[cogl] winsys-glx: Fix synchronisation behaviour in _cogl_winsys_onscreen_swap_region

[Robert Bragg <rbragg src gnome org>]

commit 671a4dfb3425beaa6553776da9286164df87ec62
Author: Adel Gadllah <adel gadllah gmail com>
Date:   Sun Sep 18 00:46:07 2011 +0200

    winsys-glx: Fix synchronisation behaviour in _cogl_winsys_onscreen_swap_region
    
    This patch basically restores the logic from 1.6.  There we assumed that
    glXCopySubBuffer won't tear and thus only needs to be throttled to the
    framerate, while glBlitFramebuffer needs to always wait to avoid
    tearing.
    
    With Nvidia drivers specifically we have seen that glBlitFramebuffer is
    not synchronized. Eventually the plan is that Cogl will actually take
    into consideration the underlying driver/hw vendor and driver version
    and we may want to only mark glBlitFramebuffer un-synchronized on
    Nvidia.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=659360
    
    Reviewed-by: Robert Bragg <robert linux intel com>

 cogl/cogl-types.h                               |    4 ++++
 cogl/winsys/cogl-winsys-glx-feature-functions.h |    2 +-
 cogl/winsys/cogl-winsys-glx.c                   |   12 +++++++++++-
 3 files changed, 16 insertions(+), 2 deletions(-)
---
diff --git a/cogl/cogl-types.h b/cogl/cogl-types.h
index 0d17b93..df9a286 100644
--- a/cogl/cogl-types.h
+++ b/cogl/cogl-types.h
@@ -668,6 +668,10 @@ typedef enum _CoglWinsysFeature
    * to the vblank frequency. */
   COGL_WINSYS_FEATURE_SWAP_REGION_THROTTLE,
 
+  /* Available if the swap region implementation won't tear and thus
+   * only needs to be throttled to the framerate */
+  COGL_WINSYS_FEATURE_SWAP_REGION_SYNCHRONIZED,
+
   COGL_WINSYS_FEATURE_N_FEATURES
 } CoglWinsysFeature;
 
diff --git a/cogl/winsys/cogl-winsys-glx-feature-functions.h b/cogl/winsys/cogl-winsys-glx-feature-functions.h
index 50df562..b903ebd 100644
--- a/cogl/winsys/cogl-winsys-glx-feature-functions.h
+++ b/cogl/winsys/cogl-winsys-glx-feature-functions.h
@@ -89,7 +89,7 @@ COGL_WINSYS_FEATURE_BEGIN (copy_sub_buffer,
                            "copy_sub_buffer\0",
                            0,
                            0,
-                           0)
+                           COGL_WINSYS_FEATURE_SWAP_REGION_SYNCHRONIZED)
 COGL_WINSYS_FEATURE_FUNCTION (void, glXCopySubBuffer,
                               (Display *dpy,
                                GLXDrawable drawable,
diff --git a/cogl/winsys/cogl-winsys-glx.c b/cogl/winsys/cogl-winsys-glx.c
index 80e262c..0eda135 100644
--- a/cogl/winsys/cogl-winsys-glx.c
+++ b/cogl/winsys/cogl-winsys-glx.c
@@ -1127,6 +1127,16 @@ _cogl_winsys_onscreen_swap_region (CoglOnscreen *onscreen,
   guint32 end_frame_vsync_counter = 0;
   gboolean have_counter;
   gboolean can_wait;
+
+  /*
+   * We assume that glXCopySubBuffer is synchronized which means it won't prevent multiple
+   * blits per retrace if they can all be performed in the blanking period. If that's the
+   * case then we still want to use the vblank sync menchanism but
+   * we only need it to throttle redraws.
+   */
+  gboolean blit_sub_buffer_is_synchronized =
+     _cogl_winsys_has_feature (COGL_WINSYS_FEATURE_SWAP_REGION_SYNCHRONIZED);
+
   int framebuffer_height =  cogl_framebuffer_get_height (framebuffer);
   int *rectangles = g_alloca (sizeof (int) * n_rectangles * 4);
   int i;
@@ -1194,7 +1204,7 @@ _cogl_winsys_onscreen_swap_region (CoglOnscreen *onscreen,
    */
   context->glFinish ();
 
-  if (have_counter && can_wait)
+  if (blit_sub_buffer_is_synchronized && have_counter && can_wait)
     {
       end_frame_vsync_counter = _cogl_winsys_get_vsync_counter ();