[bce08ef] | 1 | <?xml version="1.0" encoding="ISO-8859-1"?>
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[1770019] | 2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN" "http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd" [
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[bce08ef] | 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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| 7 | <title>Device and Module Handling on an LFS System</title>
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| 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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[81fd230] | 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 works,
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| 16 | a brief history of previous methods of handling devices is in
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| 17 | order.</para>
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[bce08ef] | 18 |
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[81fd230] | 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 contains a number of
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| 25 | calls to the <command>mknod</command> 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. Because 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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| 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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[d47f763] | 33 | used is negligible.</para>
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[81fd230] | 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
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| 40 | kernel and was made 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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| 42 | this method of creating devices dynamically never received
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| 43 | overwhelming support from the core 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.
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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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| 65 | realistic.</para>
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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. Drivers that
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| 75 | 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 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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[4ccba55] | 80 | <filename class="directory">/sys</filename>), data which the
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[81fd230] | 81 | built-in drivers registered with <systemitem
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| 82 | class="filesystem">sysfs</systemitem> are available to userspace
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| 83 | processes and to <command>udev</command> for 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 with
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[f18250c] | 87 | registering <command>/sbin/udevsend</command> as a hotplug event handler.
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[81fd230] | 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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| 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> 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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[32f1773] | 97 | <emphasis>7</emphasis> and minor <emphasis>0</emphasis>. The names and
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| 98 | permissions of the nodes created under the
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| 99 | <filename class="directory">/dev</filename> directory are configured according
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| 100 | to the rules specified in the files within the
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| 101 | <filename class="directory">/etc/udev/rules.d/</filename> directory. These are
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| 102 | numbered in a similar fashion to the LFS bootscripts. If
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| 103 | <command>udev</command> can't find a rule for the device it is creating, it will
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| 104 | default permissions to <emphasis>660</emphasis> and ownership to
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| 105 | <emphasis>root:root</emphasis>.</para>
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[81fd230] | 106 |
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| 107 | <para>Once the above stage is complete, all devices that were already
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| 108 | present and have compiled-in drivers will be available for use. What
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| 109 | about those 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
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| 113 | kernel, the kernel will generate a hotplug event and look at the file
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| 114 | <filename>/proc/sys/kernel/hotplug</filename> to find out the
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| 115 | userspace program that handles the device's connection. The
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[551f481] | 116 | <command>udev</command> initscript registered <command>udevsend</command>
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[81fd230] | 117 | as this handler. When these hotplug events are generated, the kernel
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| 118 | will tell <command>udev</command> to check the <filename
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| 119 | class="directory">/sys</filename> filesystem for the information
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| 120 | pertaining to this new device and create the <filename
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| 121 | class="directory">/dev</filename> entry for it.</para>
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| 122 |
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| 123 | <para>This brings us to one problem that exists with
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| 124 | <command>udev</command>, and likewise with <systemitem
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| 125 | class="filesystem">devfs</systemitem> before it. It is commonly
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| 126 | referred to as the <quote>chicken and egg</quote> problem. Most Linux
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[b9248a9] | 127 | distributions handle loading modules via entries in
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[81fd230] | 128 | <filename>/etc/modules.conf</filename>. Access to a device node causes
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| 129 | the appropriate kernel module to load. With <command>udev</command>,
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| 130 | this method will not work because the device node does not exist until
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| 131 | the module is loaded. To solve this, the
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| 132 | <command>S05modules</command> bootscript was added to the
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[4ccba55] | 133 | LFS-Bootscripts package, along with the
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[81fd230] | 134 | <filename>/etc/sysconfig/modules</filename> file. By
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| 135 | adding module
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| 136 | names to the <filename>modules</filename> file, these modules will be
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| 137 | loaded when the computer is starting up. This allows
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| 138 | <command>udev</command> to detect the devices and create the
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| 139 | appropriate device nodes.</para>
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| 140 |
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| 141 | <para>Note that on slower machines or for drivers that create a lot
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| 142 | of device nodes, the process of creating devices may take a few
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| 143 | seconds to complete. This means that some device nodes may not be
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| 144 | immediately accessible.</para>
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| 145 | </sect2>
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| 146 |
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| 147 | <sect2>
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| 148 | <title>Handling Hotpluggable/Dynamic Devices</title>
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| 149 |
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[d47f763] | 150 | <para>When you plug in a device, such as a Universal Serial Bus (USB) MP3 player, the kernel
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[81fd230] | 151 | recognizes that the device is now connected and generates a hotplug
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| 152 | event. If the driver is already loaded (either because it was compiled
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| 153 | into the kernel or because it was loaded via the
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| 154 | <command>S05modules</command> bootscript), <command>udev</command> will
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| 155 | be called upon to create the relevant device node(s) according to the
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| 156 | <systemitem class="filesystem">sysfs</systemitem> data available in
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[01fd09f] | 157 | <filename class="directory">/sys</filename>.</para>
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| 158 |
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| 159 | <para>If the driver for the just plugged in device is available as a module but
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| 160 | currently unloaded, the Hotplug package will load the appropriate module
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| 161 | and make this device available by creating the device node(s) for it.</para>
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[81fd230] | 162 | </sect2>
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| 163 |
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| 164 | <sect2>
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| 165 | <title>Problems with Creating Devices</title>
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| 166 |
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| 167 | <para>There are a few known problems when it comes to automatically creating
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[c66cbd7] | 168 | device nodes:</para>
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[81fd230] | 169 |
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| 170 | <para>1) A kernel driver may not export its data to <systemitem
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| 171 | class="filesystem">sysfs</systemitem>.</para>
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| 172 |
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| 173 | <para>This is most common with third party drivers from outside the
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| 174 | kernel tree. These drivers will not end up having their device nodes
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| 175 | created. Use the
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| 176 | <filename>/etc/sysconfig/createfiles</filename> configuration file to
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| 177 | manually create the devices. Consult the
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| 178 | <filename>devices.txt</filename> file inside the kernel documentation
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| 179 | or the documentation for that driver to find the proper major/minor
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| 180 | numbers.</para>
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| 181 |
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| 182 | <para>2) A non-hardware device is required. This is most common with
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| 183 | the Advanced Linux Sound Architecture (ALSA) project's Open Sound
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| 184 | System (OSS) compatibility module. These types of devices can be
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| 185 | handled in one of two ways:</para>
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| 186 |
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| 187 | <itemizedlist>
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| 188 |
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| 189 | <listitem><para>Adding the module names to
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[f6b3d49] | 190 | <filename>/etc/sysconfig/modules</filename></para><beginpage/></listitem>
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[81fd230] | 191 | <listitem><para>Using an
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| 192 | <quote>install</quote> line in
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| 193 | <filename>/etc/modprobe.conf</filename>. This tells the
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| 194 | <command>modprobe</command> command <quote>when loading this module,
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| 195 | also load this other module, at the same time.</quote> 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></listitem>
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| 204 | </itemizedlist>
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| 205 | </sect2>
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| 206 |
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| 207 | <sect2>
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| 208 | <title>Useful Reading</title>
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| 209 |
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| 210 | <para>Additional helpful documentation is available at the following
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| 211 | sites:</para>
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| 212 |
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| 213 | <itemizedlist>
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[fe5ec46] | 214 | <listitem><para remap="verbatim">A Userspace Implementation of <systemitem class="filesystem">devfs</systemitem>
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[845846e0] | 215 | <ulink url="http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf"/></para></listitem>
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[81fd230] | 216 |
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[fe5ec46] | 217 | <listitem><para remap="verbatim">udev FAQ
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[81fd230] | 218 | <ulink url="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev-FAQ"/></para></listitem>
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| 219 |
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[fe5ec46] | 220 | <listitem><para remap="verbatim">The Linux Kernel Driver Model
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[845846e0] | 221 | <ulink url="http://public.planetmirror.com/pub/lca/2003/proceedings/papers/Patrick_Mochel/Patrick_Mochel.pdf"/></para></listitem>
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[81fd230] | 222 | </itemizedlist>
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| 223 | </sect2>
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[bce08ef] | 224 |
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| 225 | </sect1>
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| 226 |
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