[BUG] linux-cachyos 7.2.0: silent loss of HDMI signal on RX 6800 XT + Samsung 4K TV at boot (LTS works)

# [BUG] linux-cachyos 7.2.0: silent loss of HDMI signal on RX 6800 XT + Samsung 4K TV at boot (LTS works)

## Summary

After upgrading `linux-cachyos` **7.1.8-1 → 7.2.0-1**, the machine boots (BIOS/GRUB visible on TV) but the TV loses signal entirely ("No Signal") when the kernel takes over the display. **No drm/amdgpu errors appear anywhere in the journal** — it is a completely silent signal loss. The system itself keeps running (NetworkManager continues normal operation until power-cycle).

Booting `linux-cachyos-lts` **6.18.42-1** with identical userspace and identical kernel command line works perfectly.

Three consecutive attempts on 7.2.0 reproduced it every time.

## Hardware

| Component | Detail |
|---|---|
| GPU | AMD Radeon RX 6800 XT (Navi 21 / SIENNA_CICHLID, `1002:73BF` sub `1EAE:6705`) |
| Display | SAMSUNG 4K TV, 3840x2160, connected via **HDMI-A-1** |
| CPU | AMD Ryzen 7 5700X3D |
| Board | SATA-less AM4 platform |

## Software

- `linux-cachyos` 7.2.0-1 (broken) / `linux-cachyos-lts` 6.18.42-1 (working)
- linux-firmware 1:20260810-2
- mesa 3:26.2.1-1, vulkan-radeon 3:26.2.1-1 (identical across both kernels)
- KDE Plasma Wayland (kwin_wayland 6.7.4), Plymouth, plasmalogin
- Kernel command line (same both kernels):
`nowatchdog nvme_load=YES splash loglevel=3 amdgpu.sg_display=0`

## How to reproduce

1. On this hardware, boot `linux-cachyos` 7.2.0-1.
2. Plymouth/early boot may or may not show briefly; once KMS handover completes, TV reports **No Signal**.
3. Machine is otherwise alive — logs continue, clean shutdown possible via short power-button press.
4. Boot `linux-cachyos-lts` from GRUB advanced menu → display works normally, every time.

## Timeline of the failing session (2026-08-21)

```
14:07:07 pacman -Syu: linux-cachyos 7.1.8-1 -> 7.2.0-1 (+44 other pkgs, mostly Qt6/KDE)
14:09:43 reboot #1 begins
14:09:47 amdgpu init OK: SIENNA_CICHLID detected, VBIOS fetched, no errors
14:09:49 DMUB firmware loaded via PSP: version=0x02020022
14:09:49 [drm] DP-HDMI FRL PCON supported
14:09:55 kwin_wayland starts, chooses drm backend
--> TV shows "No Signal" from this point; journal stays error-free
14:12:50 NetworkManager still operating normally when user power-cycled
(reboots #2 and #3 same result; recovered by selecting LTS in GRUB)
```

Notable: amdgpu initializes cleanly, DMUB loads, connector enumeration shows
no failures — there are simply no diagnostics at all after modeset handover.

## Expected vs actual

- **Expected**: desktop appears on TV over HDMI as on 7.1.8-1 and 6.18.42-1
- **Actual**: persistent no-signal after KMS handover on 7.2.0-1, silently

## Additional notes

- `amdgpu.sg_display=0` was already set on the command line during all tests.
- The regression window is strictly 7.1.8 → 7.2.0 (7.1.8 displayed fine immediately before the update).
- Suspect areas given recent upstream history: HDMI clock bump (`fee50077656`) fallout and/or the HDMI DDC/SCDC read-write reworks landing in stable — happy to test patched kernels or bisect if pointed at candidate commits.
- Willing to capture `drm.debug=0x3e` logs from a failing boot on request.

---

*Reporter note: full journals of both failing and working boots retained.*

Start by comparing the retained failing and working boot journals, then capture a failing boot with drm.debug=0x3e as requested. Review the HDMI clock bump (fee50077656) and HDMI DDC/SCDC rework candidates against the 7.1.8-to-7.2.0 kernel changes. Done means the RX 6800 XT produces a stable HDMI signal on 7.2.0 while the system remains otherwise unchanged.

Written by the indexing model from the issue text.