Dell Precision 3551: keyboard backlight control not working

[mtu]

The keyboard backlight is working, and can be controlled by the BIOS/hardware (Fn+F5 key).

However, it _cannot_ be controlled by either:

- the GUI (power settings), which have no effect, or
- the /sys/devices/platform/dell-laptop/leds/dell::kbd_backlight/ kernel interface.

Specifically, /sys/devices/platform/dell-laptop/leds/dell::kbd_backlight/brightness always contains "0", irrespective of the true state of the hardware (three possible levels: off, medium, maximum). Writing "1" or "2" to /sys/devices/platform/dell-laptop/leds/dell::kbd_backlight/brightness as root has no effect.

Interestingly, /sys/devices/platform/dell-laptop/leds/dell::kbd_backlight/max_brightness reports the correct value of 2.

Looking at /sys/devices/platform/dell-laptop/leds/dell::kbd_backlight/brightness_hw_changed yields:
cat: brightness_hw_changed: No data available

ProblemType: Bug
DistroRelease: Ubuntu 21.10
Package: linux-image-5.13.0-28-generic 5.13.0-28.31
ProcVersionSignature: Ubuntu 5.13.0-28.31-generic 5.13.19
Uname: Linux 5.13.0-28-generic x86_64
ApportVersion: 2.20.11-0ubuntu71
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: mbueker 1444 F.... pulseaudio
CasperMD5CheckResult: pass
CurrentDesktop: KDE
Date: Tue Feb 8 14:08:49 2022
InstallationDate: Installed on 2022-01-22 (16 days ago)
InstallationMedia: Kubuntu 21.10 "Impish Indri" - Release amd64 (20211012)
MachineType: Dell Inc. Precision 3551
ProcFB: 0 i915drmfb
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-5.13.0-28-generic root=/dev/mapper/vgkubuntu-root ro quiet splash vt.handoff=7
RelatedPackageVersions:
 linux-restricted-modules-5.13.0-28-generic N/A
 linux-backports-modules-5.13.0-28-generic N/A
 linux-firmware 1.201.3
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 12/10/2021
dmi.bios.release: 1.10
dmi.bios.vendor: Dell Inc.
dmi.bios.version: 1.10.0
dmi.board.name: 033T3Y
dmi.board.vendor: Dell Inc.
dmi.board.version: A00
dmi.chassis.type: 10
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvr1.10.0:bd12/10/2021:br1.10:svnDellInc.:pnPrecision3551:pvr:rvnDellInc.:rn033T3Y:rvrA00:cvnDellInc.:ct10:cvr:sku09C2:
dmi.product.family: Precision
dmi.product.name: Precision 3551
dmi.product.sku: 09C2
dmi.sys.vendor: Dell Inc.

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This change was made by a bot.