[806d0c2] | 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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[775d9a7] | 8 | <sect1 id="ch-config-eudev"> |
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[82fc053] | 9 | <?dbhtml filename="eudev.html"?> |
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[806d0c2] | 10 | |
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| 11 | <title>Device and Module Handling on a CLFS System</title> |
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| 12 | |
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[775d9a7] | 13 | <indexterm zone="ch-config-eudev"> |
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[82fc053] | 14 | <primary sortas="a-Eudev">Eudev</primary> |
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| 15 | <secondary>usage</secondary> |
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[806d0c2] | 16 | </indexterm> |
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| 17 | |
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[82fc053] | 18 | <para>In <xref linkend="chapter-building-system"/>, we installed the Eudev |
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| 19 | package. Before we go into the details regarding how this works, |
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| 20 | a brief history of previous methods of handling devices is in |
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| 21 | order.</para> |
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[806d0c2] | 22 | |
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| 23 | <sect2> |
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| 24 | <title>History</title> |
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| 25 | |
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[f6721f7] | 26 | <sect3> |
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| 27 | <title>Static Device Nodes</title> |
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[806d0c2] | 28 | |
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[f6721f7] | 29 | <para>Linux systems in general traditionally use a static device creation |
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| 30 | method, whereby a great many device nodes are created under <filename |
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[bbf3c21] | 31 | class="directory">/dev</filename> (sometimes literally thousands of |
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| 32 | nodes), regardless of whether the corresponding hardware devices actually |
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| 33 | exist. This is typically done via a <command>MAKEDEV</command> script, |
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| 34 | which contains a number of calls to the <command>mknod</command> program |
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| 35 | with the relevant major and minor device numbers for every possible |
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| 36 | device that might exist in the world.</para> |
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[806d0c2] | 37 | |
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[f6721f7] | 38 | </sect3> |
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| 39 | |
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| 40 | <sect3> |
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| 41 | <title>Devfs</title> |
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| 42 | |
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| 43 | <para>In February 2000, a new filesystem called <systemitem |
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[bbf3c21] | 44 | class="filesystem">devfs</systemitem>, which dynamically created device |
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| 45 | nodes as devices were found by the kernel, was merged into the |
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| 46 | 2.3.46 kernel and was made available during the 2.4 series of stable |
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| 47 | kernels. Although it was present in the kernel source itself, this method |
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| 48 | of creating devices dynamically never received overwhelming support from |
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| 49 | the core kernel developers.</para> |
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[f6721f7] | 50 | |
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| 51 | <para>The main problem with the approach adopted by <systemitem |
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| 52 | class="filesystem">devfs</systemitem> was the way it handled device |
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| 53 | detection, creation, and naming. The latter issue, that of device node |
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| 54 | naming, was perhaps the most critical. It is generally accepted that if |
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| 55 | device names are allowed to be configurable, then the device naming policy |
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| 56 | should be up to a system administrator, not imposed on them by any |
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| 57 | particular developer(s). The <systemitem |
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| 58 | class="filesystem">devfs</systemitem> file system also suffered from race |
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[bbf3c21] | 59 | conditions that were inherent in its design and could not be fixed without |
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| 60 | a substantial revision to the kernel. It was marked deprecated with the |
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[f6721f7] | 61 | release of the 2.6 kernel series, and was removed entirely as of version |
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| 62 | 2.6.18.</para> |
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| 63 | |
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| 64 | </sect3> |
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[806d0c2] | 65 | |
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| 66 | <sect3> |
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| 67 | <title>Sysfs</title> |
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| 68 | |
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[f6721f7] | 69 | <para>With the development of the unstable 2.5 kernel tree, later released |
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| 70 | as the 2.6 series of stable kernels, a new virtual filesystem called |
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| 71 | <systemitem class="filesystem">sysfs</systemitem> came to be. The job of |
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| 72 | <systemitem class="filesystem">sysfs</systemitem> is to export a view of |
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| 73 | the system's hardware configuration to userspace processes. Drivers that |
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[806d0c2] | 74 | have been compiled into the kernel directly register their objects with |
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| 75 | <systemitem class="filesystem">sysfs</systemitem> as they are detected by |
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| 76 | the kernel. For drivers compiled as modules, this registration will happen |
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| 77 | when the module is loaded. Once the <systemitem |
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| 78 | class="filesystem">sysfs</systemitem> filesystem is mounted (on <filename |
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| 79 | class="directory">/sys</filename>), data which the built-in drivers |
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| 80 | registered with <systemitem class="filesystem">sysfs</systemitem> are |
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[bbf3c21] | 81 | available to userspace processes. With this userspace-visible |
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| 82 | representation, the possibility of seeing a userspace replacement for |
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| 83 | <systemitem class="filesystem">devfs</systemitem> became much more |
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| 84 | realistic.</para> |
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[806d0c2] | 85 | |
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| 86 | </sect3> |
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| 87 | |
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[82fc053] | 88 | <sect3> |
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[f6721f7] | 89 | <title>Udev Implementation</title> |
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[82fc053] | 90 | |
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[bbf3c21] | 91 | <para>Shortly after the introduction of |
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| 92 | <systemitem class="filesystem">sysfs</systemitem>, work began on a |
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| 93 | program called Udev to advantage of it. The <command>udev</command> |
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| 94 | daemon made calls to <function>mknod()</function> to create device nodes |
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| 95 | in <filename class="directory">/dev</filename> dynamically, based on the |
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[f6721f7] | 96 | information from <systemitem class="filesystem">sysfs</systemitem>, in |
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| 97 | <filename class="directory">/sys</filename>. For example, |
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[806d0c2] | 98 | <filename>/sys/class/tty/vcs/dev</filename> contains the string |
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[bbf3c21] | 99 | <quote>7:0</quote>. This string was used by <command>udev</command> |
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[f6721f7] | 100 | to create a device node with major number <emphasis>7</emphasis> and |
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[bbf3c21] | 101 | minor number <emphasis>0</emphasis>.</para> |
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[f6721f7] | 102 | |
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| 103 | <para>Linux kernel version 2.6.32 introduced a new virtual file system |
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[bbf3c21] | 104 | called <systemitem class="filesystem">devtmpfs</systemitem>, an improved |
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| 105 | replacement for <systemitem class="filesystem">devfs</systemitem>. This |
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| 106 | allows device nodes to once again be dynamically created by the kernel, |
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| 107 | without many of the problems of |
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| 108 | <systemitem class="filesystem">devfs</systemitem>. As of version 176, |
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| 109 | Udev no longer creates device nodes itself, instead relying on |
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[f6721f7] | 110 | <systemitem class="filesystem">devtmpfs</systemitem> to do so.</para> |
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| 111 | |
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[806d0c2] | 112 | </sect3> |
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| 113 | |
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| 114 | <sect3> |
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[f6721f7] | 115 | <title>Systemd and Eudev</title> |
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| 116 | |
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[bbf3c21] | 117 | <para>In 2010, development began on systemd, an alternate |
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| 118 | <command>init</command> implementation. Starting with Udev 183, Udev's |
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| 119 | source tree was merged with systemd. Several Gentoo |
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| 120 | developers who disagreed with this merge announced a project fork |
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| 121 | called Eudev in December 2012, created by extracting the |
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| 122 | Udev code from systemd. One of the goals of Eudev is to allow for |
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| 123 | easier installation and usage of <command>udevd</command> without |
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| 124 | the need for the rest of systemd.</para> |
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[806d0c2] | 125 | </sect3> |
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| 126 | |
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[f6721f7] | 127 | </sect2> |
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| 128 | |
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[bbf3c21] | 129 | <sect2> |
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| 130 | <title>Device Node Creation</title> |
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| 131 | |
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| 132 | <para>By default, device nodes created by the kernel in a |
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| 133 | <systemitem class="filesystem">devtmpfs</systemitem> are owned by |
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| 134 | <emphasis>root:root</emphasis> and have <emphasis>600</emphasis> |
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| 135 | permissions. <command>udevd</command> can modify ownership and permissions |
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| 136 | of the nodes under the <filename class="directory">/dev</filename> |
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| 137 | directory, and can also create additional symlinks, based on rules |
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| 138 | specified in the files within the |
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| 139 | <filename class="directory">/etc/udev/rules.d</filename>, |
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| 140 | <filename class="directory">/lib/udev/rules.d</filename>, |
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| 141 | and <filename class="directory">/run/udev/rules.d</filename> directories. |
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| 142 | The names for these files start with a number, to indicate the order in |
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| 143 | which they are run, and they have a <filename>.rules</filename> |
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| 144 | extension (<command>udevd</command> will ignore files with any other |
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| 145 | extension). All of the rules files from these directories are combined into |
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| 146 | a single list, sorted by filename, and run in that order. In the event of |
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| 147 | a conflict, where a rules file with the same name exists in two or more of |
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| 148 | these directories, the rules in <filename class="directory">/etc</filename> |
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| 149 | take the highest priority, followed by rules files in |
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| 150 | <filename class="directory">/run</filename>, and finally |
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| 151 | <filename class="directory">/lib</filename>. Any device for which a rule |
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| 152 | cannot be found will just be ignored by <command>udevd</command> |
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[ae9c901] | 153 | and be left at the defaults defined by the kernel, as described above. <!-- For |
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[bbf3c21] | 154 | more details about writing Udev rules, see |
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[ae9c901] | 155 | <command>man 7 udev</command>. --></para> |
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[bbf3c21] | 156 | |
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| 157 | </sect2> |
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| 158 | |
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[f6721f7] | 159 | <sect2> |
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| 160 | <title>Module Loading</title> |
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| 161 | |
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| 162 | <para>Device drivers compiled as modules may have aliases built into them. |
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| 163 | Aliases are visible in the output of the <command>modinfo</command> |
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| 164 | program and are usually related to the bus-specific identifiers of devices |
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| 165 | supported by a module. For example, the <emphasis>snd-fm801</emphasis> |
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| 166 | driver supports PCI devices with vendor ID 0x1319 and device ID 0x0801, |
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| 167 | and has an alias of <quote>pci:v00001319d00000801sv*sd*bc04sc01i*</quote>. |
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| 168 | For most devices, the bus driver exports the alias of the driver that |
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| 169 | would handle the device via <systemitem |
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| 170 | class="filesystem">sysfs</systemitem>. E.g., the |
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| 171 | <filename>/sys/bus/pci/devices/0000:00:0d.0/modalias</filename> file |
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| 172 | might contain the string |
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| 173 | <quote>pci:v00001319d00000801sv00001319sd00001319bc04sc01i00</quote>. |
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| 174 | The default rules provided by Eudev will cause <command>udevd</command> |
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| 175 | to call out to <command>/sbin/modprobe</command> with the contents of the |
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| 176 | <envar>MODALIAS</envar> uevent environment variable (that should be the |
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| 177 | same as the contents of the <filename>modalias</filename> file in sysfs), |
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| 178 | thus loading all modules whose aliases match this string after wildcard |
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| 179 | expansion.</para> |
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| 180 | |
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| 181 | <para>In this example, this means that, in addition to |
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| 182 | <emphasis>snd-fm801</emphasis>, the obsolete (and unwanted) |
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| 183 | <emphasis>forte</emphasis> driver will be loaded if it is |
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| 184 | available. See below for ways in which the loading of unwanted drivers can |
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| 185 | be prevented.</para> |
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| 186 | |
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| 187 | <para>The kernel itself is also able to load modules for network |
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| 188 | protocols, filesystems and NLS support on demand.</para> |
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[806d0c2] | 189 | |
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[f6721f7] | 190 | </sect2> |
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[806d0c2] | 191 | |
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| 192 | <sect2> |
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| 193 | <title>Problems with Loading Modules and Creating Devices</title> |
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| 194 | |
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| 195 | <para>There are a few possible problems when it comes to automatically |
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| 196 | creating device nodes.</para> |
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| 197 | |
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| 198 | <sect3> |
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| 199 | <title>A kernel module is not loaded automatically</title> |
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| 200 | |
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[82fc053] | 201 | <para>Eudev will only load a module if it has a bus-specific alias and the |
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[806d0c2] | 202 | bus driver properly exports the necessary aliases to <systemitem |
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| 203 | class="filesystem">sysfs</systemitem>. In other cases, one should |
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[7349750] | 204 | arrange module loading by other means. With Linux-&linux-version2;, Eudev |
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| 205 | is known to load properly-written drivers for INPUT, IDE, PCI, USB, SCSI, |
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| 206 | SERIO and FireWire devices.</para> |
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[806d0c2] | 207 | |
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| 208 | <para>To determine if the device driver you require has the necessary |
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[82fc053] | 209 | support for Eudev, run <command>modinfo</command> with the module name as |
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[806d0c2] | 210 | the argument. Now try locating the device directory under |
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| 211 | <filename class="directory">/sys/bus</filename> and check whether there is |
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| 212 | a <filename>modalias</filename> file there.</para> |
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| 213 | |
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| 214 | <para>If the <filename>modalias</filename> file exists in <systemitem |
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| 215 | class="filesystem">sysfs</systemitem>, the driver supports the device and |
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| 216 | can talk to it directly, but doesn't have the alias, it is a bug in the |
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[82fc053] | 217 | driver. Load the driver without the help from Eudev and expect the issue |
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[806d0c2] | 218 | to be fixed later.</para> |
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| 219 | |
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| 220 | <para>If there is no <filename>modalias</filename> file in the relevant |
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| 221 | directory under <filename class="directory">/sys/bus</filename>, this |
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| 222 | means that the kernel developers have not yet added modalias support to |
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[7349750] | 223 | this bus type. With Linux-&linux-version2;, this is the |
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[87c0d57] | 224 | case with ISA busses. Expect this issue to be fixed in later kernel |
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| 225 | versions.</para> |
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[806d0c2] | 226 | |
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[82fc053] | 227 | <para>Eudev is not intended to load <quote>wrapper</quote> drivers such as |
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[806d0c2] | 228 | <emphasis>snd-pcm-oss</emphasis> and non-hardware drivers such as |
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| 229 | <emphasis>loop</emphasis> at all.</para> |
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| 230 | |
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| 231 | </sect3> |
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| 232 | |
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| 233 | <sect3> |
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[82fc053] | 234 | <title>A kernel module is not loaded automatically, and Eudev is not |
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[806d0c2] | 235 | intended to load it</title> |
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| 236 | |
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| 237 | <para>If the <quote>wrapper</quote> module only enhances the functionality |
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| 238 | provided by some other module (e.g., <emphasis>snd-pcm-oss</emphasis> |
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| 239 | enhances the functionality of <emphasis>snd-pcm</emphasis> by making the |
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| 240 | sound cards available to OSS applications), configure |
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[82fc053] | 241 | <command>modprobe</command> to load the wrapper after Eudev loads the |
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[bbf3c21] | 242 | wrapped module. To do this, add an <quote>install</quote> line to a file |
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| 243 | in <filename>/etc/modprobe.d</filename>. For example:</para> |
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[806d0c2] | 244 | |
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| 245 | <screen role="nodump"><literal>install snd-pcm /sbin/modprobe -i snd-pcm ; \ |
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| 246 | /sbin/modprobe snd-pcm-oss ; true</literal></screen> |
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| 247 | |
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[82fc053] | 248 | <para>If the module in question is not a wrapper and is useful by itself, |
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| 249 | configure the <command>S05modules</command> bootscript to load this |
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| 250 | module on system boot. To do this, add the module name to the |
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| 251 | <filename>/etc/sysconfig/modules</filename> file on a separate line. |
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| 252 | This works for wrapper modules too, but is suboptimal in that case.</para> |
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| 253 | |
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[806d0c2] | 254 | </sect3> |
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| 255 | |
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| 256 | <sect3> |
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[82fc053] | 257 | <title>Eudev loads some unwanted module</title> |
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[806d0c2] | 258 | |
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| 259 | <para>Either don't build the module, or blacklist it in |
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[bbf3c21] | 260 | <filename>/etc/modprobe.d</filename> file as done with the |
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[806d0c2] | 261 | <emphasis>forte</emphasis> module in the example below:</para> |
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| 262 | |
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| 263 | <screen role="nodump"><literal>blacklist forte</literal></screen> |
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| 264 | |
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| 265 | <para>Blacklisted modules can still be loaded manually with the |
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| 266 | explicit <command>modprobe</command> command.</para> |
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| 267 | |
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| 268 | </sect3> |
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| 269 | |
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| 270 | <sect3> |
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[bbf3c21] | 271 | <title>Eudev makes a wrong symlink</title> |
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[806d0c2] | 272 | |
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| 273 | <para>This usually happens if a rule unexpectedly matches a device. For |
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[5e420ce] | 274 | example, a poorly-written rule can match both a SCSI disk (as desired) |
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[806d0c2] | 275 | and the corresponding SCSI generic device (incorrectly) by vendor. |
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| 276 | Find the offending rule and make it more specific, with the help of |
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| 277 | <command>udevadm info</command>.</para> |
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| 278 | |
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| 279 | </sect3> |
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| 280 | |
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| 281 | <sect3> |
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[82fc053] | 282 | <title>Eudev rule works unreliably</title> |
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[806d0c2] | 283 | |
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| 284 | <para>This may be another manifestation of the previous problem. If not, |
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| 285 | and your rule uses <systemitem class="filesystem">sysfs</systemitem> |
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| 286 | attributes, it may be a kernel timing issue, to be fixed in later kernels. |
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| 287 | For now, you can work around it by creating a rule that waits for the used |
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| 288 | <systemitem class="filesystem">sysfs</systemitem> attribute and appending |
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| 289 | it to the <filename>/etc/udev/rules.d/10-wait_for_sysfs.rules</filename> |
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| 290 | file. Please notify the CLFS Development list if you do so and it |
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| 291 | helps.</para> |
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| 292 | |
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| 293 | </sect3> |
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| 294 | |
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| 295 | <sect3> |
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| 296 | <title>Device naming order changes randomly after rebooting</title> |
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| 297 | |
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[82fc053] | 298 | <para>This is due to the fact that Eudev, by design, handles uevents and |
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[806d0c2] | 299 | loads modules in parallel, and thus in an unpredictable order. This will |
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| 300 | never be <quote>fixed</quote>. You should not rely upon the kernel device |
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| 301 | names being stable. Instead, create your own rules that make symlinks with |
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| 302 | stable names based on some stable attributes of the device, such as a |
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[82fc053] | 303 | serial number or the output of various *_id utilities installed by Eudev. |
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[775d9a7] | 304 | See <xref linkend="ch-config-symlinks"/> and |
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[806d0c2] | 305 | <xref linkend="chapter-network"/> for examples.</para> |
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| 306 | |
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| 307 | </sect3> |
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| 308 | |
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| 309 | </sect2> |
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| 310 | |
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| 311 | <sect2> |
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| 312 | <title>Useful Reading</title> |
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| 313 | |
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| 314 | <para>Additional helpful documentation is available at the following |
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| 315 | sites:</para> |
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| 316 | |
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| 317 | <itemizedlist> |
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| 318 | |
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| 319 | <listitem> |
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| 320 | <para remap="verbatim">A Userspace Implementation of <systemitem class="filesystem">devfs</systemitem> |
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| 321 | <ulink url="http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf"/></para> |
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| 322 | </listitem> |
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| 323 | |
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| 324 | <listitem> |
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| 325 | <para remap="verbatim">The <systemitem class="filesystem">sysfs</systemitem> Filesystem |
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| 326 | <ulink url="http://www.kernel.org/pub/linux/kernel/people/mochel/doc/papers/ols-2005/mochel.pdf"/></para> |
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| 327 | </listitem> |
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| 328 | |
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| 329 | </itemizedlist> |
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| 330 | |
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| 331 | </sect2> |
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| 332 | |
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| 333 | </sect1> |
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