EliteBook 8570p, VESA, hw.acpi.video.lcd0.brightness no go

# date ; uname -v
Sat 20 Oct 2018 03:30:01 BST
FreeBSD 12.0-BETA1 r339438 GENERIC
# sysctl hw.acpi.video
hw.acpi.video.tv2.active: 0
hw.acpi.video.tv1.active: 0
hw.acpi.video.tv0.active: 0
hw.acpi.video.crt0.active: 0
hw.acpi.video.lcd0.levels: 100 50 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
hw.acpi.video.lcd0.economy: 50
hw.acpi.video.lcd0.fullpower: 100
hw.acpi.video.lcd0.brightness: 100
hw.acpi.video.lcd0.active: 0
# sysctl hw.acpi.video.lcd0.brightness=50
hw.acpi.video.lcd0.brightness: 100 -> 50
#