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+<?xml version="1.0" encoding="ISO-8859-1"?>
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+<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
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+ "http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd" [
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+ <!ENTITY % general-entities SYSTEM "../general.ent">
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+ %general-entities;
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+]>
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+
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+<sect1 id="ch-scripts-symlinks">
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+ <?dbhtml filename="symlinks.html"?>
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+
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+ <title>Creating custom symlinks to devices</title>
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+
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+ <sect2>
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+
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+ <title>CD-ROM symlinks</title>
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+
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+ <para>Some software that you may want to install later (e.g., various
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+ media players) expect the /dev/cdrom and /dev/dvd symlinks to exist.
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+ Also, it may be convenient to put references to those symlinks into
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+ <filename>/etc/fstab</filename>. For each of your CD-ROM devices,
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+ find the corresponding directory under
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+ <filename class="directory">/sys</filename> (e.g., this can be
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+ <filename class="directory">/sys/block/hdd</filename>) and
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+ run a command similar to the following:</para>
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+
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+<screen role="nodump"><userinput>udevtest /block/hdd</userinput></screen>
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+
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+ <para>Look at the lines containing the output of various *_id programs.</para>
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+
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+ <para>There are two approaches to creating symlinks. The first one is to
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+ use the model name and the serial number, the second one is based on the
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+ location of the device on the bus. If you are going to use the first
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+ approach, create a file similar to the following:</para>
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+
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+<screen role="nodump"><userinput>cat >/etc/udev/rules.d/82-cdrom.rules <<"EOF"
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+<literal>
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+# Custom CD-ROM symlinks
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+SUBSYSTEM=="block", ENV{ID_MODEL}=="SAMSUNG_CD-ROM_SC-148F", ENV{ID_REVISION}=="PS05", SYMLINK+="cdrom"
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+SUBSYSTEM=="block", ENV{ID_MODEL}=="PHILIPS_CDD5301", ENV{ID_SERIAL}=="5VO1306DM00190", SYMLINK+="cdrom1 dvd"
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+</literal>
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+EOF</userinput></screen>
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+
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+ <para>This way, the symlinks will stay correct even if you move the drives
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+ to different positions on the IDE bus, but the
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+ <filename>/dev/cdrom</filename> symlink won't be created if you replace
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+ the old SAMSUNG CD-ROM with a new drive.</para>
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+<!-- The symlinks in the first approach survive even the transition
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+ to libata for IDE drives, but that is not for the book. -->
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+
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+ <para>The SUBSYSTEM=="block" key is needed in order to avoid
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+ matching SCSI generic devices. Without it, in the case with SCSI
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+ CD-ROMs, the symlinks will sometimes point to the correct
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+ <filename>/dev/srX</filename> devices, and sometimes to
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+ <filename>/dev/sgX</filename>, which is wrong.</para>
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+
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+ <para>The second approach yields:</para>
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+
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+<screen role="nodump"><userinput>cat >/etc/udev/rules.d/82-cdrom.rules <<"EOF"
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+<literal>
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+# Custom CD-ROM symlinks
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+SUBSYSTEM=="block", ENV{ID_TYPE}=="cd", ENV{ID_PATH}=="pci-0000:00:07.1-ide-0:1", SYMLINK+="cdrom"
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+SUBSYSTEM=="block", ENV{ID_TYPE}=="cd", ENV{ID_PATH}=="pci-0000:00:07.1-ide-1:1", SYMLINK+="cdrom1 dvd"
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+</literal>
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+EOF</userinput></screen>
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+
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+ <para>This way, the symlinks will stay correct even if you replace drives
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+ with different models, but place them to the old positions on the IDE
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+ bus. The ENV{ID_TYPE}=="cd" key makes sure that the symlink
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+ disappears if you put something other than a CD-ROM in that position on
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+ the bus.</para>
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+
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+ <para>Of course, it is possible to mix the two approaches.</para>
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+
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+ </sect2>
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+
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+ <sect2>
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+
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+ <title>Dealing with duplicate devices</title>
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+
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+ <para>As explained in <xref linkend="ch-scripts-udev"/>, the order in
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+ which devices with the same function appear in
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+ <filename class="directory">/dev</filename> is essentially random.
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+ E.g., if you have a USB web camera and a TV tuner, sometimes
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+ <filename>/dev/video0</filename> refers to the camera and
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+ <filename>/dev/video1</filename> refers to the tuner, and sometimes
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+ after a reboot the order changes to the opposite one.
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+ For all classes of hardware except sound cards and network cards, this is
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+ fixable by creating udev rules for custom persistent symlinks.
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+ The case of network cards is covered separately in
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+ <xref linkend="ch-scripts-network"/>, and sound card configuration can
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+ be found in <ulink url="&blfs-root;">BLFS</ulink>.</para>
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+
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+ <para>For each of your devices that is likely to have this problem
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+ (even if the problem doesn't exist in your current Linux distribution),
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+ find the corresponding directory under
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+ <filename class="directory">/sys/class</filename> or
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+ <filename class="directory">/sys/block</filename>.
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+ For video devices, this may be
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+ <filename
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+ class="directory">/sys/class/video4linux/video<replaceable>X</replaceable></filename>.
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+ Figure out the attributes that identify the device uniquely (usually,
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+ vendor and product IDs and/or serial numbers work):</para>
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+
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+<screen role="nodump"><userinput>udevinfo -a -p /sys/class/video4linux/video0</userinput></screen>
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+
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+ <para>Then write rules that create the symlinks, e.g.:</para>
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+
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+<screen role="nodump"><userinput>cat >/etc/udev/rules.d/83-duplicate_devs.rules <<"EOF"
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+<literal>
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+# Persistent symlinks for webcam and tuner
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+KERNEL=="video*", SYSFS{idProduct}=="1910", SYSFS{idVendor}=="0d81", SYMLINK+="webcam"
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+KERNEL=="video*", SYSFS{device}=="0x036f", SYSFS{vendor}=="0x109e", SYMLINK+="tvtuner"
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+</literal>
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+EOF</userinput></screen>
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+
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+ <para>The result is that <filename>/dev/video0</filename> and
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+ <filename>/dev/video1</filename> devices still refer randomly to the tuner
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+ and the web camera (and thus should never be used directly), but there are
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+ symlinks <filename>/dev/tvtuner</filename> and
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+ <filename>/dev/webcam</filename> that always point to the correct
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+ device.</para>
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+
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+ <para>More information on writing Udev rules can be found in
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+ <filename>/usr/share/doc/udev-&udev-version;/index.html</filename>.</para>
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+
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+ </sect2>
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+
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+</sect1>
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