| | 1 | <?xml version="1.0" encoding="ISO-8859-1"?> |
| | 2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN" |
| | 3 | "http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd" [ |
| | 4 | <!ENTITY % general-entities SYSTEM "../general.ent"> |
| | 5 | %general-entities; |
| | 6 | ]> |
| | 7 | |
| | 8 | <sect1 id="ch-scripts-symlinks"> |
| | 9 | <?dbhtml filename="symlinks.html"?> |
| | 10 | |
| | 11 | <title>Creating custom symlinks to devices</title> |
| | 12 | <sect2> |
| | 13 | <title>CD-ROM symlinks</title> |
| | 14 | <para>Some software that you may want to install later (e.g., various |
| | 15 | media players) expect the /dev/cdrom and /dev/dvd symlinks to exist. |
| | 16 | Also, it may be convenient to put references to those symlinks into |
| | 17 | <filename>/etc/fstab</filename>. For each of your CD-ROM devices, |
| | 18 | find the corresponding directory under |
| | 19 | <filename class="directory">/sys</filename> (e.g., this can be |
| | 20 | <filename class="directory">/sys/block/hdd</filename>) and |
| | 21 | run a command similar to the following:</para> |
| | 22 | |
| | 23 | <screen role="nodump"><userinput>udevtest /block/hdd</userinput></screen> |
| | 24 | |
| | 25 | <para>Look at the lines containing the output of various *_id programs.</para> |
| | 26 | |
| | 27 | <para>There are two approaches to creating symlinks. The first one is to |
| | 28 | use the model name and the serial number, the second one is based on the |
| | 29 | location of the device on the bus. If you are going to use the first |
| | 30 | approach, create a file similar to the following:</para> |
| | 31 | |
| | 32 | <screen role="nodump"><userinput>cat >/etc/udev/rules.d/82-cdrom.rules <<"EOF" |
| | 33 | <literal> |
| | 34 | # Custom CD-ROM symlinks |
| | 35 | SUBSYSTEM=="block", ENV{ID_MODEL}=="SAMSUNG_CD-ROM_SC-148F", ENV{ID_REVISION}=="PS05", SYMLINK+="cdrom" |
| | 36 | SUBSYSTEM=="block", ENV{ID_MODEL}=="PHILIPS_CDD5301", ENV{ID_SERIAL}=="5VO1306DM00190", SYMLINK+="cdrom1 dvd" |
| | 37 | </literal> |
| | 38 | EOF</userinput></screen> |
| | 39 | |
| | 40 | <para>This way, the symlinks will stay correct even if you move the drives |
| | 41 | to different positions on the IDE bus, but the |
| | 42 | <filename>/dev/cdrom</filename> symlink won't be created if you replace |
| | 43 | the old SAMSUNG CD-ROM with a new drive.</para> |
| | 44 | <!-- The symlinks in the first approach survive even the transition |
| | 45 | to libata for IDE drives, but that is not for the book. --> |
| | 46 | |
| | 47 | <para>The SUBSYSTEM=="block" key is needed in order to avoid matching SCSI |
| | 48 | generic devices. Without it, in the case with SCSI CD-ROMs, the symlinks |
| | 49 | will sometimes point to the correct <filename>/dev/srX</filename> |
| | 50 | devices, and sometimes to <filename>/dev/sgX</filename>, which is |
| | 51 | wrong.</para> |
| | 52 | |
| | 53 | <para>The second approach yields:</para> |
| | 54 | |
| | 55 | <screen role="nodump"><userinput>cat >/etc/udev/rules.d/82-cdrom.rules <<"EOF" |
| | 56 | <literal> |
| | 57 | # Custom CD-ROM symlinks |
| | 58 | SUBSYSTEM=="block", ENV{ID_TYPE}=="cd", ENV{ID_PATH}=="pci-0000:00:07.1-ide-0:1", SYMLINK+="cdrom" |
| | 59 | SUBSYSTEM=="block", ENV{ID_TYPE}=="cd", ENV{ID_PATH}=="pci-0000:00:07.1-ide-1:1", SYMLINK+="cdrom1 dvd" |
| | 60 | </literal> |
| | 61 | EOF</userinput></screen> |
| | 62 | |
| | 63 | <para>This way, the symlinks will stay correct even if you repace drives |
| | 64 | with different models, but place them to the old positions on the IDE |
| | 65 | bus. The ENV{ID_TYPE}=="cd" key makes sure that the symlink disappears if |
| | 66 | you put something other than a CD-ROM in that position on the bus.</para> |
| | 67 | |
| | 68 | <para>Of course, it is possible to mix the two approaches.</para> |
| | 69 | </sect2> |
| | 70 | |
| | 71 | <sect2> |
| | 72 | <title>Dealing with duplicate devices</title> |
| | 73 | <para>As explained in <xref linkend="ch-scripts-udev"/>, the order in which |
| | 74 | devices with the same function appear in |
| | 75 | <filename class="directory">/dev</filename> is essentially random. |
| | 76 | E.g., if you have a USB web camera and a TV tuner, sometimes |
| | 77 | <filename>/dev/video0</filename> refers to the camera and |
| | 78 | <filename>/dev/video1</filename> refers to te tuner, and sometimes |
| | 79 | after a reboot the order changes to the opposite one. |
| | 80 | For all classes of hardware except sound cards and network cards, this is |
| | 81 | fixable by creating udev rules for custom persistent symlinks. |
| | 82 | The case of network cards is covered separately in |
| | 83 | <xref linkend="ch-scripts-network"/>, and sound card configuration can |
| | 84 | be found in BLFS.</para> |
| | 85 | |
| | 86 | <para>For each of your devices that is likely to have this problem |
| | 87 | (even if the problem doesn't exist in your current Linux distribution), |
| | 88 | find the corresponding directory under |
| | 89 | <filename class="directory">/sys/class</filename> or |
| | 90 | <filename class="directory">/sys/block</filename>. |
| | 91 | For video devices, this may be |
| | 92 | <filename class="directory">/sys/class/video4linux/video<replaceable>X</replaceable></filename>. |
| | 93 | Figure out the attributes that identify the device uniquely (usually, |
| | 94 | vendor and product IDs and/or serial numbers work):</para> |
| | 95 | <screen role="nodump"><userinput>udevinfo -a -p /sys/class/video4linux/video0</userinput></screen> |
| | 96 | <para>Then write rules that create the symlinks, e.g.:</para> |
| | 97 | <screen role="nodump"><userinput>cat >/etc/udev/rules.d/83-duplicate_devs.rules <<"EOF" |
| | 98 | <literal> |
| | 99 | # Persistent symlinks for webcam and tuner |
| | 100 | KERNEL=="video*", SYSFS{idProduct}=="1910", SYSFS{idVendor}=="0d81", SYMLINK+="webcam" |
| | 101 | KERNEL=="video*", SYSFS{device}=="0x036f", SYSFS{vendor}=="0x109e", SYMLINK+="tvtuner" |
| | 102 | </literal> |
| | 103 | EOF</userinput></screen> |
| | 104 | <para>The result is that <filename>/dev/video0</filename> and |
| | 105 | <filename>/dev/video1</filename> devices still refer randomly to the tuner |
| | 106 | and the web camera (and thus should never be used directly), but there are |
| | 107 | symlinks <filename>/dev/tvtuner</filename> and |
| | 108 | <filename>/dev/webcam</filename> that always point to the correct |
| | 109 | device.</para> |
| | 110 | |
| | 111 | <para>More information on writing Udev rules can be found in |
| | 112 | <filename>/usr/share/doc/udev-&udev-version;/index.html</filename>.</para> |
| | 113 | </sect2> |
| | 114 | |
| | 115 | </sect1> |
