1 | <?xml version="1.0" encoding="ISO-8859-1"?> |
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2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" |
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3 | "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [ |
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4 | <!ENTITY % general-entities SYSTEM "../../general.ent"> |
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5 | %general-entities; |
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6 | ]> |
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7 | |
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8 | <sect1 id="ch-scripts-symlinks"> |
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9 | <?dbhtml filename="symlinks.html"?> |
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10 | |
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11 | <title>Creating custom symlinks to devices</title> |
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12 | |
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13 | <sect2> |
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14 | |
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15 | <title>CD-ROM symlinks</title> |
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16 | |
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17 | <para>Some software that you may want to install later (e.g., various |
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18 | media players) expect the /dev/cdrom and /dev/dvd symlinks to exist. |
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19 | Also, it may be convenient to put references to those symlinks into |
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20 | <filename>/etc/fstab</filename>. For each of your CD-ROM devices, |
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21 | find the corresponding directory under |
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22 | <filename class="directory">/sys</filename> (e.g., this can be |
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23 | <filename class="directory">/sys/block/hdd</filename>) and |
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24 | run a command similar to the following:</para> |
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25 | |
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26 | <screen role="nodump"><userinput>udevadm test /sys/block/hdd</userinput></screen> |
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27 | |
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28 | <para>Look at the lines containing the output of various *_id programs.</para> |
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29 | |
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30 | <para>There are two approaches to creating symlinks. The first one is to |
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31 | use the model name and the serial number, the second one is based on the |
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32 | location of the device on the bus. If you are going to use the first |
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33 | approach, create a file similar to the following:</para> |
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34 | |
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35 | <screen role="nodump"><userinput>cat >/etc/udev/rules.d/82-cdrom.rules << EOF |
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36 | <literal> |
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37 | # Custom CD-ROM symlinks |
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38 | SUBSYSTEM=="block", ENV{ID_MODEL}=="SAMSUNG_CD-ROM_SC-148F", \ |
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39 | ENV{ID_REVISION}=="PS05", SYMLINK+="cdrom" |
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40 | SUBSYSTEM=="block", ENV{ID_MODEL}=="PHILIPS_CDD5301", \ |
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41 | ENV{ID_SERIAL}=="5VO1306DM00190", SYMLINK+="cdrom1 dvd" |
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42 | </literal> |
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43 | EOF</userinput></screen> |
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44 | |
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45 | <note> |
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46 | <para>Although the examples in this book work properly, be aware |
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47 | that Udev does not recognize the backslash for line continuation. |
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48 | If modifying Udev rules with an editor, be sure to leave each rule |
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49 | on one physical line.</para> |
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50 | </note> |
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51 | |
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52 | <para>This way, the symlinks will stay correct even if you move the drives |
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53 | to different positions on the IDE bus, but the |
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54 | <filename>/dev/cdrom</filename> symlink won't be created if you replace |
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55 | the old SAMSUNG CD-ROM with a new drive.</para> |
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56 | <!-- The symlinks in the first approach survive even the transition |
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57 | to libata for IDE drives, but that is not for the book. --> |
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58 | |
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59 | <para>The SUBSYSTEM=="block" key is needed in order to avoid |
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60 | matching SCSI generic devices. Without it, in the case with SCSI |
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61 | CD-ROMs, the symlinks will sometimes point to the correct |
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62 | <filename>/dev/srX</filename> devices, and sometimes to |
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63 | <filename>/dev/sgX</filename>, which is wrong.</para> |
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64 | |
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65 | <para>The second approach yields:</para> |
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66 | |
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67 | <screen role="nodump"><userinput>cat >/etc/udev/rules.d/82-cdrom.rules << EOF |
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68 | <literal> |
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69 | # Custom CD-ROM symlinks |
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70 | SUBSYSTEM=="block", ENV{ID_TYPE}=="cd", \ |
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71 | ENV{ID_PATH}=="pci-0000:00:07.1-ide-0:1", SYMLINK+="cdrom" |
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72 | SUBSYSTEM=="block", ENV{ID_TYPE}=="cd", \ |
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73 | ENV{ID_PATH}=="pci-0000:00:07.1-ide-1:1", SYMLINK+="cdrom1 dvd" |
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74 | </literal> |
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75 | EOF</userinput></screen> |
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76 | |
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77 | <para>This way, the symlinks will stay correct even if you replace drives |
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78 | with different models, but place them to the old positions on the IDE |
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79 | bus. The ENV{ID_TYPE}=="cd" key makes sure that the symlink |
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80 | disappears if you put something other than a CD-ROM in that position on |
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81 | the bus.</para> |
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82 | |
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83 | <para>Of course, it is possible to mix the two approaches.</para> |
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84 | |
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85 | </sect2> |
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86 | |
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87 | <sect2> |
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88 | |
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89 | <title>Dealing with duplicate devices</title> |
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90 | |
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91 | <para>As explained in <xref linkend="ch-scripts-udev"/>, the order in |
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92 | which devices with the same function appear in |
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93 | <filename class="directory">/dev</filename> is essentially random. |
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94 | E.g., if you have a USB web camera and a TV tuner, sometimes |
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95 | <filename>/dev/video0</filename> refers to the camera and |
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96 | <filename>/dev/video1</filename> refers to the tuner, and sometimes |
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97 | after a reboot the order changes to the opposite one. |
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98 | For all classes of hardware except sound cards and network cards, this is |
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99 | fixable by creating udev rules for custom persistent symlinks. |
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100 | The case of network cards is covered separately in |
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101 | <xref linkend="chapter-network"/>, and sound card configuration can |
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102 | be found in <ulink url="&cblfs-root;">CBLFS</ulink>.</para> |
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103 | |
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104 | <para>For each of your devices that is likely to have this problem |
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105 | (even if the problem doesn't exist in your current Linux distribution), |
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106 | find the corresponding directory under |
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107 | <filename class="directory">/sys/class</filename> or |
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108 | <filename class="directory">/sys/block</filename>. |
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109 | For video devices, this may be |
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110 | <filename |
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111 | class="directory">/sys/class/video4linux/video<replaceable>X</replaceable></filename>. |
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112 | Figure out the attributes that identify the device uniquely (usually, |
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113 | vendor and product IDs and/or serial numbers work):</para> |
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114 | |
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115 | <screen role="nodump"><userinput>udevadm info -a -p /sys/class/video4linux/video0</userinput></screen> |
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116 | |
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117 | <para>Then write rules that create the symlinks, e.g.:</para> |
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118 | |
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119 | <screen role="nodump"><userinput>cat >/etc/udev/rules.d/83-duplicate_devs.rules << EOF |
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120 | <literal> |
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121 | # Persistent symlinks for webcam and tuner |
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122 | KERNEL=="video*", SYSFS{idProduct}=="1910", SYSFS{idVendor}=="0d81", \ |
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123 | SYMLINK+="webcam" |
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124 | KERNEL=="video*", SYSFS{device}=="0x036f", SYSFS{vendor}=="0x109e", \ |
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125 | SYMLINK+="tvtuner" |
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126 | </literal> |
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127 | EOF</userinput></screen> |
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128 | |
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129 | <para>The result is that <filename>/dev/video0</filename> and |
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130 | <filename>/dev/video1</filename> devices still refer randomly to the tuner |
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131 | and the web camera (and thus should never be used directly), but there are |
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132 | symlinks <filename>/dev/tvtuner</filename> and |
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133 | <filename>/dev/webcam</filename> that always point to the correct |
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134 | device.</para> |
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135 | |
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136 | <para>More information on writing Udev rules can be found in |
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137 | <filename>/usr/share/doc/systemd-208/udev.html</filename>.</para> |
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138 | |
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139 | </sect2> |
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140 | |
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141 | </sect1> |
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