[1fa2099] | 1 | <?xml version="1.0" encoding="ISO-8859-1"?>
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| 2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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| 3 | "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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| 4 | <!ENTITY % general-entities SYSTEM "../general.ent">
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| 5 | %general-entities;
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| 6 | ]>
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| 7 |
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[ede49cd] | 8 | <sect1 id="ch-config-udev">
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[1fa2099] | 9 | <?dbhtml filename="udev.html"?>
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| 10 |
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| 11 | <title>Overview of Device and Module Handling</title>
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| 12 |
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[ede49cd] | 13 | <indexterm zone="ch-config-udev">
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[1fa2099] | 14 | <primary sortas="a-Udev">Udev</primary>
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| 15 | <secondary>usage</secondary>
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| 16 | </indexterm>
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| 17 |
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[37e35d2] | 18 | <para>In <xref linkend="chapter-building-system"/>, we installed the udev
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[1fa2099] | 19 | package when <phrase revision="sysv">eudev</phrase>
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| 20 | <phrase revision="systemd">systemd</phrase> was built. Before we go into the
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| 21 | details regarding how this works, a brief history of previous methods of
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| 22 | handling devices is in order.</para>
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| 23 |
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| 24 | <para>Linux systems in general traditionally used a static device creation
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| 25 | method, whereby a great many device nodes were created under <filename
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| 26 | class="directory">/dev</filename> (sometimes literally thousands of nodes),
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| 27 | regardless of whether the corresponding hardware devices actually existed. This
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| 28 | was typically done via a <command>MAKEDEV</command> script, which contains a
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| 29 | number of calls to the <command>mknod</command> program with the relevant
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| 30 | major and minor device numbers for every possible device that might exist in
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| 31 | the world.</para>
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| 32 |
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[37e35d2] | 33 | <para>Using the udev method, only those devices which are detected by the
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[1fa2099] | 34 | kernel get device nodes created for them. Because these device nodes will be
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| 35 | created each time the system boots, they will be stored on a <systemitem
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| 36 | class="filesystem">devtmpfs</systemitem> file system (a virtual file system
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| 37 | that resides entirely in system memory). Device nodes do not require much
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| 38 | space, so the memory that is used is negligible.</para>
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| 39 |
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| 40 | <sect2>
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| 41 | <title>History</title>
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| 42 |
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| 43 | <para>In February 2000, a new filesystem called <systemitem
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| 44 | class="filesystem">devfs</systemitem> was merged into the 2.3.46 kernel
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| 45 | and was made available during the 2.4 series of stable kernels. Although
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| 46 | it was present in the kernel source itself, this method of creating devices
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| 47 | dynamically never received overwhelming support from the core kernel
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| 48 | developers.</para>
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| 49 |
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| 50 | <para>The main problem with the approach adopted by <systemitem
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| 51 | class="filesystem">devfs</systemitem> was the way it handled device
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| 52 | detection, creation, and naming. The latter issue, that of device node
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| 53 | naming, was perhaps the most critical. It is generally accepted that if
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| 54 | device names are allowed to be configurable, then the device naming policy
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| 55 | should be up to a system administrator, not imposed on them by any
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| 56 | particular developer(s). The <systemitem
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| 57 | class="filesystem">devfs</systemitem> file system also suffered from race
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| 58 | conditions that were inherent in its design and could not be fixed without a
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| 59 | substantial revision to the kernel. It was marked as deprecated for a long
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| 60 | period – due to a lack of maintenance – and was finally removed
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| 61 | from the kernel in June, 2006.</para>
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| 62 |
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| 63 | <para>With the development of the unstable 2.5 kernel tree, later released
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| 64 | as the 2.6 series of stable kernels, a new virtual filesystem called
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| 65 | <systemitem class="filesystem">sysfs</systemitem> came to be. The job of
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| 66 | <systemitem class="filesystem">sysfs</systemitem> is to export a view of
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| 67 | the system's hardware configuration to userspace processes. With this
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| 68 | userspace-visible representation, the possibility of developing a userspace
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| 69 | replacement for <systemitem class="filesystem">devfs</systemitem> became
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| 70 | much more realistic.</para>
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| 71 |
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| 72 | </sect2>
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| 73 |
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| 74 | <sect2>
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| 75 | <title>Udev Implementation</title>
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| 76 |
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| 77 | <sect3>
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| 78 | <title>Sysfs</title>
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| 79 |
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| 80 | <para>The <systemitem class="filesystem">sysfs</systemitem> filesystem
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| 81 | was mentioned briefly above. One may wonder how <systemitem
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| 82 | class="filesystem">sysfs</systemitem> knows about the devices present on
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| 83 | a system and what device numbers should be used for them. Drivers that
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| 84 | have been compiled into the kernel directly register their objects with a
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| 85 | <systemitem class="filesystem">sysfs</systemitem> (devtmpfs internally)
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| 86 | as they are detected by the kernel. For drivers compiled as modules, this
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| 87 | registration will happen when the module is loaded. Once the <systemitem
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| 88 | class="filesystem">sysfs</systemitem> filesystem is mounted (on /sys),
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| 89 | data which the drivers register with <systemitem
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| 90 | class="filesystem">sysfs</systemitem> are available to userspace
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| 91 | processes and to udevd for processing (including modifications to device
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| 92 | nodes).</para>
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| 93 |
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| 94 | </sect3>
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| 95 |
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| 96 | <sect3>
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| 97 | <title>Device Node Creation</title>
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| 98 |
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| 99 | <para>Device files are created by the kernel by the <systemitem
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| 100 | class="filesystem">devtmpfs</systemitem> filesystem. Any driver that
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| 101 | wishes to register a device node will go through the <systemitem
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| 102 | class="filesystem">devtmpfs</systemitem> (via the driver core) to do it.
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| 103 | When a <systemitem class="filesystem">devtmpfs</systemitem> instance is
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| 104 | mounted on <filename class="directory">/dev</filename>, the device node
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| 105 | will initially be created with a fixed name, permissions, and
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| 106 | owner.</para>
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| 107 |
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| 108 | <para>A short time later, the kernel will send a uevent to <command>
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| 109 | udevd</command>. Based on the rules specified in the files within the
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| 110 | <filename class="directory">/etc/udev/rules.d</filename>, <filename
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| 111 | class="directory">/lib/udev/rules.d</filename>, and <filename
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| 112 | class="directory">/run/udev/rules.d</filename> directories, <command>
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| 113 | udevd</command> will create additional symlinks to the device node, or
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| 114 | change its permissions, owner, or group, or modify the internal
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| 115 | <command>udevd</command> database entry (name) for that object.</para>
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| 116 |
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| 117 | <para>The rules in these three directories are numbered and all three
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| 118 | directories are merged together. If <command>udevd</command> can't find a
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| 119 | rule for the device it is creating, it will leave the permissions and
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| 120 | ownership at whatever <systemitem
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| 121 | class="filesystem">devtmpfs</systemitem> used initially.</para> </sect3>
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| 122 |
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| 123 | <sect3 id="module-loading">
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| 124 | <title>Module Loading</title>
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| 125 |
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| 126 | <para>Device drivers compiled as modules may have aliases built into them.
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| 127 | Aliases are visible in the output of the <command>modinfo</command>
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| 128 | program and are usually related to the bus-specific identifiers of devices
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| 129 | supported by a module. For example, the <emphasis>snd-fm801</emphasis>
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| 130 | driver supports PCI devices with vendor ID 0x1319 and device ID 0x0801,
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| 131 | and has an alias of <quote>pci:v00001319d00000801sv*sd*bc04sc01i*</quote>.
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| 132 | For most devices, the bus driver exports the alias of the driver that
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| 133 | would handle the device via <systemitem
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| 134 | class="filesystem">sysfs</systemitem>. E.g., the
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| 135 | <filename>/sys/bus/pci/devices/0000:00:0d.0/modalias</filename> file
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| 136 | might contain the string
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| 137 | <quote>pci:v00001319d00000801sv00001319sd00001319bc04sc01i00</quote>.
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[37e35d2] | 138 | The default rules provided with udev will cause <command>udevd</command>
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[1fa2099] | 139 | to call out to <command>/sbin/modprobe</command> with the contents of the
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| 140 | <envar>MODALIAS</envar> uevent environment variable (which should be the
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| 141 | same as the contents of the <filename>modalias</filename> file in sysfs),
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| 142 | thus loading all modules whose aliases match this string after wildcard
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| 143 | expansion.</para>
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| 144 |
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| 145 | <para>In this example, this means that, in addition to
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| 146 | <emphasis>snd-fm801</emphasis>, the obsolete (and unwanted)
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| 147 | <emphasis>forte</emphasis> driver will be loaded if it is
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| 148 | available. See below for ways in which the loading of unwanted drivers can
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| 149 | be prevented.</para>
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| 150 |
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| 151 | <para>The kernel itself is also able to load modules for network
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[37e35d2] | 152 | protocols, filesystems, and NLS support on demand.</para>
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[1fa2099] | 153 |
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| 154 | </sect3>
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| 155 |
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| 156 | <sect3>
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| 157 | <title>Handling Hotpluggable/Dynamic Devices</title>
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| 158 |
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| 159 | <para>When you plug in a device, such as a Universal Serial Bus (USB) MP3
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| 160 | player, the kernel recognizes that the device is now connected and
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| 161 | generates a uevent. This uevent is then handled by
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| 162 | <command>udevd</command> as described above.</para>
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| 163 |
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| 164 | </sect3>
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| 165 |
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| 166 | </sect2>
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| 167 |
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| 168 | <sect2>
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| 169 | <title>Problems with Loading Modules and Creating Devices</title>
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| 170 |
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| 171 | <para>There are a few possible problems when it comes to automatically
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| 172 | creating device nodes.</para>
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| 173 |
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| 174 | <sect3>
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| 175 | <title>A kernel module is not loaded automatically</title>
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| 176 |
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| 177 | <para>Udev will only load a module if it has a bus-specific alias and the
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| 178 | bus driver properly exports the necessary aliases to <systemitem
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| 179 | class="filesystem">sysfs</systemitem>. In other cases, one should
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[37e35d2] | 180 | arrange module loading by other means. With Linux-&linux-version;, udev is
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[1fa2099] | 181 | known to load properly-written drivers for INPUT, IDE, PCI, USB, SCSI,
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| 182 | SERIO, and FireWire devices.</para>
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| 183 |
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| 184 | <para>To determine if the device driver you require has the necessary
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[37e35d2] | 185 | support for udev, run <command>modinfo</command> with the module name as
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[1fa2099] | 186 | the argument. Now try locating the device directory under
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| 187 | <filename class="directory">/sys/bus</filename> and check whether there is
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| 188 | a <filename>modalias</filename> file there.</para>
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| 189 |
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| 190 | <para>If the <filename>modalias</filename> file exists in <systemitem
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| 191 | class="filesystem">sysfs</systemitem>, the driver supports the device and
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| 192 | can talk to it directly, but doesn't have the alias, it is a bug in the
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[37e35d2] | 193 | driver. Load the driver without the help from udev and expect the issue
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[1fa2099] | 194 | to be fixed later.</para>
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| 195 |
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| 196 | <para>If there is no <filename>modalias</filename> file in the relevant
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| 197 | directory under <filename class="directory">/sys/bus</filename>, this
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| 198 | means that the kernel developers have not yet added modalias support to
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| 199 | this bus type. With Linux-&linux-version;, this is the case with ISA
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| 200 | busses. Expect this issue to be fixed in later kernel versions.</para>
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| 201 |
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| 202 | <para>Udev is not intended to load <quote>wrapper</quote> drivers such as
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| 203 | <emphasis>snd-pcm-oss</emphasis> and non-hardware drivers such as
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| 204 | <emphasis>loop</emphasis> at all.</para>
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| 205 |
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| 206 | </sect3>
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| 207 |
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| 208 | <sect3>
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[37e35d2] | 209 | <title>A kernel module is not loaded automatically, and udev is not
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[1fa2099] | 210 | intended to load it</title>
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| 211 |
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| 212 | <para>If the <quote>wrapper</quote> module only enhances the
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| 213 | functionality provided by some other module (e.g.,
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| 214 | <emphasis>snd-pcm-oss</emphasis> enhances the functionality of
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| 215 | <emphasis>snd-pcm</emphasis> by making the sound cards available to OSS
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| 216 | applications), configure <command>modprobe</command> to load the wrapper
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[37e35d2] | 217 | after udev loads the wrapped module. To do this, add a
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[1fa2099] | 218 | <quote>softdep</quote> line to the corresponding
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| 219 | <filename>/etc/modprobe.d/<replaceable><filename></replaceable>.conf</filename>
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| 220 | file. For example:</para>
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| 221 |
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| 222 | <screen role="nodump"><literal>softdep snd-pcm post: snd-pcm-oss</literal></screen>
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| 223 |
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| 224 | <para>Note that the <quote>softdep</quote> command also allows
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| 225 | <literal>pre:</literal> dependencies, or a mixture of both
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[37e35d2] | 226 | <literal>pre:</literal> and <literal>post:</literal> dependencies. See
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| 227 | the <filename>modprobe.d(5)</filename> manual page for more information
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[1fa2099] | 228 | on <quote>softdep</quote> syntax and capabilities.</para>
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| 229 |
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[37e35d2] | 230 | <para revision="sysv">If the module in question is not a wrapper and is
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| 231 | useful by itself, configure the <command>modules</command> bootscript to
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| 232 | load this module on system boot. To do this, add the module name to the
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[1fa2099] | 233 | <filename>/etc/sysconfig/modules</filename> file on a separate line.
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| 234 | This works for wrapper modules too, but is suboptimal in that case.</para>
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| 235 |
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| 236 | </sect3>
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| 237 |
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| 238 | <sect3>
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| 239 | <title>Udev loads some unwanted module</title>
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| 240 |
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| 241 | <para>Either don't build the module, or blacklist it in a
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| 242 | <filename>/etc/modprobe.d/blacklist.conf</filename> file as done with the
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| 243 | <emphasis>forte</emphasis> module in the example below:</para>
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| 244 |
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| 245 | <screen role="nodump"><literal>blacklist forte</literal></screen>
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| 246 |
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| 247 | <para>Blacklisted modules can still be loaded manually with the
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| 248 | explicit <command>modprobe</command> command.</para>
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| 249 |
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| 250 | </sect3>
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| 251 |
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| 252 | <sect3>
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| 253 | <title>Udev creates a device incorrectly, or makes a wrong symlink</title>
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| 254 |
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| 255 | <para>This usually happens if a rule unexpectedly matches a device. For
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| 256 | example, a poorly-written rule can match both a SCSI disk (as desired)
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| 257 | and the corresponding SCSI generic device (incorrectly) by vendor.
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| 258 | Find the offending rule and make it more specific, with the help of the
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| 259 | <command>udevadm info</command> command.</para>
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| 260 |
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| 261 | </sect3>
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| 262 |
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| 263 | <sect3>
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| 264 | <title>Udev rule works unreliably</title>
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| 265 |
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| 266 | <para>This may be another manifestation of the previous problem. If not,
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| 267 | and your rule uses <systemitem class="filesystem">sysfs</systemitem>
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| 268 | attributes, it may be a kernel timing issue, to be fixed in later kernels.
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| 269 | For now, you can work around it by creating a rule that waits for the used
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| 270 | <systemitem class="filesystem">sysfs</systemitem> attribute and appending
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| 271 | it to the <filename>/etc/udev/rules.d/10-wait_for_sysfs.rules</filename>
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| 272 | file (create this file if it does not exist). Please notify the LFS
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| 273 | Development list if you do so and it helps.</para>
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| 274 |
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| 275 | </sect3>
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| 276 |
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| 277 | <sect3>
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| 278 | <title>Udev does not create a device</title>
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| 279 |
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| 280 | <para>Further text assumes that the driver is built statically into the
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| 281 | kernel or already loaded as a module, and that you have already checked
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[37e35d2] | 282 | that udev doesn't create a misnamed device.</para>
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[1fa2099] | 283 |
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| 284 | <para>Udev has no information needed to create a device node if a kernel
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| 285 | driver does not export its data to
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| 286 | <systemitem class="filesystem">sysfs</systemitem>. This is most common
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| 287 | with third party drivers from outside the kernel tree. Create a static
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| 288 | device node in <filename>/lib/udev/devices</filename> with the
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| 289 | appropriate major/minor numbers (see the file
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| 290 | <filename>devices.txt</filename> inside the kernel documentation or the
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| 291 | documentation provided by the third party driver vendor). The static
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| 292 | device node will be copied to <filename class="directory">/dev</filename>
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| 293 | by <command>udev</command>.</para>
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| 294 |
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| 295 | </sect3>
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| 296 |
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| 297 | <sect3>
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| 298 | <title>Device naming order changes randomly after rebooting</title>
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| 299 |
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[37e35d2] | 300 | <para>This is due to the fact that udev, by design, handles uevents and
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[1fa2099] | 301 | loads modules in parallel, and thus in an unpredictable order. This will
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| 302 | never be <quote>fixed</quote>. You should not rely upon the kernel device
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| 303 | names being stable. Instead, create your own rules that make symlinks with
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| 304 | stable names based on some stable attributes of the device, such as a
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[37e35d2] | 305 | serial number or the output of various *_id utilities installed by udev.
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[ede49cd] | 306 | See <xref linkend="ch-config-symlinks"/> and
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| 307 | <xref linkend="ch-config-network"/> for examples.</para>
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[1fa2099] | 308 |
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| 309 | </sect3>
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| 310 |
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| 311 | </sect2>
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| 312 |
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| 313 | <sect2>
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| 314 | <title>Useful Reading</title>
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| 315 |
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| 316 | <para>Additional helpful documentation is available at the following
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| 317 | sites:</para>
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| 318 |
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| 319 | <itemizedlist>
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| 320 |
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| 321 | <listitem>
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| 322 | <para>A Userspace Implementation of <systemitem class="filesystem">devfs</systemitem>
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| 323 | <ulink url="http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf"/></para>
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| 324 | </listitem>
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| 325 |
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| 326 | <listitem>
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| 327 | <para>The <systemitem class="filesystem">sysfs</systemitem> Filesystem
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| 328 | <ulink url="http://www.kernel.org/pub/linux/kernel/people/mochel/doc/papers/ols-2005/mochel.pdf"/></para>
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| 329 | </listitem>
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| 330 |
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| 331 | <!-- No longer available
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| 332 | <listitem>
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| 333 | <para>Pointers to further reading
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| 334 | <ulink url="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev.html"/>
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| 335 | </para>
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| 336 | </listitem>
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| 337 | -->
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| 338 | </itemizedlist>
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| 339 |
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| 340 | </sect2>
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| 341 |
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| 342 | </sect1>
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