Add SteamOS distro

Adds `--distro steamos` as a first-class build target. Pattern is the
same as bazzite-deck/batocera: distros/steamos/build-rootfs.sh fetches
the upstream rootfs (Valve's Steam Deck recovery .img.bz2 from
steamdeck-images.steamos.cloud), extracts the rootfs-A + var-A btrfs
slots into $CHROOT, swaps in our linux-ps5 kernel pkg.tar.zst, and
rebuilds the initramfs with the modules amdgpu needs for the PS5
Oberon GPU.

PS5-specific fixes applied to the SteamOS rootfs:

- Replace /usr/bin/steamos-session-select with a shim that writes
  /etc/sddm.conf.d/zzz-session-override.conf directly + restarts sddm.
  The vendored helper goes through steamosctl / steamos-manager whose
  Deck-hardware code paths silently no-op on PS5, leaving the UI stuck
  at "Switching to Desktop..." with no actual session swap.

- Install a small Python dbus monitor (steam-session-switch-listener)
  that catches org.freedesktop.DisplayManager.Seat.SwitchToUser from
  Big Picture's "Switch to Desktop" button and writes the same
  override file. No -steamos3 / -steampal needed (those trigger Steam's
  A/B update reboot loop on a non-Deck box).

- Drop a Return-to-Gaming-Mode.desktop into /home/deck/Desktop pointing
  at steamos-session-select gamescope.

- chown /home/deck to deck:deck (the recovery image leaves it root:root,
  which breaks ~/.config writability on first Plasma launch).

- Enable sshd + set deck:deck so the box is reachable for further
  iteration without dragging a USB keyboard around.

- Patch /etc/fstab: comment out the by-partsets atomic mount entries
  (those paths don't exist on a flat-rootfs flash), append a LABEL-based
  /efi mount so the FAT boot partition comes up cleanly. Required for
  ps5-iw620-firmware.service to find the WLAN blob.

- Append amdgpu.dpm=0 + amdgpu.gpu_recovery=0 to the kernel cmdline.
  modprobe.d alone isn't enough — amdgpu loads before the rootfs conf
  is read, so the cmdline is the only reliable path. Without these
  Switch-to-Desktop produces a black screen on the PS5 HDMI output.

- Polkit-deny the deck user reboot/poweroff to break Steam's update
  reboot loop on first boot.

- Patch steam-jupiter to skip its OOBE rm -rf path. The OOBE assumes
  Deck factory layout and wipes /home/deck/.local if it runs.

Verified end-to-end on real PS5 (HW-rev EA-1102): autologin lands in
gamescope, Switch-to-Desktop and Return-to-Gaming both work, sshd is
reachable, ps5-iw620 brings up internal WiFi after firmware staging.

Files:
- distros/steamos/build-rootfs.sh  (646 lines)
- distros/steamos/README.md
- distros/steamos/return-to-gaming-mode.desktop
- distros/steamos/steam-session-switch-listener.{py,service}
- build_image.sh: steamos → FORMAT=arch (uses linux-ps5 pkg.tar.zst)
- .github/workflows/build-image.yml: steamos in matrix options + distros list

.github/workflows/build-image.yml

@@ -8,7 +8,7 @@ on:
         description: 'Distribution'
         default: 'ubuntu2604'
         type: choice
-        options: [all, ubuntu2604, arch, cachyos, fedora, proxmox, debian, bazzite, bazzite-deck]
+        options: [all, ubuntu2604, arch, cachyos, fedora, proxmox, debian, bazzite, bazzite-deck, steamos]
   workflow_call:
     inputs:
       distro:
@@ -34,7 +34,7 @@ jobs:
         run: |
           INPUT="${{ inputs.distro || 'ubuntu2604' }}"
           if [ "$INPUT" = "all" ]; then
-            echo 'distros=["ubuntu2604","arch","cachyos","fedora","proxmox","debian","bazzite","bazzite-deck"]' >> "$GITHUB_OUTPUT"
+            echo 'distros=["ubuntu2604","arch","cachyos","fedora","proxmox","debian","bazzite","bazzite-deck","steamos"]' >> "$GITHUB_OUTPUT"
           else
             echo "distros=[\"$INPUT\"]" >> "$GITHUB_OUTPUT"
           fi

build_image.sh

@@ -85,7 +85,7 @@ esac
 
 if [ -z "$FORMAT" ]; then
     case "$DISTRO" in
-        arch|cachyos)        FORMAT="arch" ;;
+        arch|cachyos|steamos) FORMAT="arch" ;;
         fedora|bazzite*)     FORMAT="rpm"  ;;
         all)                 FORMAT="all"  ;;
         *)                   FORMAT="deb"  ;;

distros/steamos/README.md

@@ -0,0 +1,42 @@
+# steamos
+
+SteamOS 3 image variant for PS5, built by extracting the rootfs from
+Valve's official Steam Deck recovery image and swapping in our
+`linux-ps5` kernel.
+
+## Source
+
+- Upstream: <https://steamdeck-images.steamos.cloud/recovery/steamdeck-repair-latest.img.bz2>
+  (Steam Deck recovery image; SteamOS Holo Arch-based.)
+- ~3.4 GB compressed bz2, ~10 GB decompressed.
+- Valve CDN — no per-IP throttle observed; first-time cache pull on a
+  cold runner is ~3-5 min, subsequent runs are instant via
+  `/data/cache/ps5/downloads/`.
+
+## What the build does
+
+1. Resolve `steamdeck-repair-latest.img.bz2` → its dated filename so
+   the cache key is stable.
+2. Decompress → losetup -P.
+3. Mount the first **erofs** partition (rootfs-A; A and B are
+   identical) read-only, `cp -a` to `$CHROOT`.
+4. Mount the first **ext4 < 2 GB** partition that contains `/lib`
+   (var-A), `cp -a` to `$CHROOT/var` — SteamOS splits `/var` off the
+   rootfs and first-boot fails without it.
+5. Extract our `linux-ps5-*.pkg.tar.zst` and merge `usr/`, `boot/`,
+   `etc/` into `$CHROOT`.
+6. Delete SteamOS's `linux-neptune` kernel + modules so grub boots
+   ours.
+7. `depmod -a $KVER`.
+
+The standard image-builder flow then packs `$CHROOT` into an ext4
+image with our EFI partition.
+
+## Caveats
+
+- The Steam UI session expects the Deck's `jupiter` hardware quirks
+  (gamepad, ALS, fan curve daemons) — most of those won't apply on
+  PS5. Falling back to plasma desktop on the first session is normal.
+- SteamOS is read-only by design (steamos-readonly enable). The image
+  we build here is regular ext4; if you want the read-only A/B atomic
+  experience, that needs a separate variant.

distros/steamos/return-to-gaming-mode.desktop

@@ -0,0 +1,8 @@
+[Desktop Entry]
+Type=Application
+Name=Return to Gaming Mode
+Comment=Switch the autologin session back to gamescope / Steam Deck UI
+Exec=bash -c 'steamos-session-select gamescope'
+Icon=steamdeck-gaming-return
+Terminal=false
+Categories=System;

distros/steamos/steam-session-switch-listener.py

@@ -0,0 +1,84 @@
+#!/usr/bin/env python3
+"""Listens on the system bus for Steam Big Picture's Switch-to-Desktop
+dbus call (org.freedesktop.DisplayManager.Seat.SwitchToUser) and performs
+the session swap directly — runs as root so no pkexec / polkit needed.
+
+Flow:
+  1. Steam UI's Power-Menu Switch-to-Desktop fires
+        dbus-send --system --dest=org.freedesktop.DisplayManager \\
+            /org/freedesktop/DisplayManager/Seat0 \\
+            org.freedesktop.DisplayManager.Seat.SwitchToUser \\
+            string:doorstop string:plasma
+  2. This listener catches the call (BecomeMonitor mode on system bus)
+  3. Writes /etc/sddm.conf.d/zzt-steamos-temp-login.conf for plasma
+  4. Runs 'steam -shutdown' as the deck user — gamescope-session ends
+     cleanly because Steam (gamescope's child) exits, DRM master is
+     released the gentle way, sddm bounces and autologs into plasma.
+  5. The zzt- conf is one-shot — sddm's ExecStartPre wipes it on the
+     next sddm restart, so closing plasma drops back to gamescope.
+"""
+import dbus, dbus.mainloop.glib, subprocess, sys, traceback
+from gi.repository import GLib
+
+LOG = "[steam-session-switch-listener]"
+# zzz- sorts AFTER zz-steamos-autologin.conf (so it overrides), AND it
+# doesn't match the ExecStartPre rm pattern that only deletes 'zzt-...'
+# — so the conf survives `systemctl restart sddm` cycles.
+ZZT = "/etc/sddm.conf.d/zzz-session-override.conf"
+
+SESSION_MAP = {
+    "plasma": "plasma.desktop",
+    "plasma-wayland": "plasma.desktop",
+    "plasma-wayland-persistent": "plasma.desktop",
+    "desktop": "plasma.desktop",
+    "plasma-x11": "plasmax11.desktop",
+    "plasma-x11-persistent": "plasmax11.desktop",
+    "gamescope": "gamescope-wayland.desktop",
+}
+
+def handle_switch(session):
+    target = SESSION_MAP.get(session, "gamescope-wayland.desktop")
+    print(f"{LOG} writing zzt- conf -> {target}", flush=True)
+    with open(ZZT, "w") as f:
+        f.write(f"[Autologin]\nUser=deck\nSession={target}\nRelogin=true\n")
+
+    # Restart sddm directly. Since we used zzz-session-override.conf,
+    # the ExecStartPre that only kills zzt-... doesn't wipe it. sddm
+    # restart reads the conf (zzz- sorts after zz-steamos-autologin.conf,
+    # so it wins) and autologs into the requested session.
+    print(f"{LOG} systemctl restart sddm", flush=True)
+    subprocess.Popen(["systemctl", "restart", "sddm"], stdin=subprocess.DEVNULL)
+
+def on_message(_bus, message):
+    try:
+        if message.get_interface() != "org.freedesktop.DisplayManager.Seat":
+            return
+        if message.get_member() != "SwitchToUser":
+            return
+        args = list(message.get_args_list())
+        if len(args) < 2:
+            return
+        user, session = str(args[0]), str(args[1])
+        print(f"{LOG} caught SwitchToUser({user!r}, {session!r})", flush=True)
+        handle_switch(session)
+    except Exception:
+        # NEVER let an exception kill the monitor — log and continue
+        traceback.print_exc()
+
+def main():
+    dbus.mainloop.glib.DBusGMainLoop(set_as_default=True)
+    bus = dbus.SystemBus()
+    bus_obj = bus.get_object("org.freedesktop.DBus", "/org/freedesktop/DBus")
+    monitoring = dbus.Interface(bus_obj, "org.freedesktop.DBus.Monitoring")
+    monitoring.BecomeMonitor(
+        ["type='method_call',"
+         "interface='org.freedesktop.DisplayManager.Seat',"
+         "member='SwitchToUser'"],
+        dbus.UInt32(0),
+    )
+    bus.add_message_filter(on_message)
+    print(f"{LOG} listening on system bus for SwitchToUser", flush=True)
+    GLib.MainLoop().run()
+
+if __name__ == "__main__":
+    sys.exit(main())

distros/steamos/steam-session-switch-listener.service

@@ -0,0 +1,14 @@
+[Unit]
+Description=Translate Steam Big Picture Switch-to-Desktop dbus call into steamos-session-select
+Documentation=https://gist.github.com/Rishikant181/e26fb23d4c57db74bddaa0a57b26cd26
+After=dbus.service sddm.service
+PartOf=sddm.service
+
+[Service]
+Type=simple
+ExecStart=/usr/local/sbin/steam-session-switch-listener
+Restart=on-failure
+RestartSec=2
+
+[Install]
+WantedBy=multi-user.target