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+<?xml version="1.0" encoding="ISO-8859-1"?>
+<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
+  <!ENTITY % general-entities SYSTEM "../general.ent">
+  %general-entities;
+]>
+
+<sect1 id="ch-config-udev">
+  <?dbhtml filename="udev.html"?>
+
+  <title>Overview of Device and Module Handling</title>
+
+  <indexterm zone="ch-config-udev">
+    <primary sortas="a-Udev">Udev</primary>
+    <secondary>usage</secondary>
+  </indexterm>
+
+  <para>In <xref linkend="chapter-building-system"/>, we installed the Udev
+  package when <phrase revision="sysv">eudev</phrase>
+  <phrase revision="systemd">systemd</phrase> was built. Before we go into the
+  details regarding how this works, a brief history of previous methods of
+  handling devices is in order.</para>
+
+  <para>Linux systems in general traditionally used a static device creation
+  method, whereby a great many device nodes were created under <filename
+  class="directory">/dev</filename> (sometimes literally thousands of nodes),
+  regardless of whether the corresponding hardware devices actually existed. This
+  was typically done via a <command>MAKEDEV</command> script, which contains a
+  number of calls to the <command>mknod</command> program with the relevant
+  major and minor device numbers for every possible device that might exist in
+  the world.</para>
+
+  <para>Using the Udev method, only those devices which are detected by the
+  kernel get device nodes created for them. Because these device nodes will be
+  created each time the system boots, they will be stored on a <systemitem
+  class="filesystem">devtmpfs</systemitem> file system (a virtual file system
+  that resides entirely in system memory). Device nodes do not require much
+  space, so the memory that is used is negligible.</para>
+
+  <sect2>
+    <title>History</title>
+
+    <para>In February 2000, a new filesystem called <systemitem
+    class="filesystem">devfs</systemitem> was merged into the 2.3.46 kernel
+    and was made available during the 2.4 series of stable kernels. Although
+    it was present in the kernel source itself, this method of creating devices
+    dynamically never received overwhelming support from the core kernel
+    developers.</para>
+
+    <para>The main problem with the approach adopted by <systemitem
+    class="filesystem">devfs</systemitem> was the way it handled device
+    detection, creation, and naming. The latter issue, that of device node
+    naming, was perhaps the most critical. It is generally accepted that if
+    device names are allowed to be configurable, then the device naming policy
+    should be up to a system administrator, not imposed on them by any
+    particular developer(s). The <systemitem
+    class="filesystem">devfs</systemitem> file system also suffered from race
+    conditions that were inherent in its design and could not be fixed without a
+    substantial revision to the kernel. It was marked as deprecated for a long
+    period &ndash; due to a lack of maintenance &ndash; and was finally removed
+    from the kernel in June, 2006.</para>
+
+    <para>With the development of the unstable 2.5 kernel tree, later released
+    as the 2.6 series of stable kernels, a new virtual filesystem called
+    <systemitem class="filesystem">sysfs</systemitem> came to be. The job of
+    <systemitem class="filesystem">sysfs</systemitem> is to export a view of
+    the system's hardware configuration to userspace processes. With this
+    userspace-visible representation, the possibility of developing a userspace
+    replacement for <systemitem class="filesystem">devfs</systemitem> became
+    much more realistic.</para>
+
+  </sect2>
+
+  <sect2>
+    <title>Udev Implementation</title>
+
+    <sect3>
+      <title>Sysfs</title>
+
+      <para>The <systemitem class="filesystem">sysfs</systemitem> filesystem
+      was mentioned briefly above. One may wonder how <systemitem
+      class="filesystem">sysfs</systemitem> knows about the devices present on
+      a system and what device numbers should be used for them. Drivers that
+      have been compiled into the kernel directly register their objects with a
+      <systemitem class="filesystem">sysfs</systemitem> (devtmpfs internally)
+      as they are detected by the kernel. For drivers compiled as modules, this
+      registration will happen when the module is loaded. Once the <systemitem
+      class="filesystem">sysfs</systemitem> filesystem is mounted (on /sys),
+      data which the drivers register with <systemitem
+      class="filesystem">sysfs</systemitem> are available to userspace
+      processes and to udevd for processing (including modifications to device
+      nodes).</para>
+
+    </sect3>
+
+    <sect3>
+      <title>Device Node Creation</title>
+
+      <para>Device files are created by the kernel by the <systemitem
+      class="filesystem">devtmpfs</systemitem> filesystem.  Any driver that
+      wishes to register a device node will go through the <systemitem
+      class="filesystem">devtmpfs</systemitem> (via the driver core) to do it.
+      When a <systemitem class="filesystem">devtmpfs</systemitem> instance is
+      mounted on <filename class="directory">/dev</filename>, the device node
+      will initially be created with a fixed name, permissions, and
+      owner.</para>
+
+      <para>A short time later, the kernel will send a uevent to <command>
+      udevd</command>.  Based on the rules specified in the files within the
+      <filename class="directory">/etc/udev/rules.d</filename>, <filename
+      class="directory">/lib/udev/rules.d</filename>, and <filename
+      class="directory">/run/udev/rules.d</filename> directories, <command>
+      udevd</command> will create additional symlinks to the device node, or
+      change its permissions, owner, or group, or modify the internal
+      <command>udevd</command> database entry (name) for that object.</para>
+
+      <para>The rules in these three directories are numbered and all three
+      directories are merged together. If <command>udevd</command> can't find a
+      rule for the device it is creating, it will leave the permissions and
+      ownership at whatever <systemitem
+      class="filesystem">devtmpfs</systemitem> used initially.</para> </sect3>
+      
+    <sect3 id="module-loading">
+      <title>Module Loading</title>
+
+      <para>Device drivers compiled as modules may have aliases built into them.
+      Aliases are visible in the output of the <command>modinfo</command>
+      program and are usually related to the bus-specific identifiers of devices
+      supported by a module. For example, the <emphasis>snd-fm801</emphasis>
+      driver supports PCI devices with vendor ID 0x1319 and device ID 0x0801,
+      and has an alias of <quote>pci:v00001319d00000801sv*sd*bc04sc01i*</quote>.
+      For most devices, the bus driver exports the alias of the driver that
+      would handle the device via <systemitem
+      class="filesystem">sysfs</systemitem>. E.g., the
+      <filename>/sys/bus/pci/devices/0000:00:0d.0/modalias</filename> file
+      might contain the string
+      <quote>pci:v00001319d00000801sv00001319sd00001319bc04sc01i00</quote>.
+      The default rules provided with Udev will cause <command>udevd</command>
+      to call out to <command>/sbin/modprobe</command> with the contents of the
+      <envar>MODALIAS</envar> uevent environment variable (which should be the
+      same as the contents of the <filename>modalias</filename> file in sysfs),
+      thus loading all modules whose aliases match this string after wildcard
+      expansion.</para>
+
+      <para>In this example, this means that, in addition to
+      <emphasis>snd-fm801</emphasis>, the obsolete (and unwanted)
+      <emphasis>forte</emphasis> driver will be loaded if it is
+      available. See below for ways in which the loading of unwanted drivers can
+      be prevented.</para>
+
+      <para>The kernel itself is also able to load modules for network
+      protocols, filesystems and NLS support on demand.</para>
+
+    </sect3>
+
+    <sect3>
+      <title>Handling Hotpluggable/Dynamic Devices</title>
+
+      <para>When you plug in a device, such as a Universal Serial Bus (USB) MP3
+      player, the kernel recognizes that the device is now connected and
+      generates a uevent. This uevent is then handled by
+      <command>udevd</command> as described above.</para>
+
+    </sect3>
+
+  </sect2>
+
+  <sect2>
+    <title>Problems with Loading Modules and Creating Devices</title>
+
+    <para>There are a few possible problems when it comes to automatically
+    creating device nodes.</para>
+
+    <sect3>
+      <title>A kernel module is not loaded automatically</title>
+
+      <para>Udev will only load a module if it has a bus-specific alias and the
+      bus driver properly exports the necessary aliases to <systemitem
+      class="filesystem">sysfs</systemitem>. In other cases, one should
+      arrange module loading by other means. With Linux-&linux-version;, Udev is
+      known to load properly-written drivers for INPUT, IDE, PCI, USB, SCSI,
+      SERIO, and FireWire devices.</para>
+
+      <para>To determine if the device driver you require has the necessary
+      support for Udev, run <command>modinfo</command> with the module name as
+      the argument.  Now try locating the device directory under
+      <filename class="directory">/sys/bus</filename> and check whether there is
+      a <filename>modalias</filename> file there.</para>
+
+      <para>If the <filename>modalias</filename> file exists in <systemitem
+      class="filesystem">sysfs</systemitem>, the driver supports the device and
+      can talk to it directly, but doesn't have the alias, it is a bug in the
+      driver. Load the driver without the help from Udev and expect the issue
+      to be fixed later.</para>
+
+      <para>If there is no <filename>modalias</filename> file in the relevant
+      directory under <filename class="directory">/sys/bus</filename>, this
+      means that the kernel developers have not yet added modalias support to
+      this bus type. With Linux-&linux-version;, this is the case with ISA
+      busses. Expect this issue to be fixed in later kernel versions.</para>
+
+      <para>Udev is not intended to load <quote>wrapper</quote> drivers such as
+      <emphasis>snd-pcm-oss</emphasis> and non-hardware drivers such as
+      <emphasis>loop</emphasis> at all.</para>
+
+    </sect3>
+
+    <sect3>
+      <title>A kernel module is not loaded automatically, and Udev is not
+      intended to load it</title>
+
+      <para>If the <quote>wrapper</quote> module only enhances the
+      functionality provided by some other module (e.g.,
+      <emphasis>snd-pcm-oss</emphasis> enhances the functionality of
+      <emphasis>snd-pcm</emphasis> by making the sound cards available to OSS
+      applications), configure <command>modprobe</command> to load the wrapper
+      after Udev loads the wrapped module. To do this, add a
+      <quote>softdep</quote> line to the corresponding
+      <filename>/etc/modprobe.d/<replaceable>&lt;filename&gt;</replaceable>.conf</filename>
+      file. For example:</para>
+
+<screen role="nodump"><literal>softdep snd-pcm post: snd-pcm-oss</literal></screen>
+
+      <para>Note that the <quote>softdep</quote> command also allows
+      <literal>pre:</literal> dependencies, or a mixture of both
+      <literal>pre:</literal> and <literal>post:</literal>.  See the
+      <filename>modprobe.d(5)</filename> manual page for more information
+      on <quote>softdep</quote> syntax and capabilities.</para>
+
+      <para>If the module in question is not a wrapper and is useful by itself,
+      configure the <command>modules</command> bootscript to load this
+      module on system boot. To do this, add the module name to the
+      <filename>/etc/sysconfig/modules</filename> file on a separate line.
+      This works for wrapper modules too, but is suboptimal in that case.</para>
+
+    </sect3>
+
+    <sect3>
+      <title>Udev loads some unwanted module</title>
+
+      <para>Either don't build the module, or blacklist it in a
+      <filename>/etc/modprobe.d/blacklist.conf</filename> file as done with the
+      <emphasis>forte</emphasis> module in the example below:</para>
+
+<screen role="nodump"><literal>blacklist forte</literal></screen>
+
+      <para>Blacklisted modules can still be loaded manually with the
+      explicit <command>modprobe</command> command.</para>
+
+    </sect3>
+
+    <sect3>
+      <title>Udev creates a device incorrectly, or makes a wrong symlink</title>
+
+      <para>This usually happens if a rule unexpectedly matches a device. For
+      example, a poorly-written rule can match both a SCSI disk (as desired)
+      and the corresponding SCSI generic device (incorrectly) by vendor.
+      Find the offending rule and make it more specific, with the help of the
+      <command>udevadm info</command> command.</para>
+
+    </sect3>
+
+    <sect3>
+      <title>Udev rule works unreliably</title>
+
+      <para>This may be another manifestation of the previous problem. If not,
+      and your rule uses <systemitem class="filesystem">sysfs</systemitem>
+      attributes, it may be a kernel timing issue, to be fixed in later kernels.
+      For now, you can work around it by creating a rule that waits for the used
+      <systemitem class="filesystem">sysfs</systemitem> attribute and appending
+      it to the <filename>/etc/udev/rules.d/10-wait_for_sysfs.rules</filename>
+      file (create this file if it does not exist). Please notify the LFS
+      Development list if you do so and it helps.</para>
+
+    </sect3>
+
+    <sect3>
+      <title>Udev does not create a device</title>
+
+      <para>Further text assumes that the driver is built statically into the
+      kernel or already loaded as a module, and that you have already checked
+      that Udev doesn't create a misnamed device.</para>
+
+      <para>Udev has no information needed to create a device node if a kernel
+      driver does not export its data to
+      <systemitem class="filesystem">sysfs</systemitem>. This is most common
+      with third party drivers from outside the kernel tree. Create a static
+      device node in <filename>/lib/udev/devices</filename> with the
+      appropriate major/minor numbers (see the file
+      <filename>devices.txt</filename> inside the kernel documentation or the
+      documentation provided by the third party driver vendor). The static
+      device node will be copied to <filename class="directory">/dev</filename>
+      by <command>udev</command>.</para>
+
+    </sect3>
+
+    <sect3>
+      <title>Device naming order changes randomly after rebooting</title>
+
+      <para>This is due to the fact that Udev, by design, handles uevents and
+      loads modules in parallel, and thus in an unpredictable order. This will
+      never be <quote>fixed</quote>. You should not rely upon the kernel device
+      names being stable. Instead, create your own rules that make symlinks with
+      stable names based on some stable attributes of the device, such as a
+      serial number or the output of various *_id utilities installed by Udev.
+      See <xref linkend="ch-config-symlinks"/> and
+      <xref linkend="ch-config-network"/> for examples.</para>
+
+    </sect3>
+
+  </sect2>
+
+  <sect2>
+    <title>Useful Reading</title>
+
+    <para>Additional helpful documentation is available at the following
+    sites:</para>
+
+    <itemizedlist>
+
+      <listitem>
+        <para>A Userspace Implementation of <systemitem class="filesystem">devfs</systemitem>
+        <ulink url="http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf"/></para>
+      </listitem>
+
+      <listitem>
+        <para>The <systemitem class="filesystem">sysfs</systemitem> Filesystem
+        <ulink url="http://www.kernel.org/pub/linux/kernel/people/mochel/doc/papers/ols-2005/mochel.pdf"/></para>
+      </listitem>
+
+<!--  No longer available
+      <listitem>
+        <para>Pointers to further reading
+        <ulink url="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev.html"/>
+        </para>
+      </listitem>
+-->
+    </itemizedlist>
+
+  </sect2>
+
+</sect1>
