DRM/KMS defaults to limited color range (16-235) for RGB planes, should default to full (0-255)

[diegoalejandrogomez]

The NVIDIA DRM/KMS output path defaults to limited color range (16-235) for RGB content when the COLOR_RANGE plane property is not explicitly set. The X11 DDX driver defaults to full range (0-255) for the same hardware. This causes every Wayland compositor to produce washed-out output compared to X11.

How to reproduce

1. Boot to a Wayland compositor (KDE Plasma, Sway, any wlroots compositor)
2. Open any application (Firefox, VS Code, terminal)
3. Switch to X11 (KDE Plasma X11 session)
4. Compare the same application — X11 is noticeably more vivid

The difference can be confirmed with nvidia-settings on X11:

• ColorRange = 0 (Full): correct output
• ColorRange = 1 (Limited): identical to Wayland appearance

Root cause

In kernel-open/nvidia-drm/nvidia-drm-crtc.c, the function nv_drm_color_range_to_nvkms_color_range() maps:

• DRM_COLOR_YCBCR_FULL_RANGE → NVKMS_INPUT_COLOR_RANGE_FULL
• DRM_COLOR_YCBCR_LIMITED_RANGE → NVKMS_INPUT_COLOR_RANGE_LIMITED
• default → NVKMS_INPUT_COLOR_RANGE_DEFAULT

When compositors don't set COLOR_RANGE (which the DRM spec says is for YCbCr only), the driver uses NVKMS_INPUT_COLOR_RANGE_DEFAULT, which results in limited range output on DisplayPort connections.

Additionally, the standard DRM connector property Broadcast RGB (which controls full vs limited RGB range) is not exposed by the NVIDIA driver. Both Intel and AMD expose this: https://drmdb.emersion.fr/properties/3233857728/Broadcast%20RGB

This is likely the correct property for this use case, as pointed out by a wlroots maintainer. Without it, there is no proper way to control RGB output range on the DRM path.

Expected behavior

For RGB content, the DRM/KMS path should default to full range (0-255), matching the X11 DDX behavior.

Workaround

Compositors can explicitly set COLOR_RANGE = DRM_COLOR_YCBCR_FULL_RANGE on RGB planes in their atomic commits. This has been submitted to KWin: https://invent.kde.org/plasma/kwin/-/merge_requests/9075

Environment

• GPU: RTX 5070 Ti
• Driver: 595.58.03
• Kernel: 6.19.11-1-cachyos
• Monitors: Dell S2716DG (DP), Acer XB273K V3 (DP)
• Both monitors affected