[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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[b568bbaf] | 15 | package. Before we go into the details regarding how this works,
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[81fd230] | 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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[b568bbaf] | 19 | <para>Linux systems in general traditionally use a static device creation
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| 20 | method, whereby a great many device nodes are created under <filename
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| 21 | class="directory">/dev</filename> (sometimes literally thousands of nodes),
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| 22 | regardless of whether the corresponding hardware devices actually exist. This is
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| 23 | typically done via a <command>MAKEDEV</command> script, which contains a number
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| 24 | of calls to the <command>mknod</command> program with the relevant major and
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| 25 | minor device numbers for every possible device that might exist in the world.
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| 26 | Using the Udev method, only those devices which are detected by the kernel get
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| 27 | device nodes created for them. Because these device nodes will be created each
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| 28 | time the system boots, they will be stored on a <systemitem
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| 29 | class="filesystem">tmpfs</systemitem> file system (a virtual file system that
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| 30 | resides entirely in system memory). Device nodes do not require much space, so
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| 31 | the memory that is used is negligible.</para>
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[81fd230] | 32 |
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| 33 | <sect2>
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| 34 | <title>History</title>
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| 35 |
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| 36 | <para>In February 2000, a new filesystem called <systemitem
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| 37 | class="filesystem">devfs</systemitem> was merged into the 2.3.46
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| 38 | kernel and was made available during the 2.4 series of
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| 39 | stable kernels. Although it was present in the kernel source itself,
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| 40 | this method of creating devices dynamically never received
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| 41 | overwhelming support from the core kernel developers.</para>
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| 42 |
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| 43 | <para>The main problem with the approach adopted by <systemitem
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| 44 | class="filesystem">devfs</systemitem> was the way it handled
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| 45 | device detection, creation, and naming. The latter issue, that of
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| 46 | device node naming, was perhaps the most critical. It is generally
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| 47 | accepted that if device names are allowed to be configurable, then
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| 48 | the device naming policy should be up to a system administrator, not
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| 49 | imposed on them by any particular developer(s). The <systemitem
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| 50 | class="filesystem">devfs</systemitem> file system also suffers from race
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| 51 | conditions that are inherent in its design and cannot be fixed
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| 52 | without a substantial revision to the kernel. It has also been marked
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| 53 | as deprecated due to a lack of recent maintenance.</para>
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| 54 |
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[b568bbaf] | 55 | <para>With the development of the unstable 2.5 kernel tree, later released as
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| 56 | the 2.6 series of stable kernels, a new virtual filesystem called <systemitem
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| 57 | class="filesystem">sysfs</systemitem> came to be. The job of <systemitem
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| 58 | class="filesystem">sysfs</systemitem> is to export a view of the system's
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| 59 | hardrware configuration to userspace processes. With this userspace-visible
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| 60 | representation, the possibility of seeing a userspace replacement for
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| 61 | <systemitem class="filesystem">devfs</systemitem> became much more
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[81fd230] | 62 | realistic.</para>
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[b568bbaf] | 63 |
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[81fd230] | 64 | </sect2>
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| 65 |
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| 66 | <sect2>
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| 67 | <title>Udev Implementation</title>
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| 68 |
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[b568bbaf] | 69 | <para>The <systemitem class="filesystem">sysfs</systemitem> filesystem was
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| 70 | mentioned briefly above. One may wonder how <systemitem
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| 71 | class="filesystem">sysfs</systemitem> knows about the devices present on a
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| 72 | system and what device numbers should be used for them. Drivers that have been
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| 73 | compiled into the kernel directly register their objects with <systemitem
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| 74 | class="filesystem">sysfs</systemitem> as they are detected by the kernel. For
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| 75 | drivers compiled as modules, this registration will happen when the module is
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| 76 | loaded. Once the <systemitem class="filesystem">sysfs</systemitem> filesystem is
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| 77 | mounted (on <filename class="directory">/sys</filename>), data which the
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[81fd230] | 78 | built-in drivers registered with <systemitem
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[b568bbaf] | 79 | class="filesystem">sysfs</systemitem> are available to userspace processes and
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| 80 | to <command>udev</command> for device node creation.</para>
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[81fd230] | 81 |
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[aabd480] | 82 | <para>The <command>S10udev</command> initscript takes care of creating these
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[b568bbaf] | 83 | device nodes when Linux is booted. This script starts by registering
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| 84 | <command>/sbin/udevsend</command> as a hotplug event handler. Hotplug events
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| 85 | (discussed below) are not usually generated during this stage, but
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| 86 | <command>udev</command> is registered just in case they do occur. The
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[aabd480] | 87 | <command>udevstart</command> program then walks through the <systemitem
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| 88 | class="filesystem">/sys</systemitem> filesystem and creates devices under
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[b568bbaf] | 89 | <filename class="directory">/dev</filename> that match the descriptions. For
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[aabd480] | 90 | example, <filename>/sys/class/tty/vcs/dev</filename> contains the string
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| 91 | <quote>7:0</quote> This string is used by <command>udevstart</command> to create
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| 92 | <filename>/dev/vcs</filename> with major number <emphasis>7</emphasis> and minor
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[b568bbaf] | 93 | <emphasis>0</emphasis>. The names and permissions of the nodes created under
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[aabd480] | 94 | the <filename class="directory">/dev</filename> directory are configured
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| 95 | according to the rules specified in the files within the <filename
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| 96 | class="directory">/etc/udev/rules.d/</filename> directory. These are numbered in
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| 97 | a similar fashion to the LFS-Bootscripts package. If <command>udev</command>
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| 98 | can't find a rule for the device it is creating, it will default permissions to
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| 99 | <emphasis>660</emphasis> and ownership to <emphasis>root:root</emphasis>.</para>
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[81fd230] | 100 |
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[b568bbaf] | 101 | <para>Once the above stage is complete, all devices that were already present
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| 102 | and have compiled-in drivers will be available for use. This leads us to the
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| 103 | devices that have modular drivers.</para>
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[81fd230] | 104 |
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| 105 | <para>Earlier, we mentioned the concept of a <quote>hotplug event
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[b568bbaf] | 106 | handler.</quote> When a new device connection is detected by the kernel, the
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| 107 | kernel will generate a hotplug event and look at the file
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| 108 | <filename>/proc/sys/kernel/hotplug</filename> to determine the userspace program
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| 109 | that handles the device's connection. The <command>udev</command> bootscript
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| 110 | registered <command>udevsend</command> as this handler. When these hotplug
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| 111 | events are generated, the kernel will tell <command>udev</command> to check the
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| 112 | <filename class="directory">/sys</filename> filesystem for the information
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[81fd230] | 113 | pertaining to this new device and create the <filename
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| 114 | class="directory">/dev</filename> entry for it.</para>
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| 115 |
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[b568bbaf] | 116 | <para>This brings us to one problem that exists with <command>udev</command>,
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| 117 | and likewise with <systemitem class="filesystem">devfs</systemitem> before it.
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| 118 | It is commonly referred to as the <quote>chicken and egg</quote> problem. Most
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| 119 | Linux distributions handle loading modules via entries in
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| 120 | <filename>/etc/modules.conf</filename>. Access to a device node causes the
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| 121 | appropriate kernel module to load. With <command>udev</command>, this method
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| 122 | will not work because the device node does not exist until the module is loaded.
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| 123 | To solve this, the <command>S05modules</command> bootscript was added to the
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[4ccba55] | 124 | LFS-Bootscripts package, along with the
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[b568bbaf] | 125 | <filename>/etc/sysconfig/modules</filename> file. By adding module names to the
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| 126 | <filename>modules</filename> file, these modules will be loaded when the
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[b8a3fb2] | 127 | computer starts up. This allows <command>udev</command> to detect the devices
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[b568bbaf] | 128 | and create the appropriate device nodes.</para>
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[81fd230] | 129 |
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| 130 | <para>Note that on slower machines or for drivers that create a lot
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| 131 | of device nodes, the process of creating devices may take a few
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| 132 | seconds to complete. This means that some device nodes may not be
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| 133 | immediately accessible.</para>
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| 134 | </sect2>
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| 135 |
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| 136 | <sect2>
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| 137 | <title>Handling Hotpluggable/Dynamic Devices</title>
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| 138 |
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[d47f763] | 139 | <para>When you plug in a device, such as a Universal Serial Bus (USB) MP3 player, the kernel
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[81fd230] | 140 | recognizes that the device is now connected and generates a hotplug
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| 141 | event. If the driver is already loaded (either because it was compiled
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| 142 | into the kernel or because it was loaded via the
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| 143 | <command>S05modules</command> bootscript), <command>udev</command> will
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| 144 | be called upon to create the relevant device node(s) according to the
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| 145 | <systemitem class="filesystem">sysfs</systemitem> data available in
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[01fd09f] | 146 | <filename class="directory">/sys</filename>.</para>
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| 147 |
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| 148 | <para>If the driver for the just plugged in device is available as a module but
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| 149 | currently unloaded, the Hotplug package will load the appropriate module
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| 150 | and make this device available by creating the device node(s) for it.</para>
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[81fd230] | 151 | </sect2>
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| 152 |
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| 153 | <sect2>
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| 154 | <title>Problems with Creating Devices</title>
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| 155 |
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| 156 | <para>There are a few known problems when it comes to automatically creating
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[c66cbd7] | 157 | device nodes:</para>
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[81fd230] | 158 |
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| 159 | <para>1) A kernel driver may not export its data to <systemitem
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| 160 | class="filesystem">sysfs</systemitem>.</para>
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| 161 |
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[b568bbaf] | 162 | <para>This is most common with third party drivers from outside the kernel tree.
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| 163 | Udev will be unable to automatically create device nodes for such drivers. Use
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| 164 | the <filename>/etc/sysconfig/createfiles</filename> configuration file to
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| 165 | manually create the devices. Consult the <filename>devices.txt</filename> file
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| 166 | inside the kernel documentation or the documentation for that driver to find the
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| 167 | proper major/minor numbers.</para>
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[81fd230] | 168 |
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| 169 | <para>2) A non-hardware device is required. This is most common with
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| 170 | the Advanced Linux Sound Architecture (ALSA) project's Open Sound
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| 171 | System (OSS) compatibility module. These types of devices can be
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| 172 | handled in one of two ways:</para>
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| 173 |
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| 174 | <itemizedlist>
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| 175 |
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| 176 | <listitem><para>Adding the module names to
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[f6b3d49] | 177 | <filename>/etc/sysconfig/modules</filename></para><beginpage/></listitem>
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[81fd230] | 178 | <listitem><para>Using an
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| 179 | <quote>install</quote> line in
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| 180 | <filename>/etc/modprobe.conf</filename>. This tells the
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| 181 | <command>modprobe</command> command <quote>when loading this module,
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| 182 | also load this other module, at the same time.</quote> For example:</para>
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| 183 |
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| 184 | <screen><userinput>install snd-pcm modprobe -i snd-pcm ; modprobe \
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| 185 | snd-pcm-oss ; true</userinput></screen>
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| 186 |
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| 187 | <para>This will cause the system to load both the
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| 188 | <emphasis>snd-pcm</emphasis> and <emphasis>snd-pcm-oss</emphasis>
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| 189 | modules when any request is made to load the driver
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| 190 | <emphasis>snd-pcm</emphasis>.</para></listitem>
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| 191 | </itemizedlist>
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| 192 | </sect2>
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| 193 |
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| 194 | <sect2>
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| 195 | <title>Useful Reading</title>
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| 196 |
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| 197 | <para>Additional helpful documentation is available at the following
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| 198 | sites:</para>
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| 199 |
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| 200 | <itemizedlist>
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[fe5ec46] | 201 | <listitem><para remap="verbatim">A Userspace Implementation of <systemitem class="filesystem">devfs</systemitem>
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[845846e0] | 202 | <ulink url="http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf"/></para></listitem>
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[81fd230] | 203 |
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[fe5ec46] | 204 | <listitem><para remap="verbatim">udev FAQ
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[81fd230] | 205 | <ulink url="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev-FAQ"/></para></listitem>
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| 206 |
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[fe5ec46] | 207 | <listitem><para remap="verbatim">The Linux Kernel Driver Model
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[845846e0] | 208 | <ulink url="http://public.planetmirror.com/pub/lca/2003/proceedings/papers/Patrick_Mochel/Patrick_Mochel.pdf"/></para></listitem>
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[81fd230] | 209 | </itemizedlist>
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| 210 | </sect2>
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[bce08ef] | 211 |
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| 212 | </sect1>
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| 213 |
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