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345 lines
16 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/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-config-udev">
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<?dbhtml filename="udev.html"?>
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<title>Overview of Device and Module Handling</title>
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<indexterm zone="ch-config-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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daemon when <phrase revision="sysv">udev</phrase>
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<phrase revision="systemd">systemd</phrase> was built. Before we go into the
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details regarding how udev works, a brief history of previous methods of
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handling devices is in order.</para>
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<para>Linux systems in general traditionally used a static device creation
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method, whereby a great many device nodes were 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 existed. This
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was typically done via a <command>MAKEDEV</command> script, which contained a
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number of calls to the <command>mknod</command> program with the relevant
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major and minor device numbers for every possible device that might exist in
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the world.</para>
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<para>Using the udev method, device nodes are only created for those devices
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which are detected by the kernel. These device nodes are
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created each time the system boots; they are stored in a <systemitem
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class="filesystem">devtmpfs</systemitem> file system (a virtual file system
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that resides entirely in system memory). Device nodes do not require much
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space, so 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 configurable, the device naming policy
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should be chosen by system administrators, and not imposed on them by the
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developer(s). The <systemitem
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class="filesystem">devfs</systemitem> file system also suffered from race
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conditions that were inherent in its design; these could not be fixed without a
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substantial revision of the kernel. <systemitem class="filesystem">devfs</systemitem>
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was marked as deprecated for a long
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time, and was finally removed
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from the kernel in June, 2006.</para>
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<para>With the development of the unstable 2.5 kernel tree, later released
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as the 2.6 series of stable kernels, a new virtual filesystem called
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<systemitem class="filesystem">sysfs</systemitem> came to be. The job of
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<systemitem class="filesystem">sysfs</systemitem> is to provide information about
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the system's hardware configuration to userspace processes. With this
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userspace-visible representation, it became possible to develop a userspace
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replacement for <systemitem class="filesystem">devfs</systemitem>.</para>
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</sect2>
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<sect2>
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<title>Udev Implementation</title>
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<sect3>
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<title>Sysfs</title>
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<para>The <systemitem class="filesystem">sysfs</systemitem> filesystem
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was 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
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have been compiled into the kernel register their objects in
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<systemitem class="filesystem">sysfs</systemitem> (devtmpfs internally)
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as they are detected by the kernel. For drivers compiled as modules,
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registration happens when the module is loaded. Once the <systemitem
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class="filesystem">sysfs</systemitem> filesystem is mounted (on
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<filename class="directory">/sys</filename>),
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data which the drivers have registered with <systemitem
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class="filesystem">sysfs</systemitem> are available to userspace
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processes and to udevd for processing (including modifications to device
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nodes).</para>
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</sect3>
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<sect3 id='ch-config-udev-device-node-creation'>
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<title>Device Node Creation</title>
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<para>Device files are created by the kernel in the <systemitem
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class="filesystem">devtmpfs</systemitem> file system. Any driver that
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wishes to register a device node will use the <systemitem
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class="filesystem">devtmpfs</systemitem> (via the driver core) to do it.
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When a <systemitem class="filesystem">devtmpfs</systemitem> instance is
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mounted on <filename class="directory">/dev</filename>, the device node
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will initially be exposed to userspace with a fixed name, permissions, and
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owner.</para>
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<para>A short time later, the kernel will send a uevent to <command>
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udevd</command>. Based on the rules specified in the files within the
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<filename class="directory">/etc/udev/rules.d</filename>, <filename
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class="directory">/usr/lib/udev/rules.d</filename>, and <filename
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class="directory">/run/udev/rules.d</filename> directories, <command>
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udevd</command> will create additional symlinks to the device node, or
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change its permissions, owner, or group, or modify the internal
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<command>udevd</command> database entry (name) for that object.</para>
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<para>The rules in these three directories are numbered and all three
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directories are merged together. If <command>udevd</command> can't find a
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rule for the device it is creating, it will leave the permissions and
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ownership at whatever <systemitem
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class="filesystem">devtmpfs</systemitem> used initially.</para> </sect3>
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<sect3 id="module-loading">
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<title>Module Loading</title>
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<para>Device drivers compiled as modules may have aliases built into them.
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Aliases are visible in the output of the <command>modinfo</command>
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program and are usually related to the bus-specific identifiers of devices
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supported by a module. For example, the <emphasis>snd-fm801</emphasis>
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driver supports PCI devices with vendor ID 0x1319 and device ID 0x0801,
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and has an alias of <literal>pci:v00001319d00000801sv*sd*bc04sc01i*</literal>.
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For most devices, the bus driver exports the alias of the driver that
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would handle the device via <systemitem
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class="filesystem">sysfs</systemitem>. E.g., the
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<filename>/sys/bus/pci/devices/0000:00:0d.0/modalias</filename> file
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might contain the string
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<literal>pci:v00001319d00000801sv00001319sd00001319bc04sc01i00</literal>.
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The default rules provided with udev will cause <command>udevd</command>
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to call out to <command>/sbin/modprobe</command> with the contents of the
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<envar>MODALIAS</envar> uevent environment variable (which should be the
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same as the contents of the <filename>modalias</filename> file in sysfs),
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thus loading all modules whose aliases match this string after wildcard
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expansion.</para>
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<para>In this example, this means that, in addition to
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<emphasis>snd-fm801</emphasis>, the obsolete (and unwanted)
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<emphasis>forte</emphasis> driver will be loaded if it is
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available. See below for ways in which the loading of unwanted drivers can
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be prevented.</para>
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<para>The kernel itself is also able to load modules for network
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protocols, filesystems, and NLS support on demand.</para>
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</sect3>
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<sect3>
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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
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generates a uevent. This uevent is then handled by
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<command>udevd</command> as described above.</para>
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</sect3>
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</sect2>
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<sect2>
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<title>Problems with Loading Modules and Creating Devices</title>
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<para>There are a few possible problems when it comes to automatically
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creating device nodes.</para>
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<sect3>
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<title>A Kernel Module Is Not Loaded Automatically</title>
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<para>Udev will only load a module if it has a bus-specific alias and the
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bus driver properly exports the necessary aliases to <systemitem
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class="filesystem">sysfs</systemitem>. In other cases, one should
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arrange module loading by other means. With Linux-&linux-version;, udev is
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known to load properly-written drivers for INPUT, IDE, PCI, USB, SCSI,
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SERIO, and FireWire devices.</para>
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<para>To determine if the device driver you require has the necessary
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support for udev, run <command>modinfo</command> with the module name as
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the argument. Now try locating the device directory under
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<filename class="directory">/sys/bus</filename> and check whether there is
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a <filename>modalias</filename> file there.</para>
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<para>If the <filename>modalias</filename> file exists in <systemitem
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class="filesystem">sysfs</systemitem>, the driver supports the device and
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can talk to it directly, but doesn't have the alias, it is a bug in the
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driver. Load the driver without the help from udev and expect the issue
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to be fixed later.</para>
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<para>If there is no <filename>modalias</filename> file in the relevant
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directory under <filename class="directory">/sys/bus</filename>, this
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means that the kernel developers have not yet added modalias support to
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this bus type. With Linux-&linux-version;, this is the case with ISA
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busses. Expect this issue to be fixed in later kernel versions.</para>
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<para>Udev is not intended to load <quote>wrapper</quote> drivers such as
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<emphasis>snd-pcm-oss</emphasis> and non-hardware drivers such as
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<emphasis>loop</emphasis> at all.</para>
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</sect3>
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<sect3>
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<title>A Kernel Module Is Not Loaded Automatically, and Udev Is Not
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Intended to Load It</title>
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<para>If the <quote>wrapper</quote> module only enhances the
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functionality provided by some other module (e.g.,
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<emphasis>snd-pcm-oss</emphasis> enhances the functionality of
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<emphasis>snd-pcm</emphasis> by making the sound cards available to OSS
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applications), configure <command>modprobe</command> to load the wrapper
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after udev loads the wrapped module. To do this, add a
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<quote>softdep</quote> line to the corresponding
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<filename>/etc/modprobe.d/<replaceable><filename></replaceable>.conf</filename>
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file. For example:</para>
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<screen role="nodump"><literal>softdep snd-pcm post: snd-pcm-oss</literal></screen>
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<para>Note that the <quote>softdep</quote> command also allows
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<literal>pre:</literal> dependencies, or a mixture of both
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<literal>pre:</literal> and <literal>post:</literal> dependencies. See
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the <ulink role='man' url='&man;modprobe.d.5'>modprobe.d(5)</ulink>
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manual page for more information on <quote>softdep</quote> syntax and
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capabilities.</para>
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<para revision="sysv">If the module in question is not a wrapper and is
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useful by itself, configure the <command>modules</command> bootscript to
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load this module on system boot. To do this, add the module name to the
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<filename>/etc/sysconfig/modules</filename> file on a separate line.
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This works for wrapper modules too, but is suboptimal in that case.</para>
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</sect3>
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<sect3>
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<title>Udev Loads Some Unwanted Module</title>
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<para>Either don't build the module, or blacklist it in a
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<filename>/etc/modprobe.d/blacklist.conf</filename> file as done with the
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<emphasis>forte</emphasis> module in the example below:</para>
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<screen role="nodump"><literal>blacklist forte</literal></screen>
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<para>Blacklisted modules can still be loaded manually with the
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explicit <command>modprobe</command> command.</para>
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</sect3>
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<sect3>
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<title>Udev Creates a Device Incorrectly, or Makes the Wrong Symlink</title>
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<para>This usually happens if a rule unexpectedly matches a device. For
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example, a poorly-written rule can match both a SCSI disk (as desired)
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and the corresponding SCSI generic device (incorrectly) by vendor.
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Find the offending rule and make it more specific, with the help of the
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<command>udevadm info</command> command.</para>
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</sect3>
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<sect3>
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<title>Udev Rule Works Unreliably</title>
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<para>This may be another manifestation of the previous problem. If not,
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and your rule uses <systemitem class="filesystem">sysfs</systemitem>
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attributes, it may be a kernel timing issue, to be fixed in later kernels.
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For now, you can work around it by creating a rule that waits for the used
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<systemitem class="filesystem">sysfs</systemitem> attribute and appending
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it to the <filename>/etc/udev/rules.d/10-wait_for_sysfs.rules</filename>
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file (create this file if it does not exist). Please notify the LFS
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Development list if you do so and it helps.</para>
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</sect3>
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<sect3>
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<title>Udev Does Not Create a Device</title>
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<para>First, be certain that the driver is built into the
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kernel or already loaded as a module, and that
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udev isn't creating a misnamed device.</para>
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<para>If a kernel driver does not export its data to
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<systemitem class="filesystem">sysfs</systemitem>, udev lacks the
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information needed to create a device node. This is most likely to happen
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with third party drivers from outside the kernel tree. Create a static
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device node in <filename>/usr/lib/udev/devices</filename> with the
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appropriate major/minor numbers (see the file
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<filename>devices.txt</filename> inside the kernel documentation or the
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documentation provided by the third party driver vendor). The static
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device node will be copied to <filename class="directory">/dev</filename>
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by <command>udev</command>.</para>
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</sect3>
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<sect3>
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<title>Device Naming Order Changes Randomly After Rebooting</title>
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<para>This is due to the fact that udev, by design, handles uevents and
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loads modules in parallel, and thus in an unpredictable order. This will
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never be <quote>fixed.</quote> You should not rely upon the kernel device
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names being stable. Instead, create your own rules that make symlinks with
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stable names based on some stable attributes of the device, such as a
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serial number or the output of various *_id utilities installed by udev.
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See <xref linkend="ch-config-symlinks"/> and
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<xref linkend="ch-config-network"/> for examples.</para>
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</sect3>
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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>The <systemitem class="filesystem">sysfs</systemitem> Filesystem
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<ulink url="https://www.kernel.org/pub/linux/kernel/people/mochel/doc/papers/ols-2005/mochel.pdf"/></para>
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</listitem>
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<!-- No longer available
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<listitem>
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<para>Pointers to further reading
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<ulink url="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev.html"/>
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</para>
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</listitem>
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-->
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</itemizedlist>
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</sect2>
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</sect1>
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