lfs-book/chapter07/symlinks.xml

<?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-symlinks">
  <?dbhtml filename="symlinks.html"?>

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

  <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;view/svn/postlfs/devices.html">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>

 </sect2>

</sect1>