cc: Adding BeginFrameTracker object and removing Now() from LTHI.

cc/scheduler/begin_frame_tracker.cc

Index: cc/scheduler/begin_frame_tracker.cc
diff --git a/cc/scheduler/begin_frame_tracker.cc b/cc/scheduler/begin_frame_tracker.cc
new file mode 100644
index 0000000000000000000000000000000000000000..c41000cf4face21055f57b8647f0f4f3c1900f7a
--- /dev/null
+++ b/cc/scheduler/begin_frame_tracker.cc
@@ -0,0 +1,81 @@
+// Copyright 2014 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.
+
+#include "cc/scheduler/begin_frame_tracker.h"
+
+namespace cc {
+
+BeginFrameTracker::BeginFrameTracker(const tracked_objects::Location& location)
+    : location_(location),
+      location_string_(location.ToString()),
+      current_updated_at_(),
+      current_args_(),
+      current_finished_at_(base::TimeTicks::FromInternalValue(-1)) {
+}
+
+BeginFrameTracker::~BeginFrameTracker() {
+}
+
+void BeginFrameTracker::Start(BeginFrameArgs new_args) {
+  // Trace the frame time being passed between BeginFrameTrackers.
+  TRACE_EVENT_FLOW_STEP0(
+      TRACE_DISABLED_BY_DEFAULT("cc.debug.scheduler.frames"), "BeginFrameArgs",
+      new_args.frame_time.ToInternalValue(), location_string_);
+
+  // Trace this specific begin frame tracker Start/Finish times.
+  TRACE_EVENT_ASYNC_BEGIN2(
+      TRACE_DISABLED_BY_DEFAULT("cc.debug.scheduler.frames"),
+      location_string_.c_str(), new_args.frame_time.ToInternalValue(),
+      "new args", new_args.AsValue(), "current args", current_args_.AsValue());
+
+  // Check the new BeginFrameArgs are valid and monotonically increasing.
+  DCHECK(new_args.IsValid());
+  DCHECK_LT(current_args_.frame_time, new_args.frame_time);
+
+  DCHECK(HasFinished())
+      << "Tried to start a new frame before finishing an existing frame.";
+  current_updated_at_ = base::TimeTicks::NowFromSystemTraceTime();
+  current_args_ = new_args;
+  current_finished_at_ = base::TimeTicks();
+
+  // TODO(mithro): Add UMA tracking of delta between current_updated_at_ time
+  // and the new_args.frame_time argument. This will give us how long after a
+  // BeginFrameArgs message was created before we started processing it.
+}
+
+const BeginFrameArgs& BeginFrameTracker::Get() const {
+  DCHECK(!HasFinished())
+      << "Tried to use BeginFrameArgs after marking the frame finished.";
+  DCHECK(current_args_.IsValid())
+      << "Tried to use BeginFrameArgs before starting a frame!";
+  return current_args_;
+}
+
+void BeginFrameTracker::Finish() {
+  DCHECK(!HasFinished()) << "Tried to finish an already finished frame";
+  current_finished_at_ = base::TimeTicks::NowFromSystemTraceTime();
+  TRACE_EVENT_ASYNC_END0(TRACE_DISABLED_BY_DEFAULT("cc.debug.scheduler.frames"),
+                         location_string_.c_str(),
+                         current_args_.frame_time.ToInternalValue());
+}
+
+const BeginFrameArgs& BeginFrameTracker::Last() const {
+  DCHECK(current_args_.IsValid())
+      << "Tried to use last BeginFrameArgs before starting a frame!";
+  DCHECK(HasFinished())
+      << "Tried to use last BeginFrameArgs before the frame is finished.";
+  return current_args_;
+}
+
+base::TimeDelta BeginFrameTracker::Interval() const {
+  base::TimeDelta interval = current_args_.interval;
+  // Normal interval will be ~16ms, 200Hz (5ms) screens are the fastest
+  // easily available so anything less than that is likely an error.
+  if (interval < base::TimeDelta::FromMilliseconds(4)) {

[brianderson, 2014/12/18 21:57:08]

250Hz seems like something we could hit eventually. How about error at 1kHz or
infinity?

[mithro-old, 2014/12/18 23:15:05]

I picked 250Hz because in theory 240Hz projectors exist (basically 3d ready
120Hz display). The 200Hz TV was the fastest I could find which I actually buy.
I'm pretty sure we'd collapse in a heap trying to do 250Hz (actually probably
even at 120Hz...).

Happy to change to 1kHz if you prefer.

[brianderson, 2014/12/19 01:18:25]

1kHz seems less risky.

[mithro-old, 2014/12/19 03:40:55]

Done.

+    interval = BeginFrameArgs::DefaultInterval();
+  }
+  return interval;
+}
+
+}  // namespace cc