gamescope: explicit sync off where the kernel or the stand-in node cannot signal its timelines

Adds tools/gamescope/patches/0126 (GAMESCOPE_DISABLE_EXPLICIT_SYNC=1 for the nested backends) and
sets it on a GPU that is not an Adreno when the kernel is older than 6.6 (no
DRM_IOCTL_SYNCOBJ_EVENTFD) or the guest's render node is Mali's stand-in. The graphics report
lists it with the rest of the guest's graphics environment.

app/src/main/java/com/droiddeck/launcher/session/GraphicsReport.kt

@@ -35,7 +35,7 @@ object GraphicsReport {
     /** The guest keys the report repeats: everything that decides which Vulkan driver loads. */
     private val ENV_PREFIXES = listOf(
         "VK_", "BL_VK", "BL_DRM", "BL_VKBRIDGE", "VKBRIDGE", "MESA_", "GALLIUM_", "ZINK", "TU_", "PANVK",
-        "DROIDDECK_POWERVR", "DROIDDECK_HYBRIS", "BL_GAME_LOGS", "LIBGL_",
+        "DROIDDECK_POWERVR", "DROIDDECK_HYBRIS", "BL_GAME_LOGS", "LIBGL_", "GAMESCOPE_DISABLE", "XWAYLAND",
     )
 
     /** Per-game graphics logs and the guest's Vulkan summary: on for a non-Adreno GPU, or by Setup's switch. */

app/src/main/java/com/droiddeck/launcher/session/SessionService.kt

@@ -307,6 +307,13 @@ class SessionService : Service() {
         return lines
     }
 
+    /** Whether a kernel release ("6.1.157-android14-11") is older than [major].[minor]; false if unreadable. */
+    internal fun kernelBefore(release: String, major: Int, minor: Int): Boolean {
+        val m = Regex("""^(\d+)\.(\d+)""").find(release) ?: return false
+        val (a, b) = m.destructured
+        return a.toInt() < major || (a.toInt() == major && b.toInt() < minor)
+    }
+
     /** Whether the newest finished session's log shows Xwayland itself crashing. */
     private fun lastXwaylandCrashed(): Boolean = runCatching {
         val last = SessionPaths.sessionFolders(this).filter { it != SessionPaths.current() }.lastOrNull() ?: return false
@@ -663,6 +670,20 @@ class SessionService : Service() {
             guest.add("XWAYLAND_NO_GLAMOR=1")
             Log.w(TAG, "xwayland: the last session's Xwayland crashed; glamor off for this one (XWAYLAND_NO_GLAMOR=1)")
         }
+        // gamescope's explicit sync waits on DRM syncobj timelines through
+        // DRM_IOCTL_SYNCOBJ_EVENTFD, which a kernel before 6.6 does not have (Exynos 2400e on 6.1:
+        // hundreds of "timeline ... failed" and a game stuck at black), and Mali's stand-in node
+        // (/dev/mali0) has no syncobjs at all. There clients use implicit sync instead
+        // (tools/gamescope/patches/0126). Never on an Adreno.
+        if (!com.droiddeck.launcher.core.DeviceSupport.adreno()) {
+            val kernel = System.getProperty("os.version").orEmpty()
+            val old = kernelBefore(kernel, 6, 6)
+            val maliNode = LinuxRuntime.gpuStandIn(this) == LinuxRuntime.MALI_DEVICE
+            if (old || maliNode) {
+                guest.add("GAMESCOPE_DISABLE_EXPLICIT_SYNC=1")
+                Log.i(TAG, "gamescope: explicit sync off (" + (if (old) "kernel $kernel has no SYNCOBJ_EVENTFD" else "Mali stand-in node has no syncobjs") + ")")
+            }
+        }
         if (!bridge && com.droiddeck.launcher.core.DeviceSupport.openDriverActive(this)) {
             LinuxRuntime.gpuStandIn(this)?.takeIf { it == LinuxRuntime.MALI_DEVICE }?.let { guest.add("BL_DRM_STANDIN=$it") }
         }

tools/gamescope/PATCHES.md

@@ -62,3 +62,9 @@ The system PowerVR driver uses the existing SDL/Wayland swapchain backend. Its a
 
 Sixteen more of Armada's patches are DRM/lease/HDR-on-KMS work for a native display, which this
 app's Wayland-hosted gamescope never reaches, or need a newer gamescope than the runtime has.
+- `0126-explicit-sync-switch.patch`: `GAMESCOPE_DISABLE_EXPLICIT_SYNC=1` makes the nested
+  (Wayland and SDL) backends report no explicit sync, so gamescope does not offer
+  wp_linux_drm_syncobj_manager_v1 and clients use implicit sync. The app sets it on a GPU that is
+  not an Adreno when the kernel is older than 6.6 (no DRM_IOCTL_SYNCOBJ_EVENTFD: an Exynos 2400e
+  on 6.1 logged hundreds of "timeline: DRM_IOCTL_SYNCOBJ_EVENTFD failed" and a game hung at a
+  black screen) or when the render node is Mali's stand-in (/dev/mali0 has no DRM syncobjs).

tools/gamescope/patches/0126-explicit-sync-switch.patch

@@ -0,0 +1,29 @@
+--- a/src/Backends/WaylandBackend.cpp
++++ b/src/Backends/WaylandBackend.cpp
+@@ -2309,7 +2309,10 @@
+ 
+     bool CWaylandBackend::SupportsExplicitSync() const
+     {
+-        return true;
++        // GAMESCOPE_DISABLE_EXPLICIT_SYNC=1: a kernel without DRM_IOCTL_SYNCOBJ_EVENTFD (before
++        // Linux 6.6) cannot signal the timelines, so clients use implicit sync instead.
++        const char *pszDisable = getenv( "GAMESCOPE_DISABLE_EXPLICIT_SYNC" );
++        return !( pszDisable && *pszDisable == '1' );
+     }
+ 
+     bool CWaylandBackend::IsPaused() const
+--- a/src/Backends/SDLBackend.cpp
++++ b/src/Backends/SDLBackend.cpp
+@@ -506,8 +506,10 @@
+ 
+ 	bool CSDLBackend::SupportsExplicitSync() const
+ 	{
+-		// We use a Vulkan swapchain, so yes.
+-		return true;
++		// We use a Vulkan swapchain, so yes - unless the kernel cannot signal the timelines
++		// (no DRM_IOCTL_SYNCOBJ_EVENTFD before Linux 6.6): GAMESCOPE_DISABLE_EXPLICIT_SYNC=1.
++		const char *pszDisable = getenv( "GAMESCOPE_DISABLE_EXPLICIT_SYNC" );
++		return !( pszDisable && *pszDisable == '1' );
+ 	}
+ 
+ 	bool CSDLBackend::IsPaused() const