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NeilClack
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# DRM/KMS Direct Mode — Deferred to Post-Launch
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**Status:** Deferred (too much risk before launch)
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**Created:** 2026-08-17
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---
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## Goal
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Remove Cage (Wayland compositor) from -dev kiosk variants and run RadiantWave
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directly on DRM/KMS via SDL3's built-in `SDL_VIDEODRIVER=drm` backend. This
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simplifies the stack, removes compositor overhead, and is better suited for
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embedded/hardware kiosks.
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**Decision:** Deferring until after the current launch. Cage works, and the
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transition touches every variant's boot pipeline.
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---
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## What Cage Currently Provides
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| Feature | How Cage Does It | DRM/KMS Replacement |
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|---------|------------------|---------------------|
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| Display mode setting | Opens `/dev/dri/card*` via libseat, selects connector/CRTC/mode | SDL3 DRM backend opens DRM devices directly |
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| Fullscreen surface | Creates Wayland surface, manages lifecycle | SDL3 creates DRM framebuffer surface |
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| Page flipping / VSync | Handles `drmHandlePageFlip()` | SDL3 DRM backend handles page flip |
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| Input devices | Opens `/dev/input/*` via libseat, generates Wayland events | SDL3 DRM backend reads evdev directly |
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| Keyboard mapping | XKB keymap loading, key repeat | SDL3 handles keymap translation |
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| Seat management | Connects to seatd via libseat | Not needed — group permissions (`video`, `input`) suffice |
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---
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## What Stays Safe Without Cage
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| Concern | Why It's Safe |
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|---------|---------------|
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| **Audio** | PipeWire/ALSA works independently of the compositor. App uses SDL3_mixer -> ALSA. Audio init service still runs before launch. |
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| **WiFi** | NetworkManager uses system bus (`sd_bus_open_system()` in `network_manager.cpp`). No session bus needed. `networkmanager` group handles D-Bus access. |
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| **Keyboard/input** | SDL3's DRM backend reads `/dev/input/*` evdev directly. `input` group grants access. |
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| **DRM/display** | SDL3's DRM backend opens `/dev/dri/card*` directly. `video` group grants access. |
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| **Plymouth** | `sudo -n plymouth quit --wait` still works — Plymouth runs as root on DRM/KMS. |
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| **Auto-login** | getty autologin still works on tty1. |
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| **Health check** | `pgrep -f '.radiantwave-unwrapped'` still works — no wrapper changes. |
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| **Backup service** | Runs as kiosk user, rsyncs `~/.local/radiantwave`. No compositor dependency. |
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---
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## Implementation Plan — 4 Phases
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All changes are in **kioskconfig only**. The radiantwave app needs no code
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changes — SDL3 handles DRM/KMS internally when `SDL_VIDEODRIVER=drm` is set.
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The NixOS system already provides Mesa/EGL/GBM via `hardware.graphics.enable = true`.
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Only **-dev** variants are modified. Production configs stay on Cage.
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### New module: `kiosk-drm.nix`
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A NixOS module that overrides the Wayland/cage setup:
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- `environment.systemPackages = mkForce [...]` — same list as `configuration.nix`
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but **without `pkgs.cage`**
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- `environment.loginShellInit = mkForce ''...` — same script but:
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- Sets `SDL_VIDEODRIVER=drm` instead of `wayland`
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- Removes `WLR_RENDERER`, `WLR_NO_HARDWARE_CURSORS`, `QT_QPA_PLATFORM`,
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`XDG_SESSION_TYPE`, `GODEBUG`
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- Launches `exec /run/current-system/sw/bin/radiantwave` directly
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(no `dbus-run-session cage --`)
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- Keeps: network wait, audio init, Plymouth quit
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- `services.seatd.enable = mkForce false` — nothing uses libseat anymore
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---
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### Phase 1: `radiantwavekiosk-dev` (Intel NUC)
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**Files:**
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- Create `kiosk-drm.nix`
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- Modify `flake.nix`: add `./kiosk-drm.nix` to `extraModules` for
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`radiantwavekiosk-dev`
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- `dev-iso.nix` already targets `radiantwavekiosk-dev` — no change needed
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**Verify:** `nix build .#nixosConfigurations.radiantwavekiosk-dev.config.system.build.toplevel`
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---
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### Phase 2: `radiantwavekiosk-commercial-dev` (commercial HW)
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**Files:**
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- Modify `flake.nix`: add `./kiosk-drm.nix` to `extraModules` for
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`radiantwavekiosk-commercial-dev`
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- Create `commercial-dev-iso.nix` (same pattern as `dev-iso.nix`, targeting
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`radiantwavekiosk-commercial-dev`)
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- Add `commercial-dev-iso` config to `flake.nix`
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**Verify:** `nix build .#nixosConfigurations.radiantwavekiosk-commercial-dev.config.system.build.toplevel`
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---
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### Phase 3: `radiantwavekiosk-android-dev` (rotated display)
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**Files:**
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- Create `radiantwavekiosk-android-dev` in `flake.nix` (currently only
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production `radiantwavekiosk-android` exists)
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- Create `android-kiosk-drm.nix` — DRM/KMS mode + display rotation
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**Challenge:** `rotated_display.nix` uses `wlr-randr` (Wayland tool) for
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90-degree rotation. Without Cage/Wayland, this won't work. Alternatives:
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- Option A: Use `modetest` from `libdrm` to set DRM rotation property
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- Option B: Handle rotation in the app (query display orientation, render rotated)
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- Option C: Kernel DRM rotation via modprobe params or udev rule
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This is the hardest phase — needs investigation.
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**Verify:** `nix build .#nixosConfigurations.radiantwavekiosk-android-dev.config.system.build.toplevel`
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---
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### Phase 4: `radiantwavekiosk-rpi-dev` (RPi 5)
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**Files:**
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- Modify `flake.nix`: add `./kiosk-drm.nix` to `extraModules` for
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`radiantwavekiosk-rpi-dev`
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- Modify `build-rpi.sh`: update to build `radiantwavekiosk-rpi-dev` instead of
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`radiantwavekiosk-rpi`
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RPi 5 uses VC4/V3D GPU driver — `hardware.graphics.enable = true` +
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`nixos-raspberrypi.nixosModules.raspberry-pi-5.display-vc4` already handles this.
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**Verify:** `./build-rpi.sh` (builds SD card image)
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---
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## Risks and Unknowns
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1. **SDL3 DRM backend maturity** — Less commonly used than the Wayland backend.
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Needs testing on NUC (Intel i915) and RPi 5 (VC4/V3D) hardware.
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2. **Android rotation** — `wlr-randr` won't work without Wayland. Needs an
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alternative rotation strategy.
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3. **Input device permissions** — SDL3's DRM backend reads `/dev/input/*`
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directly. Need to verify `input` group is sufficient and udev rules don't
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block access.
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4. **Multi-GPU/multi-output** — DRM/KMS needs to select the right connector.
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SDL3 handles this, but edge cases exist (e.g., multiple monitors).
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5. **Plymouth -> app handoff** — Plymouth runs on DRM/KMS too. The handoff
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should work, but timing might differ without a Wayland compositor in between.
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---
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## radiantwave Repo — No Changes Needed
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The app doesn't directly use DRM/KMS APIs. SDL3 handles the DRM backend
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internally when `SDL_VIDEODRIVER=drm` is set. The Nix package doesn't need
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changes — SDL3 dynamically loads DRM libraries at runtime.
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The only change is the environment variable in the kiosk startup script
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(`SDL_VIDEODRIVER=drm` vs `wayland`), which lives in kioskconfig.
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