source: BOOK/system-config/common/udev.xml@ 98c126f

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<sect1 id="ch-scripts-udev">
  <?dbhtml filename="udev.html"?>

  <title>Device and Module Handling on a CLFS System</title>

  <indexterm zone="ch-scripts-udev">
    <primary sortas="a-Udev">Udev</primary>
    <secondary>usage</secondary>
  </indexterm>

  <para>In <xref linkend="chapter-building-system"/>, we installed Udev,
  as one of the components of systemd. Before we go into the details regarding
  how this works, a brief history of previous methods of handling devices is in
  order.</para>

  <sect2>
    <title>History</title>

    <sect3>
      <title>Static Device Nodes</title>

      <para>Linux systems in general traditionally use a static device creation
      method, whereby a great many device nodes are created under <filename
      class="directory">/dev</filename> (sometimes literally thousands of nodes),
      regardless of whether the corresponding hardware devices actually exist.
      This is 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>

    </sect3>

    <sect3>
      <title>Devfs</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 deprecated with the
      release of the 2.6 kernel series, and was removed entirely as of version
      2.6.18.</para>

    </sect3>

    <sect3>
      <title>Sysfs</title>

      <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. Drivers that
      have been compiled into the kernel directly register their objects with
      <systemitem class="filesystem">sysfs</systemitem> 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 <filename
      class="directory">/sys</filename>), data which the built-in drivers
      registered with <systemitem class="filesystem">sysfs</systemitem> are
      available to userspace processes. With this
      userspace-visible representation, the possibility of seeing a userspace
      replacement for <systemitem class="filesystem">devfs</systemitem> became
      much more realistic.</para>

    </sect3>

    <sect3>
      <title>Udev Implementation</title>

<!--      <title>Device Node Creation</title> -->

      <para>When Udev was introduced, the <command>udevd</command> daemon made
      calls to mknod() to create device nodes in
      <filename class="directory">/dev</filename> dynamically, based on the
      information from <systemitem class="filesystem">sysfs</systemitem>, in
      <filename class="directory">/sys</filename>. For example,
      <filename>/sys/class/tty/vcs/dev</filename> contains the string
      <quote>7:0</quote>. This string was used by <command>udevd</command>
      to create a device node with major number <emphasis>7</emphasis> and
      minor number <emphasis>0</emphasis>. Using the Udev method
      only those devices which are detected by the kernel would get device
      nodes created for them. Because these device nodes were created each time
      the system boots, they were stored on a
      <systemitem class="filesystem">tmpfs</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>

      <para>Linux kernel version 2.6.32 introduced a new virtual file system
      called <systemitem class="filesystem">devtmpfs</systemitem>, a
      replacement for <systemitem class="filesystem">devfs</systemitem>.
      With this approach, a
      <systemitem class="filesystem">devtmpfs</systemitem> file system is
      mounted on <filename class="directory">/dev/</filename> when the system
      is booted, and all needed device nodes are created on this virtual
      file system. As of version 176, Udev no longer creates device nodes
      itself, instead relying on
      <systemitem class="filesystem">devtmpfs</systemitem> to do so.</para>

      <para>Udev also sets appropriate ownership and permissions
      for the device nodes, and creates extra symlinks as needed (such as
      <filename class="symlink">/dev/cdrom</filename>). The ownership and
      permissions of the nodes under the
      <filename class="directory">/dev</filename> directory are
      determined by rules specified in the files within the <filename
      class="directory">/etc/udev/rules.d/</filename> directory. These are
      numbered in a similar fashion to the CLFS-Bootscripts package. If
      <command>udevd</command> can't find a rule for the device it is creating,
      it will default permissions to <emphasis>660</emphasis> and ownership to
      <emphasis>root:root</emphasis>.</para>

    </sect3>

    <sect3>
      <title>Systemd and Eudev</title>

        <para>In May 2012, Udev's source was merged with systemd, an alternate
        <command>init</command> implementation. Some time later, several Gentoo
        developers took the Udev code from systemd and created a fork called
        Eudev.</para>

    </sect3>

  </sect2>

  <sect2>
    <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 by 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 (that 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>

  </sect2>

  <sect2>
    <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>

  </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 an <quote>install</quote> line in
      <filename>/etc/modprobe.conf</filename>. For example:</para>

<screen role="nodump"><literal>install snd-pcm /sbin/modprobe -i snd-pcm ; \
    /sbin/modprobe snd-pcm-oss ; true</literal></screen>

      <para>If the module in question is not a wrapper and is useful by itself,
      configure the <command>S05modules</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
      <filename>/etc/modprobe.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-writen 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
      <command>udevadm info</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. Please notify the CLFS Development list if you do so and it
      helps.</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-scripts-symlinks"/> and
      <xref linkend="chapter-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 remap="verbatim">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 remap="verbatim">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>

    </itemizedlist>

  </sect2>

</sect1>