lfs-book/chapter07/symlinks.xml

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

  <title>Creating Custom Symlinks to Devices</title>

  <sect2>

    <title>CD-ROM symlinks</title>

    <para>Some software that you may want to install later (e.g., various
    media players) expect the <filename class="symlink">/dev/cdrom</filename>
    and <filename class="symlink">/dev/dvd</filename> symlinks to exist, and
    to point to a CD-ROM or DVD-ROM device. Also, it may be convenient to put
    references to those symlinks into <filename>/etc/fstab</filename>. Udev
    comes with a script that will generate rules files to create these symlinks
    for you, depending on the capabilities of each device, but you need to
    decide which of two modes of operation you wish to have the script use.</para>

    <para>First, the script can operate in <quote>by-path</quote> mode (used by
    default for USB and FireWire devices), where the rules it creates depend on
    the physical path to the CD or DVD device. Second, it can operate in
    <quote>by-id</quote> mode (default for IDE and SCSI devices), where the
    rules it creates depend on identification strings stored in the CD or DVD
    device itself. The path is determined by Udev's <command>path_id</command>
    script, and the identification strings are read from the hardware by its
    <command>ata_id</command> or <command>scsi_id</command> programs, depending
    on which type of device you have.</para>

    <para>There are advantages to each approach; the correct approach to use
    will depend on what kinds of device changes may happen. If you expect the
    physical path to the device (that is, the ports and/or slots that it plugs
    into) to change, for example because you plan on moving the drive to a
    different IDE port or a different USB connector, then you should use the
    <quote>by-id</quote> mode. On the other hand, if you expect the device's
    identification to change, for example because it may die, and you would
    replace it with a different device with the same capabilities and which
    is plugged into the same connectors, then you should use the
    <quote>by-path</quote> mode.</para>

    <para>If either type of change is possible with your drive, then choose a
    mode based on the type of change you expect to happen more often.</para>

<!-- If you use by-id mode, the symlinks will survive even the transition
     to libata for IDE drives, but that is not for the book. -->

    <important><para>External devices (for example, a USB-connected CD drive)
    should not use by-path persistence, because each time the device is plugged
    into a new external port, its physical path will change. All
    externally-connected devices will have this problem if you write Udev rules
    to recognize them by their physical path; the problem is not limited to CD
    and DVD drives.</para></important>

    <para>If you wish to see the values that the Udev scripts will use, then
    for the appropriate CD-ROM device, find the corresponding directory under
    <filename class="directory">/sys</filename> (e.g., this can be
    <filename class="directory">/sys/block/hdd</filename>) and
    run a command similar to the following:</para>

<screen role="nodump"><userinput>udevadm test /sys/block/hdd</userinput></screen>

    <para>Look at the lines containing the output of various *_id programs.
    The <quote>by-id</quote> mode will use the ID_SERIAL value if it exists and
    is not empty, otherwise it will use a combination of ID_MODEL and
    ID_REVISION. The <quote>by-path</quote> mode will use the ID_PATH value.</para>

    <para>If the default mode is not suitable for your situation, then the
    following modification can be made to the
    <filename>/etc/udev/rules.d/75-cd-aliases-generator.rules</filename> file,
    as follows (where <replaceable>mode</replaceable> is one of
    <quote>by-id</quote> or <quote>by-path</quote>):</para>

<screen role="nodump"><userinput>sed -i -e 's/write_cd_rules/&amp; <replaceable>mode</replaceable>/' \
    /etc/udev/rules.d/75-cd-aliases-generator.rules</userinput></screen>

    <para>Note that it is not necessary to create the rules files or symlinks
    at this time, because you have bind-mounted the host's
    <filename class="directory">/dev</filename> directory into the LFS system,
    and we assume the symlinks exist on the host. The rules and symlinks will
    be created the first time you boot your LFS system.</para>

    <para>However, if you have multiple CD-ROM devices, then the symlinks
    generated at that time may point to different devices than they point to on
    your host, because devices are not discovered in a predictable order. The
    assignments created when you first boot the LFS system will be stable, so
    this is only an issue if you need the symlinks on both systems to point to
    the same device. If you need that, then inspect (and possibly edit) the
    generated <filename>/etc/udev/rules.d/70-persistent-cd.rules</filename>
    file after booting, to make sure the assigned symlinks match what you need.</para>

  </sect2>

  <sect2>

    <title>Dealing with duplicate devices</title>

    <para>As explained in <xref linkend="ch-scripts-udev"/>, the order in
    which devices with the same function appear in
    <filename class="directory">/dev</filename> is essentially random.
    E.g., if you have a USB web camera and a TV tuner, sometimes
    <filename>/dev/video0</filename> refers to the camera and
    <filename>/dev/video1</filename> refers to the tuner, and sometimes
    after a reboot the order changes to the opposite one.
    For all classes of hardware except sound cards and network cards, this is
    fixable by creating udev rules for custom persistent symlinks.
    The case of network cards is covered separately in
    <xref linkend="ch-scripts-network"/>, and sound card configuration can
    be found in <ulink url="&blfs-root;">BLFS</ulink>.</para>

    <para>For each of your devices that is likely to have this problem
    (even if the problem doesn't exist in your current Linux distribution),
    find the corresponding directory under
    <filename class="directory">/sys/class</filename> or
    <filename class="directory">/sys/block</filename>.
    For video devices, this may be
    <filename
    class="directory">/sys/class/video4linux/video<replaceable>X</replaceable></filename>.
    Figure out the attributes that identify the device uniquely (usually,
    vendor and product IDs and/or serial numbers work):</para>

<screen role="nodump"><userinput>udevadm info -a -p /sys/class/video4linux/video0</userinput></screen>

    <para>Then write rules that create the symlinks, e.g.:</para>

<screen role="nodump"><userinput>cat &gt; /etc/udev/rules.d/83-duplicate_devs.rules &lt;&lt; "EOF"
<literal>
# Persistent symlinks for webcam and tuner
KERNEL=="video*", ATTRS{idProduct}=="1910", ATTRS{idVendor}=="0d81", \
    SYMLINK+="webcam"
KERNEL=="video*", ATTRS{device}=="0x036f", ATTRS{vendor}=="0x109e", \
    SYMLINK+="tvtuner"
</literal>
EOF</userinput></screen>

    <para>The result is that <filename>/dev/video0</filename> and
    <filename>/dev/video1</filename> devices still refer randomly to the tuner
    and the web camera (and thus should never be used directly), but there are
    symlinks <filename>/dev/tvtuner</filename> and
    <filename>/dev/webcam</filename> that always point to the correct
    device.</para>

    <para>More information on writing Udev rules can be found in
    <filename>/usr/share/doc/udev-&udev-version;/index.html</filename>.</para>

 </sect2>

</sect1>