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diff --git a/chapter07/symlinksd.xml b/chapter07/symlinksd.xml new file mode 100644 index 000000000..354ae319a --- /dev/null +++ b/chapter07/symlinksd.xml @@ -0,0 +1,63 @@ +<?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-book;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 > /etc/udev/rules.d/83-duplicate_devs.rules << "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> |