video: Only prefer Wayland if fifo-v1 and commit-timing-v1 are available

Wayland has a myriad of unresolved problems regarding surface suspension
blocking forever in QueuePresent/SwapBuffers when occludedand the FIFO
(vsync) implementation being fundamentally broken leading to reduced
GPU-bound performance and 'barcoding' frametimes due to swapchain
starvation.

There are two protocols used to solve these two problems together --
fifo-v1 and commit-timing-v1, which implement the commit queue on the
compositor side, and a timestamp that frames are intended to be
displayed for/discarded respectfully.

To avoid severe performance regressions for developers targeting SDL3,
only pick Wayland as the default backend when these two protocols are
supported -- otherwise fallback to X11/XWayland.

We do this by having two VideoBootStraps, one which is tests the
preferred case, "wayland_preferred" (ie. if fifo-v1 + commit-timing-v1
are available init time), and the fallback, which is just "wayland",
the same name as before, which does no such tests.
Thus, forcing with SDL_VIDEO_DRIVER=wayland will go onto the fallback
option, and pick Wayland always, as usual, so there is no behaviour
change.

In the case that X11/XWayland is not available (ie. no DISPLAY), we will
still fallback to using Wayland without these protocols available.

Signed-off-by: Joshua Ashton <[email protected]>

src/video/SDL_sysvideo.h

@@ -478,7 +478,8 @@ extern VideoBootStrap KMSDRM_bootstrap;
 extern VideoBootStrap KMSDRM_LEGACY_bootstrap;
 extern VideoBootStrap DUMMY_bootstrap;
 extern VideoBootStrap DUMMY_evdev_bootstrap;
-extern VideoBootStrap Wayland_bootstrap;
+extern VideoBootStrap Wayland_preferred_bootstrap;
+extern VideoBootStrap Wayland_fallback_bootstrap;
 extern VideoBootStrap VIVANTE_bootstrap;
 extern VideoBootStrap Emscripten_bootstrap;
 extern VideoBootStrap OFFSCREEN_bootstrap;

src/video/SDL_video.c

@@ -70,11 +70,14 @@ static VideoBootStrap *bootstrap[] = {
     &COCOA_bootstrap,
 #endif
 #ifdef SDL_VIDEO_DRIVER_WAYLAND
-    &Wayland_bootstrap,
+    &Wayland_preferred_bootstrap,
 #endif
 #ifdef SDL_VIDEO_DRIVER_X11
     &X11_bootstrap,
 #endif
+#ifdef SDL_VIDEO_DRIVER_WAYLAND
+    &Wayland_fallback_bootstrap,
+#endif
 #ifdef SDL_VIDEO_DRIVER_VIVANTE
     &VIVANTE_bootstrap,
 #endif

src/video/wayland/SDL_waylandvideo.c

@@ -65,6 +65,7 @@
 #include <libdecor.h>
 #endif
 
+#define WAYLANDVID_PREFERRED_DRIVER_NAME "wayland_preferred"
 #define WAYLANDVID_DRIVER_NAME "wayland"
 
 /* Clamp certain core protocol versions on older versions of libwayland. */
@@ -369,7 +370,53 @@ static void Wayland_DeleteDevice(SDL_VideoDevice *device)
     SDL_WAYLAND_UnloadSymbols();
 }
 
-static SDL_VideoDevice *Wayland_CreateDevice(void)
+typedef struct
+{
+    SDL_bool has_fifo_v1;
+    SDL_bool has_commit_timing_v1;
+} SDL_WaylandPreferredData;
+
+static void wayland_preferred_check_handle_global(void *data, struct wl_registry *registry, uint32_t id,
+                                                  const char *interface, uint32_t version)
+{
+    SDL_WaylandPreferredData *d = data;
+
+    if (SDL_strcmp(interface, "fifo_v1") == 0) {
+        d->has_fifo_v1 = SDL_TRUE;
+    } else if (SDL_strcmp(interface, "commit_timing_v1") == 0) {
+        d->has_commit_timing_v1 = SDL_TRUE;
+    }
+}
+
+static void wayland_preferred_check_remove_global(void *data, struct wl_registry *registry, uint32_t id)
+{
+    /* No need to do anything here. */
+}
+
+static const struct wl_registry_listener preferred_registry_listener = {
+    wayland_preferred_check_handle_global,
+    wayland_preferred_check_remove_global
+};
+
+static SDL_bool Wayland_IsPreferred(struct wl_display *display)
+{
+    struct wl_registry *registry = wl_display_get_registry(display);
+    if (!registry) {
+        SDL_SetError("Failed to get the Wayland registry");
+        return SDL_FALSE;
+    }
+
+    SDL_WaylandPreferredData preferred_data = { 0 };
+    wl_registry_add_listener(registry, &preferred_registry_listener, &preferred_data);
+
+    WAYLAND_wl_display_roundtrip(display);
+
+    wl_registry_destroy(registry);
+
+    return preferred_data.has_fifo_v1 && preferred_data.has_commit_timing_v1;
+}
+
+static SDL_VideoDevice *Wayland_CreateDevice(SDL_bool fallback)
 {
     SDL_VideoDevice *device;
     SDL_VideoData *data;
@@ -398,6 +445,18 @@ static SDL_VideoDevice *Wayland_CreateDevice(void)
         }
     }
 
+    /*
+     * If we are checking for preferred Wayland, then let's query for
+     * fifo-v1 and commit-timing-v1's existance so we don't regress
+     * GPU-bound performance and frame-pacing by default due to
+     * swapchain starvation.
+     */
+    if (!fallback && !Wayland_IsPreferred(display)) {
+        WAYLAND_wl_display_disconnect(display);
+        SDL_WAYLAND_UnloadSymbols();
+        return NULL;
+    }
+
     data = SDL_calloc(1, sizeof(*data));
     if (!data) {
         WAYLAND_wl_display_disconnect(display);
@@ -523,9 +582,25 @@ static SDL_VideoDevice *Wayland_CreateDevice(void)
     return device;
 }
 
-VideoBootStrap Wayland_bootstrap = {
+static SDL_VideoDevice *Wayland_Preferred_CreateDevice(void)
+{
+    return Wayland_CreateDevice(SDL_FALSE);
+}
+
+static SDL_VideoDevice *Wayland_Fallback_CreateDevice(void)
+{
+    return Wayland_CreateDevice(SDL_TRUE);
+}
+
+VideoBootStrap Wayland_preferred_bootstrap = {
+    WAYLANDVID_PREFERRED_DRIVER_NAME, "SDL Wayland video driver",
+    Wayland_Preferred_CreateDevice,
+    Wayland_ShowMessageBox
+};
+
+VideoBootStrap Wayland_fallback_bootstrap = {
     WAYLANDVID_DRIVER_NAME, "SDL Wayland video driver",
-    Wayland_CreateDevice,
+    Wayland_Fallback_CreateDevice,
     Wayland_ShowMessageBox
 };