Created attachment 186402 [details]
HDR

SUMMARY [Plasma 6.5.1] HDR content is severely overexposed and DDC/CI brightness control fails after enabling HDR mode

STEPS TO REPRODUCE

Start with the system in standard SDR mode. Confirm that DDC/CI brightness control is fully functional via Plasma's brightness slider or keyboard shortcuts.

Enable HDR mode in System Settings > Display & Monitor.

Play any native HDR content (e.g., an HDR video file or an HDR-enabled game).

Attempt to adjust the screen brightness using the Plasma brightness slider.

OBSERVED RESULT

Overexposure of HDR Content: Native HDR content is displayed with severe overexposure. Bright areas of the image ("highlights") are completely blown out, leading to a significant loss of detail and a "clipped" appearance.

Brightness Control Failure: Immediately after HDR is enabled, all brightness control ceases to function. DDC/CI, which was working perfectly in SDR mode, becomes completely unresponsive. The brightness slider can be moved, but it has no effect on the actual screen luminance.

Diagnostic tools confirm the issue: running kscreen-doctor -o shows that brightness control is stuck at 100% (Brightness control: supported, set to 100% and dimming to 100%).

EXPECTED RESULT

HDR content should be correctly tone-mapped without overexposing highlights.

The system's brightness control should remain functional after enabling HDR, allowing the user to adjust the overall screen brightness to a comfortable level (likely via a software dimming mechanism, since DDC/CI is often disabled by the monitor in HDR mode).

SOFTWARE/OS VERSIONS 
Operating System: Arch Linux  
KDE Plasma Version: 6.5.1
KDE Frameworks Version: 6.19.0
Qt Version: 6.10.0
Kernel Version: 6.12.56-1-lts (64-bit)
Graphics Platform: Wayland
Processors: 12 × AMD Ryzen 5 5600X 6-Core Processor
Memory: 32 GiB of RAM (31.3 GiB usable)
Graphics Processor: AMD Radeon RX 7900 XTX


ADDITIONAL INFORMATION This appears to be a critical regression in Plasma 6.5.1, where enabling HDR mode simultaneously breaks brightness control and causes incorrect tone mapping.

The key observation is the direct link between the two issues: DDC/CI brightness control works flawlessly in SDR mode but fails the instant HDR is activated.

My hypothesis is that a failure in KWin's brightness control mechanism for HDR mode is causing the monitor's backlight to remain at its maximum physical level. However, KWin's compositor seems to be rendering the HDR content based on the configured SDR reference brightness (200 nits in my case), assuming a much dimmer baseline. This severe mismatch between the software rendering intent and the actual hardware brightness level results in the final image being drastically overexposed.

This issue is separate from the UI blur saturation changes tracked in bug #510818.

Diagnostic Information (kscreen-doctor -o output):

Output: 1 DP-3 a57eaaef-a13d-451c-867f-54f3db0e791a
        enabled
        connected
        priority 1
        DisplayPort
        replication source:0
        Modes:  1:3840x2160@160.00*!  2:3840x2160@60.00  3:3840x2160@144.00  4:3840x2160@120.00  5:3840x2160@119.88  6:3
840x2160@120.00  7:3840x2160@60.00  8:3840x2160@59.94  9:3840x2160@50.00  10:3840x2160@30.00  11:3840x2160@29.97  12:256
0x1440@120.00  13:2560x1440@75.00  14:2560x1440@60.00  15:1920x1200@160.00  16:1920x1080@144.00  17:1920x1080@120.00  18
:1920x1080@119.88  19:1920x1080@60.00  20:1920x1080@60.00  21:1920x1080@59.94  22:1920x1080@50.00  23:1600x1200@160.00  
24:1680x1050@59.95  25:1280x1024@75.03  26:1280x1024@60.02  27:1440x900@59.89  28:1280x800@160.00  29:1280x720@60.00  30
:1280x720@59.94  31:1280x720@50.00  32:1024x768@75.03  33:1024x768@60.00  34:800x600@75.00  35:800x600@60.32  36:720x576
@50.00  37:720x480@60.00  38:720x480@59.94  39:640x480@75.00  40:640x480@60.00  41:640x480@59.94  42:640x480@59.94  43:7
20x400@70.08  44:1600x1200@59.87  45:1280x1024@159.75  46:1024x768@159.72  47:2560x1600@59.99  48:2560x1600@159.94  49:1
920x1200@59.88  50:1280x800@59.81  51:3200x1800@59.96  52:3200x1800@159.98  53:2880x1620@59.96  54:2880x1620@159.95  55:
2560x1440@159.91  56:1920x1080@159.88  57:1600x900@59.95  58:1600x900@159.82  59:1368x768@59.88  60:1368x768@159.86  61:
1280x720@159.78  
        Geometry: 0,0 1920x1080
        Scale: 2
        Rotation: 1
        Overscan: 0
        Vrr: Automatic
        RgbRange: unknown
        HDR: enabled
                SDR brightness: 200 nits
                SDR gamut wideness: 0%
                Peak brightness: 1350 nits, overridden with: 1350 nits
                Max average brightness: 911 nits
                Min brightness: 0 nits
        Wide Color Gamut: enabled
        ICC profile: none
        Color profile source: EDID
        Color power preference: prefer efficiency and performance
        Brightness control: supported, set to 100% and dimming to 100%
        DDC/CI: allowed
        Color resolution: automatic (10), range: [8; 16] bits per color
        Allow EDR: unsupported
        Sharpness control: unsupported