[PATCH mesa 1/2] wayland: Block for the frame callback in get_back_bo not dri2_swap_buffers

[Neil Roberts]

As per Kristian's suggestion we can avoid the problem of effectively
disabling the event queuing mechanism by only doing the sync request
when the swap interval is zero. To fix the bug of using three buffers
we can just block for the frame callback in get_back_bo instead of
swap_buffers. I was originally a bit reluctant to do this because it
will end up blocking in an unusual place (ie, usually in glClear) but
I suppose that doesn't really matter.

I've split that change out into a separate patch because it is an
independent problem. The new swap interval patch can now share the
callback variable with the frame callback because they are now blocked
for in the same place.

Just to note, in addition to these two patches I think it would be
good to apply these other two patches which are buried in this thread
(although they are not required):

http://lists.freedesktop.org/archives/wayland-devel/2013-October/011766.html
http://lists.freedesktop.org/archives/wayland-devel/2013-September/010967.html

- Neil

------- >8 --------------- (use git am --scissors to automatically chop here)

Consider a typical game-style main loop which might be like this:

while (1) {
	draw_something();
	eglSwapBuffers();
}

In this case the game is relying on eglSwapBuffers to throttle to a sensible
frame rate. Previously this game would end up using three buffers even though
it should only need two. This is because Mesa decides whether to allocate a
new buffer in get_back_bo which would be before it has tried to read any
events from the compositor so it wouldn't have seen any buffer release events
yet.

This patch just moves the block for the frame callback to get_back_bo.
Typically the compositor will send a release event immediately after one of
the attaches so if we block for the frame callback here then we can be sure to
have completed at least one roundtrip and received that release event after
attaching the previous buffer before deciding whether to allocate a new one.

dri2_swap_buffers always calls get_back_bo so even if the client doesn't
render anything we will still be sure to block to the frame callback. The code
to create the new frame callback has been moved to after this call so that we
can be sure to have cleared the previous frame callback before requesting a
new one.
---
 src/egl/drivers/dri2/platform_wayland.c | 34 +++++++++++++++++++--------------
 1 file changed, 20 insertions(+), 14 deletions(-)

diff --git a/src/egl/drivers/dri2/platform_wayland.c b/src/egl/drivers/dri2/platform_wayland.c
index f9065bb..5ce8981 100644
--- a/src/egl/drivers/dri2/platform_wayland.c
+++ b/src/egl/drivers/dri2/platform_wayland.c
@@ -263,8 +263,19 @@ get_back_bo(struct dri2_egl_surface *dri2_surf)
       dri2_egl_display(dri2_surf->base.Resource.Display);
    int i;
 
-   /* There might be a buffer release already queued that wasn't processed */
-   wl_display_dispatch_queue_pending(dri2_dpy->wl_dpy, dri2_dpy->wl_queue);
+   if (dri2_surf->frame_callback == NULL) {
+      /* There might be a buffer release already queued that wasn't processed */
+      wl_display_dispatch_queue_pending(dri2_dpy->wl_dpy, dri2_dpy->wl_queue);
+   } else {
+      /* We throttle to the frame callback here so that we can be sure to have
+       * received any release events before trying to decide whether to
+       * allocate a new buffer */
+      do {
+         if (wl_display_dispatch_queue(dri2_dpy->wl_dpy,
+                                       dri2_dpy->wl_queue) == -1)
+            return EGL_FALSE;
+      } while (dri2_surf->frame_callback != NULL);
+   }
 
    if (dri2_surf->back == NULL) {
       for (i = 0; i < ARRAY_SIZE(dri2_surf->color_buffers); i++) {
@@ -557,18 +568,7 @@ dri2_swap_buffers_with_damage(_EGLDriver *drv,
 {
    struct dri2_egl_display *dri2_dpy = dri2_egl_display(disp);
    struct dri2_egl_surface *dri2_surf = dri2_egl_surface(draw);
-   int i, ret = 0;
-
-   while (dri2_surf->frame_callback && ret != -1)
-      ret = wl_display_dispatch_queue(dri2_dpy->wl_dpy, dri2_dpy->wl_queue);
-   if (ret < 0)
-      return EGL_FALSE;
-
-   dri2_surf->frame_callback = wl_surface_frame(dri2_surf->wl_win->surface);
-   wl_callback_add_listener(dri2_surf->frame_callback,
-                            &frame_listener, dri2_surf);
-   wl_proxy_set_queue((struct wl_proxy *) dri2_surf->frame_callback,
-                      dri2_dpy->wl_queue);
+   int i;
 
    for (i = 0; i < ARRAY_SIZE(dri2_surf->color_buffers); i++)
       if (dri2_surf->color_buffers[i].age > 0)
@@ -581,6 +581,12 @@ dri2_swap_buffers_with_damage(_EGLDriver *drv,
       return EGL_FALSE;
    }
 
+   dri2_surf->frame_callback = wl_surface_frame(dri2_surf->wl_win->surface);
+   wl_callback_add_listener(dri2_surf->frame_callback,
+                            &frame_listener, dri2_surf);
+   wl_proxy_set_queue((struct wl_proxy *) dri2_surf->frame_callback,
+                      dri2_dpy->wl_queue);
+
    dri2_surf->back->age = 1;
    dri2_surf->current = dri2_surf->back;
    dri2_surf->back = NULL;
-- 
1.8.3.1