rc: add a backlight service to save/restore backlight levels

The default on my laptop is annoyingly bright, and this is a useful
feature to mitigate that.  The backlight script is largely a copy of the
mixer service which provides the same value for mixers, but this one is
specifically dependant on kld to allow DRM drivers a chance to attach.

Note that it's off by default to avoid interference with DEs, and
document the capability in backlight(8).  Set backlight_enable=YES in
rc.conf(5) to enable save/restore.

Relnotes:	maybe
Reviewed by:	bapt, ivy, manu, ziaee
Differential Revision:	https://reviews.freebsd.org/D59296

libexec/rc/rc.conf

@@ -42,6 +42,7 @@ always_force_depends="NO"	# Set to check that indicated dependencies are
 apm_enable="NO"		# Set to YES to enable APM BIOS functions (or NO).
 apmd_enable="NO"	# Run apmd to handle APM event from userland.
 apmd_flags=""		# Flags to apmd (if enabled).
+backlight_enable="NO"	# Save/restore backlight levels
 ddb_enable="NO"		# Set to YES to load ddb scripts at boot.
 ddb_config="/etc/ddb.conf"	# ddb(8) config file.
 devd_enable="YES" 	# Run devd, to trigger programs on device tree changes.

libexec/rc/rc.d/Makefile

@@ -11,6 +11,7 @@ CONFS=	DAEMON \
 	NETWORKING \
 	SERVERS \
 	adjkerntz \
+	backlight \
 	bgfsck \
 	bridge \
 	cfumass \

libexec/rc/rc.d/backlight

@@ -0,0 +1,88 @@
+#!/bin/sh -
+#
+# Copyright (c) 2026 Kyle Evans <[email protected]>
+#
+# SPDX-License-Identifier: BSD-2-Clause
+#
+
+# PROVIDE: backlight
+# REQUIRE: FILESYSTEMS kld
+# KEYWORD: nojail shutdown
+
+. /etc/rc.subr
+
+name="backlight"
+desc="Save and restore backlight values"
+rcvar="backlight_enable"
+stop_cmd="backlight_stop"
+start_cmd="backlight_start"
+reload_cmd="backlight_start"
+extra_commands="reload"
+
+backlightdb="/var/db/backlight"
+
+#
+# List current backlight devices to stdout.
+#
+list_backlights()
+{
+	( cd /dev/backlight ; ls backlight* 2>/dev/null )
+}
+
+#
+# Save state of an individual backlight specified as $1
+#
+backlight_save()
+{
+	local dev
+
+	dev="/dev/backlight/$1"
+	if [ -r "$dev" ]; then
+		/usr/bin/backlight -f "$dev" -q > "$backlightdb/$1-state" 2>/dev/null
+	fi
+}
+
+#
+# Restore the state of an individual backlight specified as $1
+#
+backlight_restore()
+{
+	local file dev
+
+	dev="/dev/backlight/$1"
+	file="$backlightdb/$1-state"
+	if [ -r "$dev" -a -r "$file" ]; then
+		/usr/bin/backlight -f "$dev" $(cat "$file") > /dev/null
+	fi
+}
+
+#
+# Restore state of all backlights
+#
+backlight_start()
+{
+	local backlight
+
+	for backlight in $(list_backlights); do
+		backlight_restore "$backlight"
+	done
+}
+
+#
+# Save the state of all backlights
+#
+backlight_stop()
+{
+	local backlight
+
+	for backlight in $(list_backlights); do
+		backlight_save "$backlight"
+	done
+}
+
+load_rc_config $name
+
+# doesn't make sense to run in a svcj: config setting
+backlight_svcj="NO"
+
+run_rc_command "$1"