Brightness slider and Fn keys fail in Hybrid mode on Acer Nitro (RTX 4050) - Incorrectly targeting nvidia_0 interface #13372
Description
Distribution
Mint 22.2 Zara (Cinnamon Edition)
Package version
Cinnamon 6.4.8
Graphics hardware in use
Intel Alder Lake-P UHD Graphics & NVIDIA GeForce RTX 4050 Laptop GPU (Optimus/Hybrid Mode)
Frequency
Always
Bug description
In Hybrid (Optimus) mode, the Cinnamon brightness slider and keyboard Fn keys fail to adjust the physical screen brightness. The system erroneously targets the nvidia_0 backlight interface, which is a "dead end" on this hardware configuration.
While the UI slider moves and updates the numerical value in /sys/class/backlight/nvidia_0/brightness, the physical screen remains at 100% brightness because the hardware is actually wired to the /sys/class/backlight/intel_backlight/ controller.
Standard kernel workarounds like acpi_backlight=native, vendor, or video do not resolve the issue on this model (Acer Nitro AN515-58) and kernel version (6.14). Manual hardware control via sudo brightnessctl --device=intel_backlight set 50% works perfectly, confirming that the hardware is functional but the Cinnamon UI is linked to the wrong interface.
Steps to reproduce
Use an Acer Nitro AN515-58 laptop with NVIDIA proprietary drivers (v580+).
Ensure the system is in Hybrid/Optimus mode (Integrated graphics driving the display).
Attempt to adjust brightness using the Cinnamon taskbar slider or laptop Fn keys.
Expected behavior
The Cinnamon brightness applet should detect and prioritize the functional intel_backlight interface over the non-functional nvidia_0 interface when the iGPU is the primary display controller.
Additional information
Hard-coding the backlight to Intel via X11 configuration (20-intel.conf) causes a critical crash of the Display Manager (LightDM) because it breaks the NVIDIA PRIME bridge. The issue appears to be a lack of proper backlight arbitration in the Cinnamon settings daemon for newer RTX 40-series mobile hardware.