[3f8be484] | 1 | <?xml version="1.0" encoding="ISO-8859-1"?> |
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| 2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN" |
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| 3 | "http://www.oasis-open.org/docbook/xml/4.4/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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| 8 | <sect1 id="ch-scripts-udev"> |
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| 9 | <?dbhtml filename="udev.html"?> |
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| 10 | |
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[fb40919] | 11 | <title>Device and Module Handling on a CLFS System</title> |
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[3f8be484] | 12 | |
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| 13 | <indexterm zone="ch-scripts-udev"> |
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| 14 | <primary sortas="a-Udev">Udev</primary> |
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| 15 | <secondary>usage</secondary></indexterm> |
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| 16 | |
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| 17 | <para>In <xref linkend="chapter-building-system"/>, we installed the Udev |
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| 18 | package. Before we go into the details regarding how this works, a brief |
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| 19 | history of previous methods of handling devices is in order.</para> |
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| 20 | |
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| 21 | <para>Linux systems in general traditionally use a static device creation |
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| 22 | method, whereby a great many device nodes are created under <filename |
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| 23 | class="directory">/dev</filename> (sometimes literally thousands of nodes), |
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| 24 | regardless of whether the corresponding hardware devices actually exist. |
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| 25 | This is typically done via a <command>MAKEDEV</command> script, which |
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| 26 | contains a number of calls to the <command>mknod</command> program with |
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| 27 | the relevant major and minor device numbers for every possible device that |
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| 28 | might exist in the world. Using the Udev method, only those devices which |
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| 29 | are detected by the kernel get device nodes created for them. Because |
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| 30 | these device nodes will be created each time the system boots, they will |
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| 31 | be stored on a <systemitem class="filesystem">tmpfs</systemitem> (a virtual |
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| 32 | file system that resides entirely in system memory). Device nodes do not |
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| 33 | require much space, so the memory that is used is negligible.</para> |
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| 34 | |
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| 35 | <sect2> |
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| 36 | <title>History</title> |
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| 37 | |
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| 38 | <para>In February 2000, a new filesystem called <systemitem |
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| 39 | class="filesystem">devfs</systemitem> was merged into the 2.3.46 kernel |
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| 40 | and was made available during the 2.4 series of stable kernels. Although |
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| 41 | it was present in the kernel source itself, this method of creating |
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| 42 | devices dynamically never received overwhelming support from the core |
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| 43 | kernel developers.</para> |
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| 44 | |
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| 45 | <para>The main problem with the approach adopted by <systemitem |
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| 46 | class="filesystem">devfs</systemitem> was the way it handled |
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| 47 | device detection, creation, and naming. The latter issue, that of |
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| 48 | device node naming, was perhaps the most critical. It is generally |
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| 49 | accepted that if device names are allowed to be configurable, then |
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| 50 | the device naming policy should be up to a system administrator, not |
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| 51 | imposed on them by any particular developer(s). The <systemitem |
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| 52 | class="filesystem">devfs</systemitem> file system also suffers from race |
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| 53 | conditions that are inherent in its design and cannot be fixed |
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| 54 | without a substantial revision to the kernel. It has also been marked |
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| 55 | as deprecated due to a lack of recent maintenance.</para> |
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| 56 | |
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| 57 | <para>With the development of the unstable 2.5 kernel tree, later |
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| 58 | released as the 2.6 series of stable kernels, a new virtual filesystem |
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| 59 | called <systemitem class="filesystem">sysfs</systemitem> came to be. The |
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| 60 | job of <systemitem class="filesystem">sysfs</systemitem> is to export a |
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| 61 | view of the system's hardware configuration to userspace processes. With |
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| 62 | this userspace-visible representation, the possibility of seeing a |
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| 63 | userspace replacement for <systemitem class="filesystem">devfs</systemitem> |
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| 64 | became much more realistic.</para> |
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| 65 | |
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| 66 | </sect2> |
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| 67 | |
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| 68 | <sect2> |
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| 69 | <title>Udev Implementation</title> |
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| 70 | |
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| 71 | <para>The <systemitem class="filesystem">sysfs</systemitem> filesystem |
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| 72 | was mentioned briefly above. One may wonder how <systemitem |
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| 73 | class="filesystem">sysfs</systemitem> knows about the devices present |
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| 74 | on a system and what device numbers should be used for them. Drivers |
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| 75 | that have been compiled into the kernel directly register their objects |
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| 76 | with <systemitem class="filesystem">sysfs</systemitem> as they are |
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| 77 | detected by the kernel. For drivers compiled as modules, this |
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| 78 | registration will happen when the module is loaded. Once the |
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| 79 | <systemitem class="filesystem">sysfs</systemitem> filesystem is mounted |
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| 80 | (on <filename class="directory">/sys</filename>), data which the built-in |
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| 81 | drivers registered with <systemitem class="filesystem">sysfs</systemitem> |
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| 82 | are available to userspace processes and to <command>udev</command> for |
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| 83 | device node creation.</para> |
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| 84 | |
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| 85 | <para>The <command>S10udev</command> initscript takes care of creating |
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| 86 | these device nodes when Linux is booted. This script starts by registering |
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| 87 | <command>/sbin/udevsend</command> as a hotplug event handler. Hotplug |
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| 88 | events (discussed below) are not usually generated during this stage, |
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| 89 | but <command>udev</command> is registered just in case they do occur. |
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| 90 | The <command>udevstart</command> program then walks through the |
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| 91 | <systemitem class="filesystem">/sys</systemitem> filesystem and creates |
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| 92 | devices under <filename class="directory">/dev</filename> that match the |
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| 93 | descriptions. For example, <filename>/sys/class/tty/vcs/dev</filename> |
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| 94 | contains the string <quote>7:0</quote> This string is used by |
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| 95 | <command>udevstart</command> to create <filename>/dev/vcs</filename> |
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| 96 | with major number <emphasis>7</emphasis> and minor <emphasis>0</emphasis>. |
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| 97 | The names and permissions of the nodes created under the <filename |
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| 98 | class="directory">/dev</filename> directory are configured according to |
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| 99 | the rules specified in the files within the <filename |
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| 100 | class="directory">/etc/udev/rules.d/</filename> directory. These are |
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[fb40919] | 101 | numbered in a similar fashion to the CLFS-Bootscripts package. If |
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[3f8be484] | 102 | <command>udev</command> can't find a rule for the device it is creating, |
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| 103 | it will default permissions to <emphasis>660</emphasis> and ownership to |
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| 104 | <emphasis>root:root</emphasis>.</para> |
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| 105 | |
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| 106 | <para>Once the above stage is complete, all devices that were already |
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| 107 | present and have compiled-in drivers will be available for use. This |
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| 108 | leads us to the devices that have modular drivers.</para> |
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| 109 | |
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| 110 | <para>Earlier, we mentioned the concept of a <quote>hotplug event |
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| 111 | handler.</quote> When a new device connection is detected by the kernel, |
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| 112 | the kernel will generate a hotplug event and look at the file |
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| 113 | <filename>/proc/sys/kernel/hotplug</filename> to determine the userspace |
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| 114 | program that handles the device's connection. The <command>udev</command> |
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| 115 | bootscript registered <command>udevsend</command> as this handler. When |
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| 116 | these hotplug events are generated, the kernel will tell |
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| 117 | <command>udev</command> to check the <filename |
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| 118 | class="directory">/sys</filename> filesystem for the information |
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| 119 | pertaining to this new device and create the <filename |
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| 120 | class="directory">/dev</filename> entry for it.</para> |
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| 121 | |
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| 122 | <para>This brings us to one problem that exists with |
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| 123 | <command>udev</command>, and likewise with <systemitem |
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| 124 | class="filesystem">devfs</systemitem> before it. It is commonly |
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| 125 | referred to as the <quote>chicken and egg</quote> problem. Most |
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| 126 | Linux distributions handle loading modules via entries in |
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| 127 | <filename>/etc/modules.conf</filename>. Access to a device node causes |
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| 128 | the appropriate kernel module to load. With <command>udev</command>, |
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| 129 | this method will not work because the device node does not exist until |
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| 130 | the module is loaded. To solve this, the <command>S05modules</command> |
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[fb40919] | 131 | bootscript was added to the CLFS-Bootscripts package, along with the |
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[3f8be484] | 132 | <filename>/etc/sysconfig/modules</filename> file. By adding module |
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| 133 | names to the <filename>modules</filename> file, these modules will be |
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| 134 | loaded when the computer starts up. This allows <command>udev</command> |
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| 135 | to detect the devices and create the appropriate device nodes.</para> |
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| 136 | |
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| 137 | <para>Note that on slower machines or for drivers that create a lot |
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| 138 | of device nodes, the process of creating devices may take a few |
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| 139 | seconds to complete. This means that some device nodes may not be |
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| 140 | immediately accessible.</para> |
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| 141 | |
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| 142 | </sect2> |
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| 143 | |
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| 144 | <sect2> |
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| 145 | <title>Handling Hotpluggable/Dynamic Devices</title> |
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| 146 | |
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| 147 | <para>When you plug in a device, such as a Universal Serial Bus (USB) |
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| 148 | MP3 player, the kernel recognizes that the device is now connected and |
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| 149 | generates a hotplug event. If the driver is already loaded (either |
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| 150 | because it was compiled into the kernel or because it was loaded via |
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| 151 | the <command>S05modules</command> bootscript), <command>udev</command> |
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| 152 | will be called upon to create the relevant device node(s) according to |
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| 153 | the <systemitem class="filesystem">sysfs</systemitem> data available in |
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| 154 | <filename class="directory">/sys</filename>.</para> |
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| 155 | |
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| 156 | <para>If the driver for the just plugged in device is available as a |
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| 157 | module but currently unloaded, the Hotplug package will load the |
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| 158 | appropriate module and make this device available by creating the |
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| 159 | device node(s) for it.</para> |
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| 160 | |
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| 161 | </sect2> |
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| 162 | |
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| 163 | <sect2> |
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| 164 | <title>Problems with Creating Devices</title> |
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| 165 | |
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| 166 | <para>There are a few known problems when it comes to automatically |
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| 167 | creating device nodes:</para> |
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| 168 | |
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| 169 | <para>1) A kernel driver may not export its data to <systemitem |
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| 170 | class="filesystem">sysfs</systemitem>.</para> |
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| 171 | |
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| 172 | <para>This is most common with third party drivers from outside the |
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| 173 | kernel tree. Udev will be unable to automatically create device nodes |
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| 174 | for such drivers. Use the <filename>/etc/sysconfig/createfiles</filename> |
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| 175 | configuration file to manually create the devices. Consult the |
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| 176 | <filename>devices.txt</filename> file inside the kernel documentation |
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| 177 | or the documentation for that driver to find the proper major/minor |
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| 178 | numbers.</para> |
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| 179 | |
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| 180 | <para>2) A non-hardware device is required. This is most common with |
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| 181 | the Advanced Linux Sound Architecture (ALSA) project's Open Sound |
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| 182 | System (OSS) compatibility module. These types of devices can be |
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| 183 | handled in one of two ways:</para> |
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| 184 | |
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| 185 | <itemizedlist> |
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| 186 | <listitem> |
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| 187 | <para>Adding the module names to |
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| 188 | <filename>/etc/sysconfig/modules</filename></para> |
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| 189 | </listitem> |
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| 190 | <listitem> |
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| 191 | <para>Using an <quote>install</quote> line in |
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| 192 | <filename>/etc/modprobe.conf</filename>. This tells the |
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| 193 | <command>modprobe</command> command <quote>when loading this |
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| 194 | module, also load this other module, at the same time.</quote> |
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| 195 | For example:</para> |
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| 196 | |
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| 197 | <screen><userinput>install snd-pcm modprobe -i snd-pcm ; modprobe \ |
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| 198 | snd-pcm-oss ; true</userinput></screen> |
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| 199 | |
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| 200 | <para>This will cause the system to load both the |
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| 201 | <emphasis>snd-pcm</emphasis> and <emphasis>snd-pcm-oss</emphasis> |
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| 202 | modules when any request is made to load the driver |
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| 203 | <emphasis>snd-pcm</emphasis>.</para> |
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| 204 | </listitem> |
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| 205 | </itemizedlist> |
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| 206 | |
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| 207 | </sect2> |
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| 208 | |
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| 209 | <sect2> |
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| 210 | <title>Useful Reading</title> |
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| 211 | |
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| 212 | <para>Additional helpful documentation is available at the following |
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| 213 | sites:</para> |
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| 214 | |
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| 215 | <itemizedlist> |
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| 216 | <listitem> |
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| 217 | <para remap="verbatim">A Userspace Implementation of <systemitem class="filesystem">devfs</systemitem> |
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| 218 | <ulink url="http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf"/></para> |
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| 219 | </listitem> |
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| 220 | <listitem> |
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| 221 | <para remap="verbatim">udev FAQ |
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| 222 | <ulink url="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev-FAQ"/></para> |
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| 223 | </listitem> |
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| 224 | <listitem> |
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| 225 | <para remap="verbatim">The Linux Kernel Driver Model |
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| 226 | <ulink url="http://public.planetmirror.com/pub/lca/2003/proceedings/papers/Patrick_Mochel/Patrick_Mochel.pdf"/></para> |
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| 227 | </listitem> |
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| 228 | </itemizedlist> |
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| 229 | |
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| 230 | </sect2> |
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| 231 | |
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| 232 | </sect1> |
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