### System details

iMac12,2 (27" Mid 2011), Intel i5-2500S, Intel HD 3000 (i915) + AMD Radeon HD 6770M (legacy `radeon` driver, no Vulkan), Omarchy 4.0.4-1, kernel 7.2.5-3-omarchy, Hyprland 0.56.2, RetroArch 1.22.2, Mesa 26.2.2

### What's wrong?

`omarchy install gaming retroarch` leaves RetroArch unable to start on this machine. Launching it from the app launcher does nothing at all, and from a terminal it exits 1 and prints nothing, because the error only appears under `--verbose`:

```
[INFO] [Input] Found input driver: "x".
[INFO] [Video] Set video size to: 960x720.
[INFO] [Vulkan] Vulkan dynamic library loaded.
[ERROR] [Vulkan] Failed to enumerate physical devices.
[ERROR] [Vulkan] Failed to set video mode.
[ERROR] [Video] Cannot open video driver. Exiting...
[ERROR] Fatal error received in: "video_driver_init_internal()"
```

The cause is `bin/omarchy-install-gaming-retroarch:64`:

```bash
# Vulkan is required for slang shaders and unlocks hardware renderers in beetle-psx-hw, parallel-n64, dolphin
set_cfg video_driver "vulkan"
```

Neither GPU in this iMac can provide a Vulkan device, so `vkEnumeratePhysicalDevices` returns zero and RetroArch treats it as fatal:

- **Radeon HD 6770M** (`AMD TURKS`, TeraScale 2) is driven by the legacy `radeon` driver. RADV only supports GCN and newer.
- **Intel HD 3000** is Sandy Bridge / Gen6. Mesa's ANV requires Gen7 (Ivy Bridge) or newer.

What makes this silent rather than obvious is that `install/hardware/vulkan.sh` matches `lspci` on vendor strings alone, so it installs `vulkan-radeon` and `vulkan-intel` here. Both ICDs are present in `/usr/share/vulkan/icd.d/`, so Vulkan looks available while enumerating zero devices:

```
$ ls /usr/share/vulkan/icd.d/
intel_hasvk_icd.json  intel_icd.json  radeon_icd.json

$ for c in /sys/class/drm/card*; do echo "$(basename $c) -> $(basename $(readlink $c/device/driver))"; done
card0 -> radeon
card1 -> i915
```

**Steps to reproduce**

1. Install Omarchy on an iMac12,2 (HD 3000 + Radeon HD 6770M on the legacy `radeon` driver).
2. Run `omarchy install gaming retroarch`.
3. Launch RetroArch. Nothing appears. `retroarch` in a terminal exits 1 silently; `retroarch --verbose` shows the Vulkan error above.

**What I tried**

| `video_driver` | Result |
|---|---|
| `vulkan` (Omarchy default) | **Fatal.** Exits 1 before opening a window |
| `gl` | Starts and runs, but uses the GLSL backend, so the installer's slang `crt-royale` preset does not apply |
| `glcore` | **Fixed.** Starts, runs, and keeps the Slang shader backend |

Under `glcore` the GL context comes up clean on the Radeon and the slang backend is retained:

```
[INFO] [GLCore] Vendor: Mesa, Renderer: AMD TURKS (DRM 2.51.0 / 7.2.5-3-omarchy).
[INFO] [GLCore] Using Slang shader backend.
[INFO] [Display] Found display driver: "glcore".
```

It then reaches the main loop and stays there, idling in `poll_schedule_timeout` at 5-8% of one core across repeated samples, with no `[ERROR]` lines in the log.

Workaround:

```bash
sed -i 's/^video_driver = .*/video_driver = "glcore"/' ~/.config/retroarch/retroarch.cfg
```

**Related**

- #13760: Chromium renders with artifacts on this same iMac12,2 for the same reason — a no-Vulkan legacy `radeon` GPU.
- #11370: Voxtype enables Vulkan on Haswell `hasvk` GPUs and crash-loops. Same root pattern of assuming a Vulkan device exists.
- #13727: Vulkan detection matches "Non-VGA" Apple T2 devices and installs `vulkan-asahi`. Also `install/hardware/vulkan.sh` inferring Vulkan support from `lspci` vendor strings.
- #12622: GTK4/Nautilus exits at startup where the default Vulkan renderer has no usable device.
- #3581 / PR #3711: Ghostty crashed on legacy GPUs and was fixed by install-time legacy-GPU detection falling back to Alacritty. That is the precedent this could follow.

**Possible fix**

Have the RetroArch installer fall back to `glcore` when no Vulkan device is actually present, rather than hardcoding `vulkan`:

```bash
if vulkan_device_available; then
  set_cfg video_driver "vulkan"
else
  set_cfg video_driver "glcore"
fi
```

`glcore` is the right fallback rather than `gl`, because the installer also writes a slang preset:

```
~/.config/retroarch/config/global.slangp -> crt-royale.slangp
```

Only `glcore` and `vulkan` consume slang shaders; `gl` uses GLSL and would silently drop `crt-royale`. So `glcore` keeps the installer's intended CRT look and only gives up the Vulkan-only hardware renderers (`beetle-psx-hw`, `parallel-n64`, `dolphin`), which this GPU could not drive anyway.

For detection, enumerating devices is more reliable than matching `lspci` vendor strings, since the failure here is specifically "ICD installed, zero devices". `vulkaninfo --summary` from `vulkan-tools` would work, though it is not installed by default; probing for a non-empty device list in the installer would avoid adding a dependency. The same check would also help #13727 and #11370.

I have not verified how `crt-royale` performs once content is loaded on this GPU — I have no ROMs on this machine, and the preset only applies on content load, not in the menu. It is a heavy shader, so it may well be too slow here even though the backend is correct. That is a separate performance question from this launch failure.

Filed by Claude Opus 5 via Claude Code.