glx: Clean up sync-to-vblank mechanism discovery

The code has gotten really complicated to follow.

As soon as we have a sync-to-vblank mechanism we should just bail out.

Also, __GL_SYNC_TO_VBLANK (which is used by nVidia) should be assumed
equivalent to a CLUTTER_VBLANK_GLX_SWAP.

clutter/glx/clutter-backend-glx.c

@@ -262,6 +262,7 @@ clutter_backend_glx_get_features (ClutterBackend *backend)
   ClutterBackendGLX *backend_glx = CLUTTER_BACKEND_GLX (backend);
   const gchar *glx_extensions = NULL;
   ClutterFeatureFlags flags;
+  gboolean use_dri = FALSE;
 
   flags = clutter_backend_x11_get_features (backend);
   flags |= CLUTTER_FEATURE_STAGE_MULTIPLE;
@@ -275,7 +276,7 @@ clutter_backend_glx_get_features (ClutterBackend *backend)
                 "  GL_VENDOR: %s\n"
                 "  GL_RENDERER: %s\n"
                 "  GL_VERSION: %s\n"
-                "  GL_EXTENSIONS: %s\n",
+                "  GL_EXTENSIONS: %s",
                 glGetString (GL_VENDOR),
                 glGetString (GL_RENDERER),
                 glGetString (GL_VERSION),
@@ -285,15 +286,26 @@ clutter_backend_glx_get_features (ClutterBackend *backend)
     glXQueryExtensionsString (clutter_x11_get_default_display (),
                               clutter_x11_get_default_screen ());
 
-  CLUTTER_NOTE (BACKEND, "GLX Extensions: %s", glx_extensions);
+  CLUTTER_NOTE (BACKEND, "  GLX Extensions: %s", glx_extensions);
+
+  use_dri = check_vblank_env ("dri");
 
   /* First check for explicit disabling or it set elsewhere (eg NVIDIA) */
-  if (getenv ("__GL_SYNC_TO_VBLANK") || check_vblank_env ("none"))
+  if (check_vblank_env ("none"))
     {
       CLUTTER_NOTE (BACKEND, "vblank sync: disabled at user request");
+      goto done;
     }
-  else
+
+  if (g_getenv ("__GL_SYNC_TO_VBLANK") != NULL)
     {
+      backend_glx->vblank_type = CLUTTER_VBLANK_GLX_SWAP;
+      flags |= CLUTTER_FEATURE_SYNC_TO_VBLANK;
+
+      CLUTTER_NOTE (BACKEND, "Using __GL_SYNC_TO_VBLANK hint");
+      goto done;
+    }
+
   /* We try two GL vblank syncing mechanisms.
    * glXSwapIntervalSGI is tried first, then glXGetVideoSyncSGI.
    *
@@ -306,7 +318,7 @@ clutter_backend_glx_get_features (ClutterBackend *backend)
    * to be investigated. glXGetVideoSyncSGI on ATI at least seems to have
    * no effect.
    */
-      if (!check_vblank_env ("dri") &&
+  if (!use_dri &&
       _cogl_check_extension ("GLX_SGI_swap_control", glx_extensions))
     {
       backend_glx->swap_interval =
@@ -314,33 +326,33 @@ clutter_backend_glx_get_features (ClutterBackend *backend)
 
       CLUTTER_NOTE (BACKEND, "attempting glXSwapIntervalSGI vblank setup");
 
-          if (backend_glx->swap_interval != NULL)
+      if (backend_glx->swap_interval != NULL &&
-            {
+          backend_glx->swap_interval (1) == 0)
-              if (backend_glx->swap_interval (1) == 0)
         {
           backend_glx->vblank_type = CLUTTER_VBLANK_GLX_SWAP;
           flags |= CLUTTER_FEATURE_SYNC_TO_VBLANK;
-                  CLUTTER_NOTE (BACKEND, "glXSwapIntervalSGI setup success");
-                }
-            }
 
-          if (!(flags & CLUTTER_FEATURE_SYNC_TO_VBLANK))
+          CLUTTER_NOTE (BACKEND, "glXSwapIntervalSGI setup success");
-            CLUTTER_NOTE (BACKEND, "glXSwapIntervalSGI vblank setup failed");
 
 #ifdef GLX_INTEL_swap_event
-          /* GLX_INTEL_swap_event allows us to avoid blocking the CPU while
+          /* GLX_INTEL_swap_event allows us to avoid blocking the CPU
-           * we wait for glXSwapBuffers to complete, and instead we get an X
+           * while we wait for glXSwapBuffers to complete, and instead
-           * event notifying us of completion... */
+           * we get an X event notifying us of completion...
+           */
           if (!(clutter_paint_debug_flags & CLUTTER_DEBUG_DISABLE_SWAP_EVENTS) &&
-              _cogl_check_extension ("GLX_INTEL_swap_event", glx_extensions) &&
+              _cogl_check_extension ("GLX_INTEL_swap_event", glx_extensions))
-              flags & CLUTTER_FEATURE_SYNC_TO_VBLANK)
             {
               flags |= CLUTTER_FEATURE_SWAP_EVENTS;
             }
 #endif /* GLX_INTEL_swap_event */
+
+          goto done;
         }
 
-      if (!check_vblank_env ("dri") &&
+      CLUTTER_NOTE (BACKEND, "glXSwapIntervalSGI vblank setup failed");
+    }
+
+  if (!use_dri &&
       !(flags & CLUTTER_FEATURE_SYNC_TO_VBLANK) &&
       _cogl_check_extension ("GLX_SGI_video_sync", glx_extensions))
     {
@@ -355,14 +367,14 @@ clutter_backend_glx_get_features (ClutterBackend *backend)
       if ((backend_glx->get_video_sync != NULL) &&
           (backend_glx->wait_video_sync != NULL))
         {
-              CLUTTER_NOTE (BACKEND,
+          CLUTTER_NOTE (BACKEND, "glXGetVideoSyncSGI vblank setup success");
-                            "glXGetVideoSyncSGI vblank setup success");
 
           backend_glx->vblank_type = CLUTTER_VBLANK_GLX;
           flags |= CLUTTER_FEATURE_SYNC_TO_VBLANK;
+
+          goto done;
         }
 
-          if (!(flags & CLUTTER_FEATURE_SYNC_TO_VBLANK))
       CLUTTER_NOTE (BACKEND, "glXGetVideoSyncSGI vblank setup failed");
     }
 
@@ -373,23 +385,25 @@ clutter_backend_glx_get_features (ClutterBackend *backend)
   if (!(flags & CLUTTER_FEATURE_SYNC_TO_VBLANK))
     {
       CLUTTER_NOTE (BACKEND, "attempting DRI vblank setup");
+
       backend_glx->dri_fd = open("/dev/dri/card0", O_RDWR);
       if (backend_glx->dri_fd >= 0)
         {
           CLUTTER_NOTE (BACKEND, "DRI vblank setup success");
+
           backend_glx->vblank_type = CLUTTER_VBLANK_DRI;
           flags |= CLUTTER_FEATURE_SYNC_TO_VBLANK;
+
+          goto done;
         }
 
-          if (!(flags & CLUTTER_FEATURE_SYNC_TO_VBLANK))
       CLUTTER_NOTE (BACKEND, "DRI vblank setup failed");
     }
 #endif /* __linux__ */
 
-      if (!(flags & CLUTTER_FEATURE_SYNC_TO_VBLANK))
   CLUTTER_NOTE (BACKEND, "no use-able vblank mechanism found");
-    }
 
+done:
   CLUTTER_NOTE (BACKEND, "backend features checked");
 
   return flags;
@@ -747,7 +761,7 @@ glx_wait_for_vblank (ClutterBackendGLX *backend_glx)
   switch (backend_glx->vblank_type)
     {
     case CLUTTER_VBLANK_GLX_SWAP:
-      /* Nothing */
+      CLUTTER_NOTE (BACKEND, "Waiting for vblank (swap)");
       break;
 
     case CLUTTER_VBLANK_GLX:
@@ -756,6 +770,7 @@ glx_wait_for_vblank (ClutterBackendGLX *backend_glx)
 
         glFinish ();
 
+        CLUTTER_NOTE (BACKEND, "Waiting for vblank (wait_video_sync)");
         backend_glx->get_video_sync (&retraceCount);
         backend_glx->wait_video_sync (2,
                                       (retraceCount + 1) % 2,
@@ -770,6 +785,7 @@ glx_wait_for_vblank (ClutterBackendGLX *backend_glx)
 
         glFinish ();
 
+        CLUTTER_NOTE (BACKEND, "Waiting for vblank (drm)");
         blank.request.type     = DRM_VBLANK_RELATIVE;
         blank.request.sequence = 1;
         blank.request.signal   = 0;
@@ -828,7 +844,6 @@ clutter_backend_glx_redraw (ClutterBackend *backend,
         stage_glx->glxwin ? stage_glx->glxwin : stage_x11->xwin;
 
       /* wait for the next vblank */
-      CLUTTER_NOTE (BACKEND, "Waiting for vblank");
       glx_wait_for_vblank (CLUTTER_BACKEND_GLX (backend));
 
       /* push on the screen */