Mike Swanson
7c8488ad14
Synced files: - Session log 2026-03-19 updated (autostart, ScreenConnect, Flarum forum, theme, Node.js) - docs/forum-posts/ recovered from old btrfs home (7 forum post guides) Machine: acg-guru-5070 Timestamp: 2026-03-19 19:25:24 Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
74 lines
2.7 KiB
Markdown
74 lines
2.7 KiB
Markdown
# Fix: KDE Plasma Brightness Stuck at ~20% on Intel/NVIDIA Hybrid Laptop
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## Environment
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- OS: CachyOS (Arch-based), kernel 6.19.7-1-cachyos
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- DE: KDE Plasma 6
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- GPU: Intel Arrow Lake-S (integrated) + NVIDIA RTX 5070 Ti Mobile
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- Laptop: ASUS (model with dual NVMe)
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## Problem
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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.
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## Diagnosis
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Two backlight interfaces exist:
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```bash
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ls /sys/class/backlight/
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# intel_backlight nvidia_0
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cat /sys/class/backlight/intel_backlight/brightness
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# 100
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cat /sys/class/backlight/intel_backlight/max_brightness
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# 496
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cat /sys/class/backlight/nvidia_0/brightness
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# 100
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cat /sys/class/backlight/nvidia_0/max_brightness
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# 100
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cat /sys/class/backlight/intel_backlight/type
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# raw
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cat /sys/class/backlight/nvidia_0/type
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# raw
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```
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**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.
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## Fix
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### Immediate Fix
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Set the correct brightness:
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```bash
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sudo sh -c 'echo 496 > /sys/class/backlight/intel_backlight/brightness'
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```
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### Permanent Fix - Hide nvidia_0 via udev
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Create `/etc/udev/rules.d/backlight.rules`:
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```bash
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sudo tee /etc/udev/rules.d/backlight.rules > /dev/null << 'EOF'
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# Disable nvidia_0 backlight - conflicts with intel_backlight on this laptop
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# KDE picks up nvidia_0 (max=100) and maps it incorrectly to intel_backlight (max=496)
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SUBSYSTEM=="backlight", KERNEL=="nvidia_0", ATTR{brightness}="0", RUN+="/bin/chmod 000 /sys/class/backlight/nvidia_0"
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EOF
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```
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Apply immediately (without reboot):
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```bash
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sudo chmod 000 /sys/class/backlight/nvidia_0
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sudo udevadm control --reload-rules
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systemctl --user restart plasma-powerdevil
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```
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## Verification
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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.
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## Why This Happens
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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.
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This is most common on newer laptops with Intel Arrow Lake + RTX 40/50 series mobile GPUs running the proprietary NVIDIA driver.
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