NetBSD 9.3 amd64 won't turn off display backlight

[Damien Boureille <damien.boureille%gmail.com@localhost>]

I'm running NetBSD 9.3-amd64 GENERIC on a Dell Precision M4600 (Sandy Bridge) mobile workstation, used as a server.

Closing the lid will not turn off the backlight, which conducts and builds up heat inside the case.

# wsconsctl -d -w backlight=0
wsconsctl: WSDISPLAYIO_PARAM_BACKLIGHT: Inappropriate ioctl for device

# sysctl -a | grep acpi.acpi
hw.acpi.acpiout1.brightness = 50
hw.acpi.acpivga0.bios_switch = 1

This works fine but only dims the display:
# sysctl -w hw.acpi.acpiout1.brightness=0

No other relevant variable seems available in sysctl.

I'm using an i915 chipset:

# pcictl pci0 list
000:00:0: Intel Sandy Bridge (mobile) Host Bridge (host bridge, revision 0x09)
000:02:0: Intel Sandy Bridge (mobile) GT2+ Integrated Graphics Device (VGA display, revision 0x09)
(...)

# dmesg | grep display
[     6.187988] wsdisplay0 at intelfb0 kbdmux 1: console (default, vt100 emulation), using wskbd0
[     6.208326] wsmux1: connecting to wsdisplay0
[    18.353375] wsdisplay0: screen 1 added (default, vt100 emulation)
[    18.363376] wsdisplay0: screen 2 added (default, vt100 emulation)

# dmesg | grep acpivga
[     1.019042] acpivga0 at acpi0 (VID): ACPI Display Adapter
[     1.019042] acpiout0 at acpivga0 (CRT, 0x0100): ACPI Display Output Device
[     1.019042] acpiout1 at acpivga0 (LCD, 0x0400): ACPI Display Output Device
[     1.019042] acpiout2 at acpivga0 (DVI, 0x0302): ACPI Display Output Device
[     1.019042] acpiout3 at acpivga0 (DVI2, 0x0303): ACPI Display Output Device
[     1.019042] acpiout4 at acpivga0 (DVI3, 0x0304): ACPI Display Output Device
[     1.019042] acpiout5 at acpivga0 (DP, 0x0300): ACPI Display Output Device
[     1.019042] acpiout6 at acpivga0 (DP2, 0x0301): ACPI Display Output Device
[     1.019042] acpiout7 at acpivga0 (DP3, 0x0305): ACPI Display Output Device
[     1.019042] acpivga0: connected output devices:
[     1.019042] acpivga0:   0x0100 (acpiout0): Ext. Monitor, head 0
[     1.019042] acpivga0:   0x0400 (acpiout1): Unknown Output Device, head 0
[     1.019042] acpivga0:   0x0302 (acpiout2): Unknown Output Device, head 0
[     1.019042] acpivga0:   0x0303 (acpiout3): Unknown Output Device, head 0
[     1.019042] acpivga0:   0x0300 (acpiout5): Unknown Output Device, head 0
[     1.019042] acpivga0:   0x0301 (acpiout6): Unknown Output Device, head 0
[     1.019042] acpivga0:   0x0304 (acpiout4): Unknown Output Device, head 0
[     1.019042] acpivga0:   0x0305 (acpiout7): Unknown Output Device, head 0

# modstat | grep wmi 
acpiwmi                    driver   builtin  -        3       - -
wmidell                    driver   builtin  -        0       - acpiwmi,sysmon_power
wmieeepc                   driver   builtin  -        0       - acpiwmi,sysmon_power
wmihp                      driver   builtin  -       -1       - acpiwmi,sysmon_envsys
wmimsi                     driver   builtin  -        0       - acpiwmi

The display turns off fine with Linux 5.10 (Alpine 3.17).

I'm not yet sure how to fix this. Any clue is appreciated.

D.B.