feat(host/steam): default the gadget Deck on for SteamOS (glass-confirmed)

The virtual Steam Deck is validated glass-to-glass on a Deck: it appears as a
distinct second Steam controller, a held A drives Steam's overlay ("Resume
Game"), and a button press registers in a real game (confirmed in-game).

gadget_preferred() now defaults ON for SteamOS hosts (/etc/os-release ID=steamos
or ID_LIKE), OFF elsewhere where the universal UHID path stays the default;
PUNKTFUNK_STEAM_GADGET=1/0 forces it. A Deck-as-host with a physical Deck never
reaches this path — resolve_gamepad's conflict gate degrades SteamDeck → DualSense
first, so the two-Deck case never happens in production (it was only a test-rig
confound on the dev Deck).

The feature is complete: a virtual Steam Deck that Steam Input recognizes +
promotes, churn-free, with input flowing to games. Workspace clippy/fmt/test
green. Not pushed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/inject/linux/steam_controller.rs

@@ -384,8 +384,8 @@ impl SteamControllerManager {
         if idx >= MAX_PADS || self.pads[idx].is_some() || self.broken {
             return;
         }
-        // Prefer the USB gadget on SteamOS (the only transport Steam Input promotes); fall back to the
+        // Prefer the USB gadget on SteamOS (default there — the only transport Steam Input promotes);
-        // universal UHID pad if the gadget is unavailable or not opted in.
+        // fall back to the universal UHID pad if the gadget is unavailable or disabled.
         let opened = if crate::inject::steam_gadget::gadget_preferred() {
             crate::inject::steam_gadget::ensure_modules();
             match crate::inject::steam_gadget::SteamDeckGadget::open(idx as u8) {

crates/punktfunk-host/src/inject/linux/steam_gadget.rs

@@ -606,11 +606,24 @@ pub fn ensure_modules() {
     }
 }
 
-/// Whether to prefer the USB-gadget Deck over the UHID `SteamDeckPad`. SteamOS-host only (needs the
+/// Whether to prefer the USB-gadget Deck over the UHID `SteamDeckPad` — the only transport Steam Input
-/// gadget modules + root) and opt-in for now (`PUNKTFUNK_STEAM_GADGET=1`) while the full Steam-Input
+/// promotes (validated glass-to-glass on a Deck). Defaults **on for SteamOS** hosts (which ship the
-/// feature contract is hardened; defaults off, so the universal UHID path stays the default.
+/// gadget modules + run Steam Input); off elsewhere, where the universal UHID path stays the default.
+/// `PUNKTFUNK_STEAM_GADGET=1`/`0` forces it on/off. A Deck-as-host with a *physical* Deck never reaches
+/// here: `resolve_gamepad`'s conflict gate degrades `SteamDeck` → DualSense before the manager is built.
 pub fn gadget_preferred() -> bool {
-    std::env::var("PUNKTFUNK_STEAM_GADGET")
+    if let Ok(v) = std::env::var("PUNKTFUNK_STEAM_GADGET") {
-        .map(|v| v == "1" || v.eq_ignore_ascii_case("true"))
+        return v == "1" || v.eq_ignore_ascii_case("true");
+    }
+    is_steamos()
+}
+
+/// True on SteamOS-class hosts (`/etc/os-release` `ID=steamos`, or `ID_LIKE` naming it).
+fn is_steamos() -> bool {
+    std::fs::read_to_string("/etc/os-release")
+        .map(|s| {
+            s.lines()
+                .any(|l| l == "ID=steamos" || (l.starts_with("ID_LIKE=") && l.contains("steamos")))
+        })
         .unwrap_or(false)
 }

design/steam-controller-deck-support.md

@@ -822,3 +822,15 @@ confirms our state reports (pressed button at byte 8) are delivered on the inter
 by hid-steam — so with M1/M2's byte-8→BTN_SOUTH decode, the input chain is proven end-to-end. The
 only piece left is a foreground-game confirmation that Steam Input maps it onto the X-Box pad (Steam
 only maps contextually), after which the gadget can default on for SteamOS hosts.
+
+### Glass confirmed + default-on for SteamOS (2026-06-29)
+
+Validated glass-to-glass on the Deck: the gadget shows up as a distinct second Steam controller, a
+held A snaps the Steam overlay shut as "Resume Game" (so Steam Input receives + acts on the gadget's
+input), and **a button press registers in a real game** — confirmed in-game. The two-Deck test
+confound (the Deck has its physical Deck + the virtual one) is a test-rig artifact, not a feature
+limit: a real non-Deck SteamOS host has only the virtual Deck, and a Deck-as-host degrades `SteamDeck`
+→ DualSense via the M6 conflict gate before the gadget is ever built. So `gadget_preferred()` now
+defaults **on for SteamOS** (`/etc/os-release` `ID=steamos`), off elsewhere (UHID stays default),
+with `PUNKTFUNK_STEAM_GADGET=1`/`0` to force. The virtual Steam Deck — recognized + promoted by Steam
+Input, churn-free, input flowing to games — is complete.

packaging/linux/steam-deck-gadget/README.md

@@ -56,14 +56,17 @@ Steam Input, which exposes its own X-Box 360 pad — exactly a real Deck's behav
   `EP_WRITE` starves the control path.
 - `dummy_hcd` + `raw_gadget` must both be loaded and `/dev/raw-gadget` present before launch.
 
-## Host backend (shipped, opt-in)
+## Host backend (shipped — default on for SteamOS)
 
 The C PoC's transport is ported to a Rust host gamepad backend:
 `crates/punktfunk-host/src/inject/linux/steam_gadget.rs` (`SteamDeckGadget`), driven by the same
 `steam_proto` serializer as the UHID `SteamDeckPad`. The Steam-Deck manager
-(`inject/linux/steam_controller.rs`) now selects per-pad between **UHID** (default, universal) and the
+(`inject/linux/steam_controller.rs`) selects per-pad between **UHID** (universal) and the **USB
-**USB gadget** (`PUNKTFUNK_STEAM_GADGET=1`, SteamOS-only — best-effort `modprobe dummy_hcd raw_gadget`,
+gadget**: the gadget is the **default on SteamOS hosts** (`gadget_preferred()` → `ID=steamos`;
-graceful fallback to UHID if `/dev/raw-gadget` is unusable).
+best-effort `modprobe dummy_hcd raw_gadget`, graceful fallback to UHID if `/dev/raw-gadget` is
+unusable), and off elsewhere where UHID stays the default. `PUNKTFUNK_STEAM_GADGET=1`/`0` forces it.
+A Deck-as-host with a *physical* Deck never uses it — `resolve_gamepad`'s conflict gate degrades
+`SteamDeck` → DualSense first.
 
 The Rust transport is **validated on the Deck** (a static musl test binary that `#[path]`-includes the
 real module): it enumerates the 3-interface Deck, hid-steam binds it + reads our serial + creates the