System: RTX 5070 Ti, Driver 595.71.05, CachyOS kernel 7.0.1-1-cachyos
Issue: Enabling HDR in KDE Display Settings, then playing fullscreen 4K video causes:
1. DRM atomic modeset rejected with “Permission denied”
2. /dev/dri/card0 access revoked
3. PCIe lanes lock up (WiFi hangs until power drain or suspend/resume)
4. KWin session killed
KDE developer confirmed this is a driver-level DRM issue, not compositor bug.
Workaround: disable HDR. Fix verified on 595.58.03 (older driver version).