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diff --git a/chapter07/symlinks.xml b/chapter07/symlinks.xml new file mode 100644 index 000000000..c3e799867 --- /dev/null +++ b/chapter07/symlinks.xml @@ -0,0 +1,128 @@ +<?xml version="1.0" encoding="ISO-8859-1"?> +<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN" + "http://www.oasis-open.org/docbook/xml/4.4/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>CD-ROM symlinks</title> + + <para>Some software that you may want to install later (e.g., various + media players) expect the /dev/cdrom and /dev/dvd symlinks to exist. + Also, it may be convenient to put references to those symlinks into + <filename>/etc/fstab</filename>. For each of your CD-ROM devices, + 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>udevtest /block/hdd</userinput></screen> + + <para>Look at the lines containing the output of various *_id programs.</para> + + <para>There are two approaches to creating symlinks. The first one is to + use the model name and the serial number, the second one is based on the + location of the device on the bus. If you are going to use the first + approach, create a file similar to the following:</para> + +<screen role="nodump"><userinput>cat >/etc/udev/rules.d/82-cdrom.rules <<"EOF" +<literal> +# Custom CD-ROM symlinks +SUBSYSTEM=="block", ENV{ID_MODEL}=="SAMSUNG_CD-ROM_SC-148F", ENV{ID_REVISION}=="PS05", SYMLINK+="cdrom" +SUBSYSTEM=="block", ENV{ID_MODEL}=="PHILIPS_CDD5301", ENV{ID_SERIAL}=="5VO1306DM00190", SYMLINK+="cdrom1 dvd" +</literal> +EOF</userinput></screen> + + <para>This way, the symlinks will stay correct even if you move the drives + to different positions on the IDE bus, but the + <filename>/dev/cdrom</filename> symlink won't be created if you replace + the old SAMSUNG CD-ROM with a new drive.</para> +<!-- The symlinks in the first approach survive even the transition + to libata for IDE drives, but that is not for the book. --> + + <para>The SUBSYSTEM=="block" key is needed in order to avoid + matching SCSI generic devices. Without it, in the case with SCSI + CD-ROMs, the symlinks will sometimes point to the correct + <filename>/dev/srX</filename> devices, and sometimes to + <filename>/dev/sgX</filename>, which is wrong.</para> + + <para>The second approach yields:</para> + +<screen role="nodump"><userinput>cat >/etc/udev/rules.d/82-cdrom.rules <<"EOF" +<literal> +# Custom CD-ROM symlinks +SUBSYSTEM=="block", ENV{ID_TYPE}=="cd", ENV{ID_PATH}=="pci-0000:00:07.1-ide-0:1", SYMLINK+="cdrom" +SUBSYSTEM=="block", ENV{ID_TYPE}=="cd", ENV{ID_PATH}=="pci-0000:00:07.1-ide-1:1", SYMLINK+="cdrom1 dvd" +</literal> +EOF</userinput></screen> + + <para>This way, the symlinks will stay correct even if you replace drives + with different models, but place them to the old positions on the IDE + bus. The ENV{ID_TYPE}=="cd" key makes sure that the symlink + disappears if you put something other than a CD-ROM in that position on + the bus.</para> + + <para>Of course, it is possible to mix the two approaches.</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>udevinfo -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*", SYSFS{idProduct}=="1910", SYSFS{idVendor}=="0d81", SYMLINK+="webcam" +KERNEL=="video*", SYSFS{device}=="0x036f", SYSFS{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> |