Index: OK/bootscripts/common/eudev.xml
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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-scripts-eudev">
-  <?dbhtml filename="eudev.html"?>
-
-  <title>Device and Module Handling on a CLFS System</title>
-
-  <indexterm zone="ch-scripts-eudev">
-    <primary sortas="a-Eudev">Eudev</primary>
-    <secondary>usage</secondary>
-  </indexterm>
-
-  <para>In <xref linkend="chapter-building-system"/>, we installed the Eudev
-  package. 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 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>
-
-  <para>Using the Eudev 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">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>
-
-  <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 suffers from race
-    conditions that are inherent in its design and cannot be fixed without a
-    substantial revision to the kernel. It has also been marked as deprecated
-    due to a lack of recent maintenance.</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 seeing a userspace
-    replacement for <systemitem class="filesystem">devfs</systemitem> became
-    much more realistic.</para>
-
-  </sect2>
-
-  <sect2>
-    <title>Eudev 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
-      <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 and to <command>udevd</command> for device
-      node creation.</para>
-
-    </sect3>
-
-    <sect3>
-      <title>Eudev Bootscript</title>
-
-      <para>The <command>S10udev</command> initscript takes care of creating
-      device nodes when Linux is booted. The script unsets the uevent handler
-      from the default of <command>/sbin/hotplug</command>.  This is done
-      because the kernel no longer needs to call out to an external binary.
-      Instead <command>udevd</command> will listen on a netlink socket for
-      uevents that the kernel raises. Next, the bootscript copies any static
-      device nodes that exist in <filename
-      class="directory">/lib/udev/devices</filename> to <filename
-      class="directory">/dev</filename>. This is necessary because some devices,
-      directories, and symlinks are needed before the dynamic device handling
-      processes are available during the early stages of booting a system.
-      Creating static device nodes in <filename
-      class="directory">/lib/udev/devices</filename> also provides an easy
-      workaround for devices that are not supported by the dynamic device
-      handling infrastructure. The bootscript then starts the Eudev daemon,
-      <command>udevd</command>, which will act on any uevents it receives.
-      Finally, the bootscript forces the kernel to replay uevents for any
-      devices that have already been registered and then waits for
-      <command>udevd</command> to handle them.</para>
-
-    </sect3>
-
-    <sect3>
-      <title>Device Node Creation</title>
-
-      <para>To obtain the right major and minor number for a device, Eudev relies
-      on the information provided by <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 is used by <command>udevd</command>
-      to create a device node with major number <emphasis>7</emphasis> and minor
-      <emphasis>0</emphasis>. The names and permissions of the nodes created
-      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>. Documentation on the syntax of the Eudev
-      rules configuration files is available in
-      <filename>/usr/share/doc/udev/writing_udev_rules/index.html</filename></para>
-
-    </sect3>
-
-    <sect3>
-      <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 Eudev 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>
-
-    </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>Eudev 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;, Eudev 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 Eudev, 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 Eudev 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>Eudev 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 Eudev 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 Eudev 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>Eudev 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>Eudev 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>Eudev 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>Eudev 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 Eudev doesn't create a misnamed device.</para>
-
-      <para>Eudev 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 the
-      <command>S10udev</command> bootscript.</para>
-
-    </sect3>
-
-    <sect3>
-      <title>Device naming order changes randomly after rebooting</title>
-
-      <para>This is due to the fact that Eudev, 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 Eudev.
-      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>
