DroidDeck/docs/development/game-environment.md

Steam game environment

Open Games → Game environment, directly below FEX preset, in Steam settings or the in-session drawer. Choose all Proton games, an installed game, or enter a Steam app ID. For a non-Steam shortcut, use its numeric compatdata directory ID (unsigned 32-bit). Changes apply on the next game launch, including when Steam is already running. A running game must be restarted. Native Linux games and Steam itself are outside this editor's scope.

The order is inherited process environment, selected FEX preset and built-in settings, shared edits, then game-specific edits. Editor entries therefore win over matching Steam launch-option variables. Remove explicitly unsets a variable. Restore inherited settings removes the override. Reset this profile clears only that profile's overrides. Values are literal strings, with no shell expansion; quotes are only needed when the consuming program expects them.

Defaults and available suggestions

The initial configuration enables Mesa shader caching with MESA_SHADER_CACHE_DISABLE=false, sets VKD3D_FEATURE_LEVEL=12_2 and VKD3D_SHADER_MODEL=6_9, and preserves the selected FEX preset. Existing edits and explicit removals take precedence over these defaults. FEX's own default remains the default preset. Shader caching in VKD3D/DXVK, synchronization, ray tracing and diagnostic logging otherwise retain the runtime's defaults.

The variable-name picker offers 24 predefined variables and a Custom entry. Like WinNative, known variables use toggles, value dropdowns, multi-select lists, or numeric/text fields. Feature-level and shader-model dropdowns also accept custom values. Custom variables have editable names and literal values.

Variable	Use
VKD3D_FEATURE_LEVEL	D3D12 capability override; defaults to 12_2.
VKD3D_SHADER_MODEL	Shader-model override; defaults to 6_9.
VKD3D_CONFIG	Per-game workarounds such as nodxr; not a universal performance preset.
MESA_SHADER_CACHE_MAX_SIZE	Storage budget for Mesa's shader cache, for example 1G.
mesa_glthread	OpenGL threading; test with the affected game.
DXVK_CONFIG	DXVK configuration options; recent builds accept dxvk.maxFrameRate = 60.
VKD3D_FRAME_RATE	D3D12 frame-rate limit.
DXVK_HUD, PROTON_LOG	Optional diagnostics; leave unset for ordinary play.
PROTON_USE_WINED3D	D3D9–11 OpenGL fallback for compatibility testing.
PROTON_USE_XALIA	Proton's gamepad-navigation helper toggle.

Forcing 12_2 or 6_9 changes reported capabilities; it cannot implement a missing Vulkan feature. Remove either entry to use automatic detection for that capability. Other picker suggestions are editable starting values and are enabled only when added and saved. Older or custom components can support a different set of options. Android Wine wrapper, ALSA-server, Box64 and patched async-DXVK options are identified in the editor as requiring a different/custom component; custom entries remain allowed. The old DXVK_FRAME_RATE environment variable was removed from current DXVK, so it is not offered as a suggestion. Prefer the session frame limiter for ordinary play.

Launch path and references

WinNative's Linux session filters Android-only options, merges user variables over the FEX preset, and passes the result to env -i before starting Steam. Its Windows/Wine graphics helper also maps the selected feature level to VKD3D_FEATURE_LEVEL. Reviewed source: WinNative Linux session, variable editor, graphics configuration.

DroidDeck instead publishes an atomic JSON snapshot at /root/.config/droiddeck/game-environment.json. Both its Valve ARM64 Proton wrapper and adopted third-party Proton wrappers execute bannerlator-game-env, which reads that snapshot for each real game launch and uses execvpe to start Proton. Probe prefix compatdata/0 and non-launch verbs are unchanged. Malformed configuration falls back to the inherited environment without evaluating its contents. Signed non-Steam prefix IDs are normalized to unsigned IDs.

Upstream references: VKD3D capability parsing, VKD3D options, Proton runtime options, Mesa variables, DXVK variables, DXVK frame-limiter changes.

Validation: ./gradlew testDebugUnitTest and python3 -m unittest discover -s tools/tests -p test_game_environment.py. Actual game compatibility depends on the installed Proton, VKD3D and Vulkan driver; these checks do not establish that every device supports feature level 12_2 or SM 6_9.