[Mesa-dev] [PATCH] st/dri: decrease input lag by syncing sooner in SwapBuffers

[Marek Olšák]

From: Marek Olšák <marek.olsak at amd.com>

It's done by:
- decrease the number of frames in flight by 1
- flush before throttling in SwapBuffers
  (instead of wait-then-flush, do flush-then-wait)

The improvement is apparent with Unigine Heaven.

Previously:
    draw frame 2
    wait frame 0
    flush frame 2
    present frame 2

    The input lag is 2 frames.

Now:
    draw frame 2
    flush frame 2
    wait frame 1
    present frame 2

    The input lag is 1 frame. Flushing is done before waiting, because
    otherwise the device would be idle after waiting.

Nine is affected because it also uses the pipe cap.
---
 src/gallium/auxiliary/util/u_screen.c         |  2 +-
 src/gallium/state_trackers/dri/dri_drawable.c | 20 +++++++++----------
 2 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/src/gallium/auxiliary/util/u_screen.c b/src/gallium/auxiliary/util/u_screen.c
index 27f51e0898e..410f17421e6 100644
--- a/src/gallium/auxiliary/util/u_screen.c
+++ b/src/gallium/auxiliary/util/u_screen.c
@@ -349,21 +349,21 @@ u_pipe_screen_get_param_defaults(struct pipe_screen *pscreen,
    case PIPE_CAP_MAX_VARYINGS:
       return 8;
 
    case PIPE_CAP_COMPUTE_GRID_INFO_LAST_BLOCK:
       return 0;
 
    case PIPE_CAP_COMPUTE_SHADER_DERIVATIVES:
       return 0;
 
    case PIPE_CAP_MAX_FRAMES_IN_FLIGHT:
-      return 2;
+      return 1;
 
    case PIPE_CAP_DMABUF:
 #ifdef PIPE_OS_LINUX
       return 1;
 #else
       return 0;
 #endif
 
    default:
       unreachable("bad PIPE_CAP_*");
diff --git a/src/gallium/state_trackers/dri/dri_drawable.c b/src/gallium/state_trackers/dri/dri_drawable.c
index 26bfdbecc53..c1de3bed9dd 100644
--- a/src/gallium/state_trackers/dri/dri_drawable.c
+++ b/src/gallium/state_trackers/dri/dri_drawable.c
@@ -555,33 +555,33 @@ dri_flush(__DRIcontext *cPriv,
        *
        * This pulls a fence off the throttling queue and waits for it if the
        * number of fences on the throttling queue has reached the desired
        * number.
        *
        * Then flushes to insert a fence at the current rendering position, and
        * pushes that fence on the queue. This requires that the st_context_iface
        * flush method returns a fence even if there are no commands to flush.
        */
       struct pipe_screen *screen = drawable->screen->base.screen;
-      struct pipe_fence_handle *fence;
+      struct pipe_fence_handle *oldest_fence, *new_fence = NULL;
 
-      fence = swap_fences_pop_front(drawable);
-      if (fence) {
-         (void) screen->fence_finish(screen, NULL, fence, PIPE_TIMEOUT_INFINITE);
-         screen->fence_reference(screen, &fence, NULL);
-      }
+      st->flush(st, flush_flags, &new_fence);
 
-      st->flush(st, flush_flags, &fence);
+      oldest_fence = swap_fences_pop_front(drawable);
+      if (oldest_fence) {
+         screen->fence_finish(screen, NULL, oldest_fence, PIPE_TIMEOUT_INFINITE);
+         screen->fence_reference(screen, &oldest_fence, NULL);
+      }
 
-      if (fence) {
-         swap_fences_push_back(drawable, fence);
-         screen->fence_reference(screen, &fence, NULL);
+      if (new_fence) {
+         swap_fences_push_back(drawable, new_fence);
+         screen->fence_reference(screen, &new_fence, NULL);
       }
    }
    else if (flags & (__DRI2_FLUSH_DRAWABLE | __DRI2_FLUSH_CONTEXT)) {
       st->flush(st, flush_flags, NULL);
    }
 
    if (drawable) {
       drawable->flushing = FALSE;
    }
 
-- 
2.17.1