# Fix: KDE Plasma Brightness Stuck at ~20% on Intel/NVIDIA Hybrid Laptop

## Environment
- OS: CachyOS (Arch-based), kernel 6.19.7-1-cachyos
- DE: KDE Plasma 6
- GPU: Intel Arrow Lake-S (integrated) + NVIDIA RTX 5070 Ti Mobile
- Laptop: ASUS (model with dual NVMe)

## Problem

KDE brightness slider showed 100%, but the screen was visibly much dimmer than it should be. Adjusting brightness via hotkeys would make it even dimmer — any hotkey press reset the panel to a dim state.

## Diagnosis

Two backlight interfaces exist:

```bash
ls /sys/class/backlight/
# intel_backlight  nvidia_0

cat /sys/class/backlight/intel_backlight/brightness
# 100
cat /sys/class/backlight/intel_backlight/max_brightness
# 496

cat /sys/class/backlight/nvidia_0/brightness
# 100
cat /sys/class/backlight/nvidia_0/max_brightness
# 100

cat /sys/class/backlight/intel_backlight/type
# raw
cat /sys/class/backlight/nvidia_0/type
# raw
```

**Root cause:** Both backlights report type `raw`, so KDE couldn't distinguish which was the "real" panel backlight. `nvidia_0` has max=100, `intel_backlight` has max=496. When KDE's brightness hotkeys set "100%" via `nvidia_0`'s scale, it wrote `100` to `intel_backlight` — which is only ~20% of its actual 496 range.

## Fix

### Immediate Fix
Set the correct brightness:
```bash
sudo sh -c 'echo 496 > /sys/class/backlight/intel_backlight/brightness'
```

### Permanent Fix - Hide nvidia_0 via udev

Create `/etc/udev/rules.d/backlight.rules`:
```bash
sudo tee /etc/udev/rules.d/backlight.rules > /dev/null << 'EOF'
# Disable nvidia_0 backlight - conflicts with intel_backlight on this laptop
# KDE picks up nvidia_0 (max=100) and maps it incorrectly to intel_backlight (max=496)
SUBSYSTEM=="backlight", KERNEL=="nvidia_0", ATTR{brightness}="0", RUN+="/bin/chmod 000 /sys/class/backlight/nvidia_0"
EOF
```

Apply immediately (without reboot):
```bash
sudo chmod 000 /sys/class/backlight/nvidia_0
sudo udevadm control --reload-rules
systemctl --user restart plasma-powerdevil
```

## Verification

After hiding `nvidia_0`, KDE only sees `intel_backlight` and maps the slider/hotkeys correctly to the 0-496 range. Full brightness is restored and hotkeys work as expected.

## Why This Happens

On Intel/NVIDIA hybrid laptops, both GPUs may expose a backlight interface. The NVIDIA driver creates `nvidia_0` even though the actual panel backlight is controlled by `intel_backlight`. Since both report type `raw`, KDE (powerdevil) can pick up the wrong one. The `nvidia_0` interface has a max of 100 — when KDE writes that value to the actual panel controller (max 496), you get ~20% brightness.

This is most common on newer laptops with Intel Arrow Lake + RTX 40/50 series mobile GPUs running the proprietary NVIDIA driver.