On Steam Deck, DRM_MODE_ROTATE_90 and DRM_MODE_ROTATE_270 seems reversed
On the Steam Deck, when hw compositing instead of client compositing is in-use, DRM_MODE_ROTATE_270 and DRM_MODE_ROTATE_90 are reversed, causing up-side-down rendering. No idea where the fault truly is but a band aid solution would be to swap DRM_MODE_ROTATE_270 and DRM_MODE_ROTATE_90 at ToDrmRotation drm/DrmPlane.cpp, hardcoded or with the help of props.
The issue is not observed on Intel devices, and I don't have another amd device to test this.
Kernel in-use can be found at https://github.com/hmtheboy154/Darkmatter-kernel/tree/gloria