Screen brightness adjustment doesn't work on Lenovo Thinkpad T14 AMD gen1

[Dehuan Xin]

Thinkpad T14 AMD uses relatively new AMD Ryzen 4000 platform.
Screen brightness is fixed at likely the brightest setting and can not be adjusted.
Keyboard shortcut keys are working as the brightness icon shows up, but the actual brightness doesn't change.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: evince 3.36.7-0ubuntu1
ProcVersionSignature: Ubuntu 5.4.0-47.51-generic 5.4.55
Uname: Linux 5.4.0-47-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.4
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Tue Sep 15 22:13:07 2020
InstallationDate: Installed on 2020-09-16 (0 days ago)
InstallationMedia: Ubuntu 20.04.1 LTS "Focal Fossa" - Release amd64 (20200731)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: evince
UpgradeStatus: No upgrade log present (probably fresh install)

[Patrick Elsen (xfbs+)]

I have this issue as well on elementaryOS based on Ubuntu 18.04:

Linux Celer 5.4.0-52-generic #57~18.04.1-Ubuntu SMP Thu Oct 15 14:04:49 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

Tried booting with

thinkpad-acpi.brightness_enable=1 acpi_backlight=vendor

But no luck either. There is something in `/sys/class/backlight` but writing to `brightness` has no effect.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.