[Mesa-dev] [PATCH] Revert "wayland: Block for the frame callback in get_back_bo not dri2_swap_buffers"

[Daniel Stone]

This reverts commit 25cc889004aad6d1cab9edd76db898658e347b97, though
since the code has changed, it was applied manually.

The intent of moving blocking from SwapBuffers to get_back_bo, was to
avoid unnecessary triple-buffering by ensuring that the compositor had
fully processed the previous frame before we started rendering. This
means that the only time we would have to resort to triple-buffering
would be when the buffer is directly scanned out, thus saving an extra
buffer for composition anyway.

The 'repaint window' changes introduced in Weston since then, however,
have narrowed the window of time between the frame event being sent and
the repaint loop needing to conclude, to 7ms by default, in order to
reduce latency. This means however that blocking in get_back_bo gives a
maximum of 7ms for the entire GL submission to begin and complete.

Not only this, but if a client is using buffer_age to avoid full
repaints, the buffer-age request will stall in get_back_bo until the
frame callback completes, meaning that the client cannot even calculate
the repaint area before the 7ms window.

The combination of the two meant that WebKit-GTK+ was failing to
achieve full framerate on a Minnowboard, due to spending a great deal of
its time attempting to query the age of the next buffer before redraw.

Revert to the previous behaviour of allowing rendering to begin but
delaying SwapBuffers, unless and until we can find a more gentle
behaviour.

Signed-off-by: Daniel Stone <daniels at collabora.com>
Cc: Neil Roberts <neil at linux.intel.com>
Cc: Kristian Høgsberg <krh at bitplanet.net>
---
 src/egl/drivers/dri2/platform_wayland.c | 30 ++++++++++++++----------------
 1 file changed, 14 insertions(+), 16 deletions(-)

diff --git a/src/egl/drivers/dri2/platform_wayland.c b/src/egl/drivers/dri2/platform_wayland.c
index ccab192..ba74d33 100644
--- a/src/egl/drivers/dri2/platform_wayland.c
+++ b/src/egl/drivers/dri2/platform_wayland.c
@@ -324,14 +324,8 @@ get_back_bo(struct dri2_egl_surface *dri2_surf)
       return -1;
    }
 
-   /* We always want to throttle to some event (either a frame callback or
-    * a sync request) after the commit so that we can be sure the
-    * compositor has had a chance to handle it and send us a release event
-    * before we look for a free buffer */
-   while (dri2_surf->throttle_callback != NULL)
-      if (wl_display_dispatch_queue(dri2_dpy->wl_dpy,
-                                    dri2_dpy->wl_queue) == -1)
-         return -1;
+   /* 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->back == NULL) {
       for (i = 0; i < ARRAY_SIZE(dri2_surf->color_buffers); i++) {
@@ -705,6 +699,11 @@ dri2_wl_swap_buffers_with_damage(_EGLDriver *drv,
    struct dri2_egl_surface *dri2_surf = dri2_egl_surface(draw);
    int i;
 
+   while (dri2_surf->throttle_callback != NULL)
+      if (wl_display_dispatch_queue(dri2_dpy->wl_dpy,
+                                    dri2_dpy->wl_queue) == -1)
+         return -1;
+
    for (i = 0; i < ARRAY_SIZE(dri2_surf->color_buffers); i++)
       if (dri2_surf->color_buffers[i].age > 0)
          dri2_surf->color_buffers[i].age++;
@@ -1464,14 +1463,8 @@ swrast_update_buffers(struct dri2_egl_surface *dri2_surf)
 
    /* find back buffer */
 
-   /* We always want to throttle to some event (either a frame callback or
-    * a sync request) after the commit so that we can be sure the
-    * compositor has had a chance to handle it and send us a release event
-    * before we look for a free buffer */
-   while (dri2_surf->throttle_callback != NULL)
-      if (wl_display_dispatch_queue(dri2_dpy->wl_dpy,
-                                    dri2_dpy->wl_queue) == -1)
-         return -1;
+   /* 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);
 
    /* try get free buffer already created */
    for (i = 0; i < ARRAY_SIZE(dri2_surf->color_buffers); i++) {
@@ -1552,6 +1545,11 @@ dri2_wl_swrast_commit_backbuffer(struct dri2_egl_surface *dri2_surf)
 {
    struct dri2_egl_display *dri2_dpy = dri2_egl_display(dri2_surf->base.Resource.Display);
 
+   while (dri2_surf->throttle_callback != NULL)
+      if (wl_display_dispatch_queue(dri2_dpy->wl_dpy,
+                                    dri2_dpy->wl_queue) == -1)
+         return -1;
+
    if (dri2_surf->base.SwapInterval > 0) {
       dri2_surf->throttle_callback =
          wl_surface_frame(dri2_surf->wl_win->surface);
-- 
2.9.3