> Did this issue start happening after an update/upgrade? Was there a
prior kernel version where you were not having this particular problem?

I can not say if brightness controls worked immediately after installing
the system. However, they were definitely broken after installing a few
standard packages and the (proprietary) graphics drivers.

The latest upstream kernel does *not* resolve the issue (neither are the
FN buttons working, nor are `backlight` classes available under
`/sys/class/backlight`).

(I guess "generic" was the proper variant to test?)

$ uname -r
4.15.0-041500-generic

$ dmesg | grep acpi     
[    0.000000] ACPI: LAPIC_NMI (acpi_id[0x01] high edge lint[0x1])
[    0.000000] ACPI: LAPIC_NMI (acpi_id[0x02] high edge lint[0x1])
[    0.000000] ACPI: LAPIC_NMI (acpi_id[0x03] high edge lint[0x1])
[    0.000000] ACPI: LAPIC_NMI (acpi_id[0x04] high edge lint[0x1])
[    0.000000] ACPI: LAPIC_NMI (acpi_id[0x05] high edge lint[0x1])
[    0.000000] ACPI: LAPIC_NMI (acpi_id[0x06] high edge lint[0x1])
[    0.000000] ACPI: LAPIC_NMI (acpi_id[0x07] high edge lint[0x1])
[    0.000000] ACPI: LAPIC_NMI (acpi_id[0x08] high edge lint[0x1])
[    0.045039] acpiphp: ACPI Hot Plug PCI Controller Driver version: 0.5
[    0.104918] acpi PNP0A08:00: _OSC: OS supports [ExtendedConfig ASPM ClockPM 
Segments MSI]
[    0.105102] acpi PNP0A08:00: _OSC: platform does not support [PCIeHotplug 
PME AER PCIeCapability]
[    0.105105] acpi PNP0A08:00: _OSC: not requesting control; platform does not 
support [PCIeCapability]
[    0.105109] acpi PNP0A08:00: _OSC: OS requested [PCIeHotplug PME AER 
PCIeCapability]
[    0.105112] acpi PNP0A08:00: _OSC: platform willing to grant []
[    0.105114] acpi PNP0A08:00: _OSC failed (AE_SUPPORT); disabling ASPM
[    0.264669] clocksource: acpi_pm: mask: 0xffffff max_cycles: 0xffffff, 
max_idle_ns: 2085701024 ns
[    1.541604] acpi device:77: hash matches
[    1.542026] acpi device:4a: hash matches
[   14.000879] acpi PNP0C14:02: duplicate WMI GUID 
05901221-D566-11D1-B2F0-00A0C9062910 (first instance was on PNP0C14:01)
[   14.000963] acpi PNP0C14:03: duplicate WMI GUID 
05901221-D566-11D1-B2F0-00A0C9062910 (first instance was on PNP0C14:01)
[   14.075152] thinkpad_acpi: ThinkPad ACPI Extras v0.25
[   14.075153] thinkpad_acpi: http://ibm-acpi.sf.net/
[   14.075154] thinkpad_acpi: ThinkPad BIOS N1UET37W (1.11 ), EC unknown
[   14.075155] thinkpad_acpi: Lenovo ThinkPad P51, model 20HJS27B00
[   14.440734] thinkpad_acpi: radio switch found; radios are enabled
[   14.639622] thinkpad_acpi: rfkill switch tpacpi_bluetooth_sw: radio is 
unblocked
[   14.650775] input: ThinkPad Extra Buttons as 
/devices/platform/thinkpad_acpi/input/input13



** Changed in: linux (Ubuntu)
       Status: Incomplete => Confirmed

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https://bugs.launchpad.net/bugs/1747273

Title:
  Can not control brightness on Thinkpad P51

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