cc: Introduce BeginFrame sequence numbers and acknowledgements.

cc/output/begin_frame_args.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_OUTPUT_BEGIN_FRAME_ARGS_H_
 #define CC_OUTPUT_BEGIN_FRAME_ARGS_H_
 
+#include <stdint.h>
+
 #include <memory>
 
 #include "base/location.h"
 #include "base/memory/ref_counted.h"
 #include "base/time/time.h"
 #include "base/values.h"
 #include "cc/base/cc_export.h"
 
 namespace base {
 namespace trace_event {
@@ skipping 24 matching lines @@
   enum BeginFrameArgsType {
     INVALID,
     NORMAL,
     MISSED,
     // Not a real type, but used by the IPC system. Should always remain the
     // *last* value in this enum.
     BEGIN_FRAME_ARGS_TYPE_MAX,
   };
   static const char* TypeToString(BeginFrameArgsType type);
 
+  static const uint64_t kInvalidFrameNumber;
+  static const uint64_t kStartingFrameNumber;
+
   // Creates an invalid set of values.
   BeginFrameArgs();
 
 #ifdef NDEBUG
   typedef const void* CreationLocation;
 #else
   typedef const tracked_objects::Location& CreationLocation;
   tracked_objects::Location created_from;
 #endif
 
   // You should be able to find all instances where a BeginFrame has been
   // created by searching for "BeginFrameArgs::Create".
   // The location argument should **always** be BEGINFRAME_FROM_HERE macro.
   static BeginFrameArgs Create(CreationLocation location,
+                               uint64_t source_id,
+                               uint64_t sequence_number,
                                base::TimeTicks frame_time,
                                base::TimeTicks deadline,
                                base::TimeDelta interval,
                                BeginFrameArgsType type);
 
   // This is the default delta that will be used to adjust the deadline when
   // proper draw-time estimations are not yet available.
   static base::TimeDelta DefaultEstimatedParentDrawTime();
 
   // This is the default interval to use to avoid sprinkling the code with
   // magic numbers.
   static base::TimeDelta DefaultInterval();
 
   bool IsValid() const { return interval >= base::TimeDelta(); }
 
   std::unique_ptr<base::trace_event::ConvertableToTraceFormat> AsValue() const;
   void AsValueInto(base::trace_event::TracedValue* dict) const;
 
+  uint64_t source_id;
+  uint64_t sequence_number;
   base::TimeTicks frame_time;
   base::TimeTicks deadline;
   base::TimeDelta interval;
   BeginFrameArgsType type;
   bool on_critical_path;
 
  private:
-  BeginFrameArgs(base::TimeTicks frame_time,
+  BeginFrameArgs(uint64_t source_id,
+                 uint64_t sequence_number,
+                 base::TimeTicks frame_time,
                  base::TimeTicks deadline,
                  base::TimeDelta interval,
                  BeginFrameArgsType type);
 };
 
+// Sent by a BeginFrameObserver as acknowledgment of completing a BeginFrame.
+struct CC_EXPORT BeginFrameAck {
+  BeginFrameAck();
+  BeginFrameAck(uint64_t source_id,
+                uint64_t sequence_number,
+                uint64_t latest_confirmed_frame,
+                uint32_t remaining_frames,
+                bool has_damages);
+
+  // Source identifier of the BeginFrame that is acknowledged.

[brianderson, 2016/12/09 01:08:11]

In this comment, can you explain why the source_id is needed?

[Eric Seckler, 2016/12/09 16:47:16]

Done. Also added a comment to BeginFrameArgs.

+  uint64_t source_id;
+
+  // Sequence number of the BeginFrame that is acknowledged.
+  uint64_t sequence_number;
+
+  // Sequence number of the latest BeginFrame that was positively acknowledged
+  // (confirmed) by the observer.
+  //
+  // There are two scenarios for a positive acknowledgment:
+  //  a) All of the observer's pending updates led to successful damages (e.g. a
+  //     CompositorFrame or a damaged surface).
+  //  b) The observer did not have any updates and thus did not need to
+  //     produce damages.
+  // A negative acknowledgment, in contrast, describes a situation in which the
+  // observer had pending updates, but was unable to successfully produce
+  // corresponding damages for all its updates in time.
+  //
+  // As a result, |latest_confirmed_frame| describes the "staleness" of the last
+  // damages that were produced by the observer. Note that even if
+  // |has_damages == true|, the damages produced as a result of the acknowledged
+  // BeginFrame may be stale (|latest_confirmed_frame < sequence_number|). In
+  // such a case, the damages that were produced may contain updates from
+  // previous BeginFrames or only part of this BeginFrame's updates.
+  //
+  // Observers aggregate the |latest_confirmed_frame| of their children: The
+  // compositor Scheduler indicates the latest BeginFrame that both impl and
+  // main thread confirmed. Likewise, the DisplayScheduler indicates the minimum
+  // |latest_confirmed_frame| that all its BeginFrameObservers acknowledged.
+  uint64_t latest_confirmed_frame;
+
+  // Number of BeginFrames queued at the observer at time of acknowledgment.
+  uint32_t remaining_frames;
+
+  // |true| if the observer has produced damages (e.g. sent a CompositorFrame or
+  // damaged a surface) as part of responding to the BeginFrame.
+  bool has_damages;
+};
+
 }  // namespace cc
 
 #endif  // CC_OUTPUT_BEGIN_FRAME_ARGS_H_