147 | immediately accessible.</para>
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| 148 | </sect2>
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| 149 |
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| 150 | <sect2>
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| 151 | <title>Handling hotpluggable/dynamic devices</title>
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| 152 |
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| 153 | <para>When you plug in a device, e.g. a USB MP3 player, the kernel
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[0cb7f8b] | 154 | recognizes that the device is now connected and generates a hotplug
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[5ba3d1d] | 155 | event. The driver will already have been loaded (either because it was
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| 156 | compiled into the kernel, or it was loaded via the
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| 157 | <command>S05modules</command> bootscript) so <command>udev</command>
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| 158 | will simply be called upon to create the relevant device node
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| 159 | according to the <systemitem class="filesystem">sysfs</systemitem>
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| 160 | data available in <filename class="directory">/sys</filename>.</para>
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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 problems when it comes to automatically creating
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| 167 | devices nodes:</para>
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| 168 |
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| 169 | <para>1) A kernel driver may not export its data to sysfs</para>
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| 170 |
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| 171 | <para>This is most common with 3rd party drivers from outside the
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| 172 | kernel tree. These drivers will not end up having their device nodes
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| 173 | created. You can use the
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| 174 | <filename>/etc/sysconfig/createfiles</filename> configuration file to
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[0cb7f8b] | 175 | manually create the devices. Consult the
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| 176 | <filename>devices.txt</filename> file inside your 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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[5ba3d1d] | 179 |
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| 180 | <para>2) A non-hardware device is required. This is most common with the
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| 181 | ALSA project's OSS compatibility module. These types of devices can
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[0cb7f8b] | 182 | be handled in one of two ways:</para>
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[5ba3d1d] | 183 |
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| 184 | <itemizedlist>
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| 185 |
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| 186 | <listitem><para>Adding the module names to
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| 187 | <filename>/etc/sysconfig/modules</filename></para></listitem>
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| 188 | <listitem><para>Use of an
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| 189 | <quote>install</quote> line in
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| 190 | <filename>/etc/modprobe.conf</filename>. Basically, this tells the
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| 191 | modprobe command <quote>when loading this module, also load this other
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[0cb7f8b] | 192 | module, at the same time.</quote> For example:</para>
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[5ba3d1d] | 193 |
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| 194 | <screen><userinput>install snd-pcm modprobe -i snd-pcm ; modprobe snd-pcm-oss ; true</userinput></screen>
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| 195 |
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| 196 | <para>This will cause the system to load both the
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| 197 | <emphasis>snd-pcm</emphasis> and <emphasis>snd-pcm-oss</emphasis>
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| 198 | modules when any request is made to load the driver
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| 199 | <emphasis>snd-pcm</emphasis>.</para></listitem>
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| 200 | </itemizedlist>
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| 201 | </sect2>
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| 202 |
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| 203 | <sect2>
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| 204 | <title>Useful reading</title>
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| 205 |
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| 206 | <para>Some additional documentation that will be useful to
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| 207 | read:</para>
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| 208 |
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| 209 | <itemizedlist>
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| 210 | <listitem><para>A Userspace Implementation of devfs -- <ulink
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| 211 | url="http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf"/></para></listitem>
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| 212 | <listitem><para>udev FAQ -- <ulink
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| 213 | url="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev-FAQ"/></para></listitem>
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| 214 | <listitem><para>The Linux Kernel Driver Model -- <ulink
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| 215 | url="http://public.planetmirror.com/pub/lca/2003/proceedings/papers/Patrick_Mochel/Patrick_Mochel.pdf"/></para></listitem>
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| 216 | </itemizedlist>
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| 217 | </sect2>
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| 218 |
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| 219 | </sect1>
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| 220 |
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