[Bug 222265] New: amdgpu: RX 7600 defaults to limited RGB range (washed-out blacks) over HDMI since kernel 7.3.4

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=222265

            Bug ID: 222265
           Summary: amdgpu: RX 7600 defaults to limited RGB range
                    (washed-out blacks) over HDMI since kernel 7.3.4
           Product: Drivers
           Version: 2.5
    Kernel Version: 7.3
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P3
         Component: Video(DRI - non Intel)
          Assignee: drivers_video-dri@kernel-bugs.osdl.org
          Reporter: mister.freeman@laposte.net
        Regression: Yes

Created attachment 311464
  --> https://bugzilla.kernel.org/attachment.cgi?id=311464&action=edit
Attaching the original unmodified 256-byte EDID binary (`original_edid.bin`)
from the Iiyama ProLite X2483/X2481 monitor

Starting with Linux kernel 7.3.4, my AMD Radeon RX 7600 outputs washed-out
blacks (greyish blacks, indicative of limited RGB range 16-235 or incorrect
color format negotiation) over HDMI (`HDMI-A-1`) immediately at early KMS boot,
prior to any display manager or Wayland compositor loading.

Hardware & Software Environment
- **Distribution:** Arch Linux (up to date)
- **Kernel version:** 7.3.4 (Regression; previous kernels / LTS work properly)
- **GPU:** AMD Radeon RX 7600 (RDNA 3 / Navi 33)
- **Connection:** HDMI (`HDMI-A-1`)
- **Monitor:** Iiyama ProLite X2483/X2481 (1920x1080 @ 60Hz, EDID 1.3 with
CTA-861 extension block)
- **Desktop Environment:** KDE Plasma (Wayland) / Plasma Login Manager

### Steps to Reproduce
1. Boot Linux kernel 7.3.4 with `amdgpu` enabled (early KMS).
2. Observe the TTY console and login screen on the HDMI monitor before any user
session overrides DRM properties.

### Actual Results
Black levels appear washed out (dark grey) right from kernel mode-setting
initialization at boot.

### Expected Results
The `amdgpu` driver should negotiate and output Full RGB range (0-255) by
default on a standard PC monitor over HDMI, producing true deep blacks from
early boot.

### Workarounds & Diagnostic Findings
1. **Desktop Compositor Override:** Manually changing the "RGB Range" property
from "Automatic" to "Full" inside KDE Plasma Display Settings immediately fixes
the blacks once the desktop session starts. This confirms the default automatic
negotiation in `amdgpu` 7.3.x is selecting Limited RGB (16-235) instead of Full
RGB (0-255).
2. **EDID Override (CTA-861 removal):** Stripping the CTA-861 extension block
from the monitor's EDID (reducing it to a 128-byte Base EDID block without
CEA/HDMI extensions) and loading it via `drm.edid_firmware=edid/custom.bin`
forces `amdgpu` to treat the display as a DVI/PC monitor and restores Full RGB
range (true blacks) immediately at boot.

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