1<?xml version="1.0"?>
2<!--*-nxml-*-->
3<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
4  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
5<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
6<refentry id="systemd-backlight@.service" conditional='ENABLE_BACKLIGHT'>
7
8  <refentryinfo>
9    <title>systemd-backlight@.service</title>
10    <productname>systemd</productname>
11  </refentryinfo>
12
13  <refmeta>
14    <refentrytitle>systemd-backlight@.service</refentrytitle>
15    <manvolnum>8</manvolnum>
16  </refmeta>
17
18  <refnamediv>
19    <refname>systemd-backlight@.service</refname>
20    <refname>systemd-backlight</refname>
21    <refpurpose>Load and save the display backlight brightness at boot and shutdown</refpurpose>
22  </refnamediv>
23
24  <refsynopsisdiv>
25    <para><filename>systemd-backlight@.service</filename></para>
26    <para><filename>/usr/lib/systemd/systemd-backlight</filename> save [backlight|leds]:DEVICE</para>
27    <para><filename>/usr/lib/systemd/systemd-backlight</filename> load [backlight|leds]:DEVICE</para>
28  </refsynopsisdiv>
29
30  <refsect1>
31    <title>Description</title>
32
33    <para><filename>systemd-backlight@.service</filename> is a service
34    that restores the display backlight brightness at early boot and
35    saves it at shutdown. On disk, the backlight brightness is stored
36    in <filename>/var/lib/systemd/backlight/</filename>. During
37    loading, if the udev property <option>ID_BACKLIGHT_CLAMP</option> is
38    not set to false, the brightness is clamped to a value of at
39    least 1 or 5% of maximum brightness, whichever is greater. This
40    restriction will be removed when the kernel allows user space to
41    reliably set a brightness value which does not turn off the
42    display.</para>
43  </refsect1>
44
45  <refsect1>
46    <title>Kernel Command Line</title>
47
48    <para><filename>systemd-backlight</filename> understands the
49    following kernel command line parameter:</para>
50
51    <variablelist class='kernel-commandline-options'>
52      <varlistentry>
53        <term><varname>systemd.restore_state=</varname></term>
54
55        <listitem><para>Takes a boolean argument. Defaults to
56        <literal>1</literal>. If <literal>0</literal>, does not
57        restore the backlight settings on boot. However, settings will
58        still be stored on shutdown. </para></listitem>
59      </varlistentry>
60    </variablelist>
61  </refsect1>
62
63  <refsect1>
64    <title>See Also</title>
65    <para>
66      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
67    </para>
68  </refsect1>
69
70</refentry>
71