Backlight controls broken for Thinkpad T14s in Cinnamon 6.6.9

[waldo22]

Distribution

Debian 14 (testing), kernel 7.1.8

Package version

6.6.9

Graphics hardware in use

Radeon 780M Graphics (Ryzen 7 integrated)

Frequency

Always

Bug description

Backlight controls for Thinkpad T14s Gen 4 (AMD) do not work on Debian 14 kernel 7.1.8, whereas they worked perfectly with Debian 13 Trixie kernel 6.12. Firmware version is R2EET48W (1.29 ), which is most recent as of 7th September 2026.

Upon installation, backlight is so dim as to be unusable. The <F5> and <F6> brightness controls have no effect, and the screen brightness OSD is not displayed. This behavior with no kernel parameter set is the same behavior as with kernel parameters acpi_backlight=native and amdgpu.backlight=0, and /sys/class/backlight/ is empty.

Adding acpi_backlight=vendor or acpi_backlight=none to the GRUB_CMDLINE_LINUX_DEFAULT parameter in /etc/default/grub/ and re-booting sets the screen to full brightness, which makes it usable, but unpleasant to look at, and controls still fail to adjust brightness and OSD is still not displayed. /sys/class/backlight/ remains empty.

Setting acpi_backlight=video instead of vendor causes the OSD to display, and the OSD graphic increases or decreases the length of the backlight bar, but the brightness itself is unchanged. The acpi_backlight=video kernel parameter also populates /sys/class/backlight/ with the virtual device acpi_video0 which is symlinked as acpi_video0 -> ../../devices/pci0000:00/0000:00:08.1/0000:c3:00.0/backlight/acpi_video0. With this parameter set as acpi_backlight=video, both the brightnessctl command and csd-backlight-helper updates the value in /sys/class/backlight/acpi_video0/brightness, but the actual brightness is unchanged.

inxi --edid yields:

Graphics:
  Device-1: Advanced Micro Devices [AMD/ATI] Phoenix1 vendor: Lenovo
    driver: amdgpu v: kernel arch: RDNA-3 code: Phoenix process: TSMC n4 (4nm)
    built: 2023+ pcie: gen: 4 speed: 16 GT/s lanes: 16 ports: active: eDP-1
    empty: DP-1, DP-2, DP-3, DP-4, DP-5, DP-6, HDMI-A-1, Writeback-1
    bus-ID: c3:00.0 chip-ID: 1002:15bf class-ID: 0300 temp: 43.0 C
  Device-2: Chicony Integrated Camera driver: uvcvideo type: USB rev: 2.0
    speed: 480 Mb/s lanes: 1 mode: 2.0 bus-ID: 3-1:2 chip-ID: 04f2:b7c0
    class-ID: fe01 serial: 01.00.00
  Display: x11 server: X.Org v: 21.1.23 driver: X: loaded: amdgpu
    unloaded: fbdev,modesetting,vesa dri: radeonsi gpu: amdgpu display-ID: :0
    screens: 1
  Screen-1: 0 s-res: 1920x1200 s-dpi: 96 s-size: 508x317mm (20.00x12.48")
    s-diag: 599mm (23.57")
  Monitor-1: eDP-1 mapped: eDP model: Lenovo 0x403d built: 2021 res:
    mode: 1920x1200 hz: 60 scale: 100% (1) dpi: 162 gamma: 1.2 chroma: red:
    x: 0.592 y: 0.349 green: x: 0.329 y: 0.557 blue: x: 0.153 y: 0.118 white:
    x: 0.314 y: 0.329 size: 301x188mm (11.85x7.4") diag: 355mm (14")
    ratio: 16:10 modes: 1920x1200, 1920x1080, 1600x1200, 1680x1050,
    1280x1024, 1440x900, 1280x800, 1280x720, 1024x768, 800x600, 640x480
  API: EGL v: 1.5 hw: drv: amd radeonsi platforms: device: 0 drv: radeonsi
    device: 1 drv: swrast gbm: drv: radeonsi surfaceless: drv: radeonsi x11:
    drv: radeonsi inactive: wayland
  API: OpenGL v: 4.6 vendor: amd mesa v: 26.1.5-1 glx-v: 1.4
    direct-render: yes renderer: AMD Radeon 780M Graphics (radeonsi phoenix ACO
    DRM 3.64 7.1.7+deb14-amd64) device-ID: 1002:15bf memory: 1000 MiB
    unified: yes
  Info: Tools: api: eglinfo,glxinfo x11: xdriinfo, xdpyinfo, xprop, xrandr

Kernel debug shows the following 3 messages for each time the brightness decrease or increase is pressed:

pkexec[4378]: PAM (polkit-1) /usr/lib/pam.d is not supported on this system; see /usr/share/doc/libpam0g/Debian-PAM-MiniPolicy.gz
pkexec[4378]: pam_unix(polkit-1:session): session opened for user root(uid=0) by wes(uid=1000)
pkexec[4378]: wes: Executing command [USER=root] [TTY=unknown] [CWD=/home/wes] [COMMAND=/usr/lib/x86_64-linux-gnu/cinnamon-settings-daemon/csd-backlight-helper --set-brightness 81 -b firmware -b platform -b raw]

Steps to reproduce

Press <Fn>+<F5> or <Fn>+<F6> to decrease or increase backlight brightness and brightness does not change.

Expected behavior

Backlight should adjust brighter with <Fn>+<F6> and dimmer with <Fn>+<F5> upon clean installation, without any intervention.

Additional information

This appears to be a problem between Cinnamon (and Mate) and the amdgpu driver in Linux kernel v 7.x.

The following Arch wiki entry may be relevant:
https://wiki.archlinux.org/title/Backlight#sysfs_modified_but_no_brightness_change

Discussion on the Debian forums is here:
https://forums.debian.net/viewtopic.php?t=167051