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238 lines
11 KiB
XML
238 lines
11 KiB
XML
<?xml version="1.0" encoding="ISO-8859-1"?>
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<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
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"http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd" [
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<!ENTITY % general-entities SYSTEM "../general.ent">
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%general-entities;
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]>
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<sect1 id="ch-scripts-udev">
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<?dbhtml filename="udev.html"?>
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<title>Device and Module Handling on an LFS System</title>
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<indexterm zone="ch-scripts-udev">
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<primary sortas="a-Udev">Udev</primary>
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<secondary>usage</secondary>
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</indexterm>
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<para>In <xref linkend="chapter-building-system"/>, we installed the Udev
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package. Before we go into the details regarding how this works,
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a brief history of previous methods of handling devices is in
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order.</para>
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<para>Linux systems in general traditionally use a static device creation
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method, whereby a great many device nodes are created under <filename
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class="directory">/dev</filename> (sometimes literally thousands of nodes),
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regardless of whether the corresponding hardware devices actually exist. This is
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typically done via a <command>MAKEDEV</command> script, which contains a number
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of calls to the <command>mknod</command> program with the relevant major and
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minor device numbers for every possible device that might exist in the world.
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Using the Udev method, only those devices which are detected by the kernel get
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device nodes created for them. Because these device nodes will be created each
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time the system boots, they will be stored on a <systemitem
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class="filesystem">tmpfs</systemitem> file system (a virtual file system that
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resides entirely in system memory). Device nodes do not require much space, so
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the memory that is used is negligible.</para>
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<sect2>
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<title>History</title>
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<para>In February 2000, a new filesystem called <systemitem
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class="filesystem">devfs</systemitem> was merged into the 2.3.46 kernel
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and was made available during the 2.4 series of stable kernels. Although
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it was present in the kernel source itself, this method of creating devices
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dynamically never received overwhelming support from the core kernel
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developers.</para>
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<para>The main problem with the approach adopted by <systemitem
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class="filesystem">devfs</systemitem> was the way it handled device
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detection, creation, and naming. The latter issue, that of device node
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naming, was perhaps the most critical. It is generally accepted that if
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device names are allowed to be configurable, then the device naming policy
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should be up to a system administrator, not imposed on them by any
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particular developer(s). The <systemitem class="filesystem">devfs</systemitem>
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file system also suffers from race conditions that are inherent in its design
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and cannot be fixed without a substantial revision to the kernel. It has also
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been marked as deprecated due to a lack of recent maintenance.</para>
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<para>With the development of the unstable 2.5 kernel tree, later released as
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the 2.6 series of stable kernels, a new virtual filesystem called <systemitem
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class="filesystem">sysfs</systemitem> came to be. The job of <systemitem
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class="filesystem">sysfs</systemitem> is to export a view of the system's
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hardrware configuration to userspace processes. With this userspace-visible
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representation, the possibility of seeing a userspace replacement for
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<systemitem class="filesystem">devfs</systemitem> became much more
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realistic.</para>
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</sect2>
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<sect2>
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<title>Udev Implementation</title>
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<para>The <systemitem class="filesystem">sysfs</systemitem> filesystem was
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mentioned briefly above. One may wonder how <systemitem
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class="filesystem">sysfs</systemitem> knows about the devices present on
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a system and what device numbers should be used for them. Drivers that have
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been compiled into the kernel directly register their objects with
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<systemitem class="filesystem">sysfs</systemitem> as they are detected by
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the kernel. For drivers compiled as modules, this registration will happen
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when the module is loaded. Once the <systemitem
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class="filesystem">sysfs</systemitem> filesystem is mounted (on <filename
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class="directory">/sys</filename>), data which the built-in drivers
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registered with <systemitem class="filesystem">sysfs</systemitem> are
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available to userspace processes and to <command>udev</command> for device
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node creation.</para>
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<para>The <command>S10udev</command> initscript takes care of creating
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these device nodes when Linux is booted. This script starts by registering
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<command>/sbin/udevsend</command> as a hotplug event handler. Hotplug events
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(discussed below) are not usually generated during this stage, but
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<command>udev</command> is registered just in case they do occur. The
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<command>udevstart</command> program then walks through the <systemitem
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class="filesystem">/sys</systemitem> filesystem and creates devices under
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<filename class="directory">/dev</filename> that match the descriptions.
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For example, <filename>/sys/class/tty/vcs/dev</filename> contains the
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string <quote>7:0</quote> This string is used by <command>udevstart</command>
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to create <filename>/dev/vcs</filename> with major number
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<emphasis>7</emphasis> and minor <emphasis>0</emphasis>. The names and
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permissions of the nodes created under the <filename
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class="directory">/dev</filename> directory are configured according to the
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rules specified in the files within the <filename
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class="directory">/etc/udev/rules.d/</filename> directory. These are
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numbered in a similar fashion to the LFS-Bootscripts package. If
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<command>udev</command> can't find a rule for the device it is creating,
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it will default permissions to <emphasis>660</emphasis> and ownership to
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<emphasis>root:root</emphasis>.</para>
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<para>Once the above stage is complete, all devices that were already present
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and have compiled-in drivers will be available for use. This leads us to the
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devices that have modular drivers.</para>
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<para>Earlier, we mentioned the concept of a <quote>hotplug event
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handler.</quote> When a new device connection is detected by the kernel,
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the kernel will generate a hotplug event and look at the file
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<filename>/proc/sys/kernel/hotplug</filename> to determine the userspace
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program that handles the device's connection. The <command>udev</command>
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bootscript registered <command>udevsend</command> as this handler. When
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these hotplug events are generated, the kernel will tell
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<command>udev</command> to check the <filename
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class="directory">/sys</filename> filesystem for the information pertaining
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to this new device and create the <filename class="directory">/dev</filename>
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entry for it.</para>
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<para>This brings us to one problem that exists with <command>udev</command>,
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and likewise with <systemitem class="filesystem">devfs</systemitem> before it.
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It is commonly referred to as the <quote>chicken and egg</quote> problem. Most
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Linux distributions handle loading modules via entries in
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<filename>/etc/modules.conf</filename>. Access to a device node causes the
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appropriate kernel module to load. With <command>udev</command>, this method
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will not work because the device node does not exist until the module is loaded.
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To solve this, the <command>S05modules</command> bootscript was added to the
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LFS-Bootscripts package, along with the
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<filename>/etc/sysconfig/modules</filename> file. By adding module names to the
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<filename>modules</filename> file, these modules will be loaded when the
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computer starts up. This allows <command>udev</command> to detect the devices
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and create the appropriate device nodes.</para>
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<para>Note that on slower machines or for drivers that create a lot of device
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nodes, the process of creating devices may take a few seconds to complete.
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This means that some device nodes may not be immediately accessible.</para>
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</sect2>
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<sect2>
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<title>Handling Hotpluggable/Dynamic Devices</title>
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<para>When you plug in a device, such as a Universal Serial Bus (USB) MP3
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player, the kernel recognizes that the device is now connected and generates
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a hotplug event. If the driver is already loaded (either because it was
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compiled into the kernel or because it was loaded via the
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<command>S05modules</command> bootscript), <command>udev</command> will be
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called upon to create the relevant device node(s) according to the
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<systemitem class="filesystem">sysfs</systemitem> data available in
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<filename class="directory">/sys</filename>.</para>
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<para>If the driver for the just plugged in device is available as a module but
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currently unloaded, the Hotplug package will load the appropriate module
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and make this device available by creating the device node(s) for it.</para>
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</sect2>
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<sect2>
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<title>Problems with Creating Devices</title>
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<para>There are a few known problems when it comes to automatically creating
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device nodes:</para>
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<para>1) A kernel driver may not export its data to <systemitem
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class="filesystem">sysfs</systemitem>.</para>
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<para>This is most common with third party drivers from outside the kernel
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tree. Udev will be unable to automatically create device nodes for such
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drivers. Use the <filename>/etc/sysconfig/createfiles</filename>
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configuration file to manually create the devices. Consult the
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<filename>devices.txt</filename> file inside the kernel documentation or
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the documentation for that driver to find the proper major/minor
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numbers.</para>
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<para>2) A non-hardware device is required. This is most common with
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the Advanced Linux Sound Architecture (ALSA) project's Open Sound
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System (OSS) compatibility module. These types of devices can be
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handled in one of two ways:</para>
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<itemizedlist>
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<listitem>
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<para>Adding the module names to
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<filename>/etc/sysconfig/modules</filename></para>
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</listitem>
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<listitem>
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<para>Using an <quote>install</quote> line in
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<filename>/etc/modprobe.conf</filename>. This tells the
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<command>modprobe</command> command <quote>when loading this module,
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also load this other module, at the same time.</quote>
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For example:</para>
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<screen role="nodump"><userinput>install snd-pcm modprobe -i snd-pcm ; modprobe \
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snd-pcm-oss ; true</userinput></screen>
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<para>This will cause the system to load both the
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<emphasis>snd-pcm</emphasis> and <emphasis>snd-pcm-oss</emphasis>
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modules when any request is made to load the driver
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<emphasis>snd-pcm</emphasis>.</para>
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</listitem>
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</itemizedlist>
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</sect2>
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<sect2>
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<title>Useful Reading</title>
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<para>Additional helpful documentation is available at the following
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sites:</para>
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<itemizedlist>
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<listitem>
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<para>A Userspace Implementation of <systemitem class="filesystem">devfs</systemitem>
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<ulink url="http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf"/></para>
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</listitem>
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<listitem>
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<para>udev FAQ
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<ulink url="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev-FAQ"/></para>
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</listitem>
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<listitem>
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<para>The Linux Kernel Driver Model
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<ulink url="http://public.planetmirror.com/pub/lca/2003/proceedings/papers/Patrick_Mochel/Patrick_Mochel.pdf"/></para>
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</listitem>
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</itemizedlist>
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</sect2>
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</sect1>
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