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This is my first post.
I tried hard to fix it myself, using the Arch Wiki and searching online. My last attempt was a long discussion with Gemini.
In the end, I asked for a report to post the issue, and the result is below.
I'm posting here because I'm not 100% sure of what I'm doing.
THANKS in advance.
1. Issue Description
On a Lenovo ThinkPad T14s Gen 1 (AMD) laptop running Arch Linux, the keyboard backlight functionality is not working as expected. While the system appears to recognize the keyboard backlight device and allows software control of its brightness value, the physical backlight does not illuminate. The keyboard backlight functions correctly when running Windows on the same hardware.
2. Hardware Information
Laptop Model: Lenovo ThinkPad T14s Gen 1 (AMD) - 20UJSORP0W.
Processor: AMD Ryzen 5 PRO 4650U,
BIOS/UEFI Version: R1CHT82W (1.51, 2024-11-05) - The latest version is 1.52.
3. Software Information
Operating System: Arch Linux
Kernel Version: 6.14.9-arch1-1
Bootloader: systemd-boot
Relevant Kernel Modules: thinkpad_acpi (and potentially amdgpu for backlight interaction)
Utilities Used: brightnessctl
4. Steps to Reproduce
Boot the ThinkPad T14s Gen 1 (AMD) into Arch Linux.
Attempt to control the keyboard backlight using brightnessctl.
5. Observed Behavior
The device /sys/class/leds/tpacpi::kbd_backlight/ is present after applying the acpi_backlight=native kernel parameter. (Initially, this device was not present without specific kernel parameters).
Executing brightnessctl --device='tpacpi::kbd_backlight' set 2 successfully updates the brightness value in sysfs. The output from brightnessctl shows:
Updated device 'tpacpi::kbd_backlight':
Device 'tpacpi::kbd_backlight' of class 'leds':
Current brightness: 2 (100%)
Max brightness: 2
Despite the software indicating 100% brightness, the physical keyboard backlight remains off.
Attempting to use the Fn + Space key combination (standard for keyboard backlight on ThinkPads) has no effect on the physical backlight.
Interestingly, the brightnessctl set 2k command (which often targets the keyboard backlight) instead controls the screen backlight (amdgpu_bl1), showing:
Updated device 'amdgpu_bl1':
Device 'amdgpu_bl1' of class 'backlight':
Current brightness: 2 (1%)
Max brightness: 255
This suggests a possible misdirection or confusion in device handling at a lower level.
6. Kernel Log Output (dmesg)
Initial dmesg output without specific acpi_backlight parameters showed:
[ 1.088568] thinkpad_acpi: This ThinkPad has standard ACPI backlight brightness control, supported by the ACPI video driver
[ 1.110221] thinkpad_acpi: Standard ACPI backlight interface available, not loading native one
[ 2050.374004] thinkpad_acpi: Cannot enable backlight brightness support, ACPI is already handling it. Refer to the acpi_backlight kernel parameter.
After adding acpi_backlight=vendor to the kernel command line:
[ 0.000000] Command line: ... acpi_backlight=vendor
[ 0.041658] Kernel command line: ... acpi_backlight=vendor
[ 5.017923] amdgpu 0000:06:00.0: amdgpu: [drm] Skipping amdgpu DM backlight registration
Note: While acpi_backlight=vendor made tpacpi::kbd_backlight appear, it caused the amdgpu driver to skip screen backlight registration.
After switching to acpi_backlight=native in the kernel command line:
[ 0.000000] Command line: ... acpi_backlight=native
[ 0.041707] Kernel command line: ... acpi_backlight=native
[ 1.151821] thinkpad_acpi: This ThinkPad has standard ACPI backlight brightness control, supported by the ACPI video driver
[ 1.167434] thinkpad_acpi: Standard ACPI backlight interface available, not loading native one
Note: With acpi_backlight=native, tpacpi::kbd_backlight is still present, amdgpu screen backlight registration is not skipped, but the keyboard backlight still does not light up.
7. Expected Behavior
The keyboard backlight should illuminate and respond to software control (brightnessctl) and hardware keys (Fn + Space) as it does on Windows.
8. Troubleshooting Performed
Verified tpacpi::kbd_backlight device in /sys/class/leds/ (appears with acpi_backlight kernel parameters).
Attempted manual brightness control via echo X > /sys/class/leds/tpacpi::kbd_backlight/brightness (no physical effect).
Used brightnessctl --device='tpacpi::kbd_backlight' set X (no physical effect, despite software reporting success).
Updated BIOS/UEFI to the latest version available from Lenovo (confirmed this has been done).
Tested different acpi_backlight kernel parameters (vendor and native), observing changes in sysfs device presence and dmesg output.
Confirmed keyboard backlight functions correctly on Windows on the same device.
9. Conclusion
Given that the device is recognized in sysfs and brightnessctl successfully updates its reported brightness, but the physical backlight does not respond, and it works correctly on Windows, this strongly suggests a kernel/driver bug or a firmware-Linux compatibility issue that needs to be addressed in the thinkpad_acpi module or related ACPI/DRM subsystems for this specific ThinkPad T14s Gen 1 (AMD) model.
Thanks!!!
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I am having the same problem, I just got the laptop and thought the keyboard backlight is broken until I read this post. Is there any update on this ?
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