maintainer-feedback requested: [Bug 286721] graphics/drm-61-kmod, graphics/drm-66-kmod: No backlight control

Bugzilla Automation <bugzilla@FreeBSD.org> has asked freebsd-x11 (Nobody)
<x11@FreeBSD.org> for maintainer-feedback:
Bug 286721: graphics/drm-61-kmod, graphics/drm-66-kmod: No backlight control
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=286721



--- Description ---
Controlling LCD brightness doesn't work on drm-61 and drm-66. Works on drm-515.
-CURRENT from today (4a38527105d5), but problem was noticed months ago.

Backlight will show that value changes:
% backlight 100
% backlight
brightness: 100
% backlight 50
% backlight
brightness: 49

But brightness of the internal LCD screen doesn't change.

acpi_video.ko is loaded.
sysctl hw.acpi.video.lcd0.brightness behaves same as backlight command.

HW: Lenovo ThinkPad X1 carbon, 5th (from 2017.)
# dmesg | grep ^CPU
CPU: Intel(R) Core(TM) i7-7500U CPU @ 2.70GHz (2900.00-MHz K8-class CPU)
# pciconf -lv | grep vgapci -A4
vgapci0@pci0:0:2:0:	class=0x030000 rev=0x02 hdr=0x00 vendor=0x8086
device=0x5916 subvendor=0x17aa subdevice=0x224f
    vendor     = 'Intel Corporation'
    device     = 'HD Graphics 620'
    class      = display
    subclass   = VGA

# backlight -i
Backlight name: intel_backlight
Backlight hardware type: Panel

Files in /dev/ are the same with drm-515 and drm-61:
/dev/backlight/
/dev/backlight/backlight0
/dev/backlight/intel_backlight0
/dev/led/kbd_backlight