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<?xml version="1.0"?>
<!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
<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>
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