[BUG] linux-cachyos 7.2.0 + mesa 26.2.1: Strix Halo HDMI black after login, kwin_wayland_drm Failed to create framebuffer (EINVAL); LTS does not help

### Pre-flight checklist

- [x] I have searched existing issues and this is not a duplicate.
- [x] I have read the Contributing Guidelines.
- [ ] I have verified the issue is reproducible with the **latest** available CachyOS kernel (`7.2.2-1` was **not** re-tested alone; see below).
- [ ] I have **not** tried Arch Linux's `linux` kernel.

### Upstream / vanilla kernel check

I have not tested with a vanilla/upstream kernel.

### Kernel variant

`linux-cachyos-bore` (also had `linux-cachyos` installed). znver4 / `x86_64_v4`.

This is **not** a duplicate of:

- #959 — `Pageflip timed out` on **7.1.6**; LTS restored a desktop.
- #992 — silent HDMI “No Signal” at boot on **RX 6800 XT** / Navi 21; LTS works; no framebuffer error.

Here: SDDM greeter works, **LTS does not restore a desktop**, distinctive log is framebuffer `EINVAL`, AMD **Strix Halo** iGPU over HDMI.

## System information (`cachyos-bugreport.sh`)

Not attached from a **failing** boot. After the 2026-08-21 regression the journals of the black-screen boots rotated; the machine is currently on the working hold below, so a fresh `cachyos-bugreport.sh` would show the **good** stack, not the failure. Write-up of the failing combo: https://paste.cachyos.org/p/7ec675f.md

Forum thread: https://discuss.cachyos.org/t/strix-halo-hdmi-black-after-login-kwin-failed-to-create-framebuffer/35155

## Manual system information

- CPU: AMD Ryzen AI MAX+ 395
- GPU: Radeon 8060S (`strix_halo` / `gfx1151`, PCI `1002:1586`), amdgpu, DCN 3.5
- Display: single **HDMI-A-1**, TCL SMART TV 1920x1080. No NVIDIA, no hybrid graphics.
- Mini PC: Bosgame BeyondMax (M5), board Sixunited AXB35-02
- DE: KDE Plasma 6 Wayland (Hyprland also installed via SDDM)
- Working kernel: `7.1.8-1-cachyos-bore`

## Bug description

On 2026-08-21 a full `pacman -Syu` left **SDDM working**, but **Plasma and Hyprland both went black immediately after login**. TTY (`Ctrl+Alt+F3`) still worked.

This looks like a **userspace DRM/Mesa + KWin + kernel** combo, not “kernel-only”. Mesa **26.2.0** had already been running here on **7.1.8** since ~15 Aug. The break was the jump to **mesa 26.2.1 + linux 7.2.0 + kwin 6.7.4-7.1 + Qt 6.11.2**.

## Steps to reproduce

1. On this Strix Halo HDMI box, install the 2026-08-21 combo: `linux-cachyos` / `linux-cachyos-bore` **7.2.0-1**, `mesa`/`vulkan-radeon` **3:26.2.1-1**, `kwin` **6.7.4-7.1**, `plasma-workspace` **6.7.4-3.1**, `qt6-base` **6.11.2**.
2. Reboot, log in at SDDM (Plasma Wayland or Hyprland).
3. Session goes black. TTY still works.

I have **not** re-tested `7.2.2-1` alone on Mesa 26.2.0. Repos as of 2026-09-02 still offered mesa **26.2.1**, kwin **6.7.4-7.1**, plasma-workspace **6.7.4-3.1**, qt6-base **6.11.2-3**. Partial downgrade (kernel/Mesa/KWin only, leaving Qt 6.11.2) restored a session but broke Plasma chrome (`Qt_6_PRIVATE_API`). I am not willing to `-Syu` the graphics stack again on this HDMI box until those userspace versions are superseded.

## Expected behavior

Wayland session starts after SDDM login, as it did on kernel 7.1.8 + mesa 26.2.0 + kwin 6.7.4-5.1 + qt6-base 6.11.1.

## Actual behavior

Black screen after login. Both Plasma and Hyprland fail. LTS kernel with the new Mesa/KWin userspace did **not** restore a desktop.

## Logs / stack traces

Locale es_ES:

```
kwin_wayland_drm: Failed to create framebuffer: Argumento inválido
```

(`Invalid argument` / `EINVAL`)

`amdgpu … optc35_disable_crtc REG_WAIT` also appears on **working** boots (known DCN 3.5 HDMI quirk) and was **not** the differentiator.

## Additional system information

### Broken vs last known good

| Package | Broken | Last known good here |
|---|---|---|
| linux-cachyos-bore / linux-cachyos | 7.2.0-1 | 7.1.8-1 |
| mesa / vulkan-radeon (and lib32) | 3:26.2.1-1 | 3:26.2.0-1 |
| kwin | 6.7.4-7.1 | 6.7.4-5.1 |
| plasma-workspace | 6.7.4-3.1 | 6.7.4-1.1 |
| qt6-base (and rest of Qt 6.11.2) | 6.11.2 | 6.11.1-1 |

Happy to capture a fresh `cachyos-bugreport.sh` if maintainers want me to temporarily install the broken set again from a TTY with a known-good fallback.

Start with the failing and last-known-good package matrix in the issue, then read the linked paste and forum thread alongside the `kwin_wayland_drm` framebuffer error. Reproduce on the Strix Halo HDMI setup if possible, comparing the listed kernel, Mesa, KWin, Plasma, and Qt versions. Done means identifying a reproducible regression boundary and documenting evidence or a confirmed fix.

Written by the indexing model from the issue text.