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**Environment:**
* **OS:** Arch Linux (Latest stable kernel)
* **Hardware:** ASUS ROG Strix Laptop (AMD CPU + NVIDIA dGPU)
* **WM/DE:** Niri (Wayland)
* **Backlight Utilities:** brightnessctl
---
**Problem Description:**
Whenever I adjust the screen brightness (either via hardware keys or manually in the terminal), **the physical brightness changes perfectly**. However, a cascading low-level permission/IO error is wrapped and dumped into the stdout/stderr or system logs simultaneously.
When adjusting, the user-space utility bubbles up the following error:
```text
failed to set brightness: write brightness: open /sys/class/backlight/nvidia_wmi_ec_backlight/brightness: permission denied
```
*(Note: Even when explicitly specifying the device with brightnessctl -d nvidia_wmi_ec_backlight or suppressing output with -q, the error still surfaces, yet the physical panel brightness reacts correctly).*
---
**Log Evidences (journalctl -f):**
Tracing the journal logs during the brightness keypress event reveals that the root cause is actually an underlying **Input/output error** triggered by missing ACPI firmware symbols, which subsequently causes systemd-backlight to crash-loop and forces user-space tools to trip over fallback mechanisms:
```text
systemd-backlight[4262]: nvidia_wmi_ec_backlight: Failed to write system 'brightness' attribute: Input/output error
systemd[1]: systemd-backlight@backlight:nvidia_wmi_ec_backlight.service: Main process exited, code=exited, status=1/FAILURE
kernel: ACPI BIOS Error (bug): Could not resolve symbol [\_SB.PCI0.SBRG.BPWM], AE_NOT_FOUND (20251212/psargs-332)
kernel: ACPI Error: Aborting method \_SB.PCI0.AWMI.WMAA due to previous error (AE_NOT_FOUND) (20251212/psparse-529)
kernel: nvidia-wmi-ec-backlight 603E9613-EF25-4338-A3D0-C46177516DB7-0: EC backlight control failed: AE_NOT_FOUND
(sd-bright)[4293]: nvidia_wmi_ec_backlight: Failed to write brightness to device: Input/output error
```
---
**Questions & Discussion:**
Based on the kernel's AE_NOT_FOUND dump, this seems to be a classic hardware/firmware-level bug where the laptop's BIOS fails to export the BPWM symbol under the current GPU mode. The physical brightness adjustments are likely executing successfully via another valid interface (like amdgpu_bl0) or through raw D-Bus calls handled by asusctl.
1. What is the most elegant way to completely disable or blacklist this ghost nvidia_wmi_ec_backlight interface without breaking the actual physical backlight control loop?
2. If I pass nvidia.NVreg_EnableBacklightHandler=0 as a kernel parameter or write a specific udev rule to ignore it, will it introduce any unintended side effects when switching graphics modes (e.g., MUX switch/Advanced Optimus) on an ROG platform?
Any insights or recommended workarounds would be greatly appreciated. Thanks in advance!
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