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1118b1757d
git-svn-id: http://svn.linuxfromscratch.org/LFS/branches/merge@11073 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689
64 lines
2.7 KiB
XML
64 lines
2.7 KiB
XML
<?xml version="1.0" encoding="ISO-8859-1"?>
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<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/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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<sect1 id="ch-scripts-symlinks">
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<?dbhtml filename="symlinks.html"?>
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<title>Creating Custom Symlinks to Devices</title>
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<sect2>
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<title>Dealing with duplicate devices</title>
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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-book;postlfs/devices.html">BLFS</ulink>.</para>
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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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<screen role="nodump"><userinput>udevadm info -a -p /sys/class/video4linux/video0</userinput></screen>
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<para>Then write rules that create the symlinks, e.g.:</para>
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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*", ATTRS{idProduct}=="1910", ATTRS{idVendor}=="0d81", \
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SYMLINK+="webcam"
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KERNEL=="video*", ATTRS{device}=="0x036f", ATTRS{vendor}=="0x109e", \
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SYMLINK+="tvtuner"
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</literal>
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EOF</userinput></screen>
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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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</sect2>
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</sect1>
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