[display] Release images before sending vsync

When handling vsync, retire the old images and signal their signal
events before sending the new vsync event, to guarantee that clients
see the old images as free by the time they receive the vsync event.

Test: display-test, run a scenic program
Bug: SCN-996
Change-Id: I9bc7ca047bc7d1ded7f6f67e2b48ce585e32711c

system/dev/display/display/controller.cpp

@@ -476,6 +476,41 @@ void Controller::OnDisplayVsync(uint64_t display_id, zx_time_t timestamp,
         }
     }
 
+    if (!info->pending_layer_change) {
+        // Since we know there are no pending layer changes, we know that every layer (i.e z_index)
+        // has an image. So every image either matches a handle (in which case it's being
+        // displayed), is older than its layer's image (i.e. in front of in the queue) and can be
+        // retired, or is newer than its layer's image (i.e. behind in the queue) and has yet to be
+        // presented.
+        uint32_t z_indices[handle_count];
+        for (unsigned i = 0; i < handle_count; i++) {
+            z_indices[i] = UINT32_MAX;
+        }
+        image_node_t* cur;
+        image_node_t* tmp;
+        list_for_every_entry_safe(&info->images, cur, tmp, image_node_t, link) {
+            bool z_already_matched = false;
+            for (unsigned i = 0; i < handle_count; i++) {
+                if (handles[i] == cur->self->info().handle) {
+                    z_indices[i] = cur->self->z_index();
+                    z_already_matched = true;
+                    break;
+                } else if (z_indices[i] == cur->self->z_index()) {
+                    z_already_matched = true;
+                    break;
+                }
+            }
+
+            // Retire any images for which we don't already have a z-match, since
+            // those are older than whatever is currently in their layer.
+            if (!z_already_matched) {
+                list_delete(&cur->link);
+                cur->self->OnRetire();
+                cur->self.reset();
+            }
+        }
+    }
+
     // Drop the vsync event if we're in the middle of switching clients, since we don't want to
     // send garbage image ids. Switching clients is rare enough that any minor timing issues that
     // this could cause aren't worth worrying about.
@@ -499,42 +534,6 @@ void Controller::OnDisplayVsync(uint64_t display_id, zx_time_t timestamp,
     } else {
         zxlogf(TRACE, "Dropping vsync\n");
     }
-
-    if (info->pending_layer_change) {
-        return;
-    }
-
-    // Since we know there are no pending layer changes, we know that every layer (i.e z_index)
-    // has an image. So every image either matches a handle (in which case it's being displayed),
-    // is older than its layer's image (i.e. in front of in the queue) and can be retired, or is
-    // newer than its layer's image (i.e. behind in the queue) and has yet to be presented.
-    uint32_t z_indices[handle_count];
-    for (unsigned i = 0; i < handle_count; i++) {
-        z_indices[i] = UINT32_MAX;
-    }
-    image_node_t* cur;
-    image_node_t* tmp;
-    list_for_every_entry_safe(&info->images, cur, tmp, image_node_t, link) {
-        bool z_already_matched = false;
-        for (unsigned i = 0; i < handle_count; i++) {
-            if (handles[i] == cur->self->info().handle) {
-                z_indices[i] = cur->self->z_index();
-                z_already_matched = true;
-                break;
-            } else if (z_indices[i] == cur->self->z_index()) {
-                z_already_matched = true;
-                break;
-            }
-        }
-
-        // Retire any images for which we don't already have a z-match, since
-        // those are older than whatever is currently in their layer.
-        if (!z_already_matched) {
-            list_delete(&cur->link);
-            cur->self->OnRetire();
-            cur->self.reset();
-        }
-    }
 }
 
 zx_status_t Controller::GetAudioFormat(uint64_t display_id, uint32_t fmt_idx,