systemd/man/systemd-backlight@.service.xml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
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<refentry id="systemd-backlight_.service" conditional='ENABLE_BACKLIGHT'
          xmlns:xi="http://www.w3.org/2001/XInclude">

  <refentryinfo>
    <title>systemd-backlight@.service</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>systemd-backlight@.service</refentrytitle>
    <manvolnum>8</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd-backlight@.service</refname>
    <refname>systemd-backlight</refname>
    <refpurpose>Load and save the display backlight brightness at boot and shutdown</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <para><filename>systemd-backlight@.service</filename></para>
    <para><filename>/usr/lib/systemd/systemd-backlight</filename> save [backlight|leds]:DEVICE</para>
    <para><filename>/usr/lib/systemd/systemd-backlight</filename> load [backlight|leds]:DEVICE</para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><filename>systemd-backlight@.service</filename> is a service that restores the brightness of
    a display backlight or LED (e.g. keyboard backlight) device at early boot, and saves it at shutdown.
    The brightness is stored in <filename>/var/lib/systemd/backlight/</filename>.</para>

    <para>On restoring brightness of a display backlight device, <command>systemd-backlight</command> reads
    <varname>ID_BACKLIGHT_CLAMP</varname> udev property, that takes a boolean value or a percentage (needs to
    be suffixed with <literal>%</literal>, e.g. <literal>30%</literal>). When a percentage is specified, the
    saved brightness is clamped to a value of at least 1 or the specified percentage of the maximum
    brightness, whichever is greater. When unset or set to true, the brightness is clamped in the same way
    with percentage 5%. When false, the saved brightness will not be clamped, and loaded as is.</para>

    <para>On restoring brightness of a LED device, <command>systemd-backlight</command> reads
    <varname>ID_LEDS_CLAMP</varname> udev property, that also takes a boolean value or a percentage. When a
    percentage is specified, the saved brightness is clamped to the specified percentage of the maximum
    brightness. When set to true, the brightness is clamped in the same way with percentage 5%. When unset or
    set to false, the saved brightness will not be clamped, and loaded as is.</para>
  </refsect1>

  <refsect1>
    <title>Kernel Command Line</title>

    <para><filename>systemd-backlight</filename> understands the
    following kernel command line parameter:</para>

    <variablelist class='kernel-commandline-options'>
      <varlistentry>
        <term><varname>systemd.restore_state=</varname></term>

        <listitem><para>Takes a boolean argument. Defaults to
        <literal>1</literal>. If <literal>0</literal>, does not
        restore the backlight settings on boot. However, settings will
        still be stored on shutdown. </para>

        <xi:include href="version-info.xml" xpointer="v209"/></listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>