[BattOr] Reduce StartTracing time by about four seconds.

tools/battor_agent/battor_agent.cc

 // Copyright 2015 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 "tools/battor_agent/battor_agent.h"
 
 #include <iomanip>
 
 #include "base/bind.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "tools/battor_agent/battor_connection_impl.h"
@@ skipping 228 matching lines @@
       PerformAction(Action::READ_CALIBRATION_FRAME);
       return;
     case Action::SEND_CURRENT_SAMPLE_REQUEST:
       num_read_attempts_ = 1;
       PerformAction(Action::READ_CURRENT_SAMPLE);
       return;
     case Action::SEND_GIT_HASH_REQUEST:
       num_read_attempts_ = 1;
       PerformAction(Action::READ_GIT_HASH);
       return;
+    case Action::RESET:
+      CompleteCommand(BATTOR_ERROR_NONE);
+      return;
     default:
       CompleteCommand(BATTOR_ERROR_UNEXPECTED_MESSAGE);
   }
 }
 
 void BattOrAgent::OnMessageRead(bool success,
                                 BattOrMessageType type,
                                 std::unique_ptr<vector<char>> bytes) {
   // Return immediately if whatever action we were trying to perform already
   // timed out.
@@ skipping 140 matching lines @@
       vector<RawBattOrSample> frame;
       if (!ParseSampleFrame(type, *bytes, next_sequence_number_++,
                             &frame_header, &frame)) {
         CompleteCommand(BATTOR_ERROR_UNEXPECTED_MESSAGE);
         return;
       }
 
       // Check for the empty frame the BattOr uses to indicate it's done
       // streaming samples.
       if (frame.empty()) {
-        CompleteCommand(BATTOR_ERROR_NONE);
+        // Proactively reset the BattOr to speed up the next trace.

[charliea (OOO until 10-5), 2017/05/09 19:19:06]

I think this could be made clearer: for people stumbling through this code in
the future, it may be unclear why resetting the BattOr now will speed up the
trace in the future. That requires a surprisingly significant amount of
knowledge about how BattOrs work.

Could you elaborate in the comment about why this speeds things up?

[aschulman, 2017/05/12 22:32:03]

agreed. I will do this as best as I can.

+        PerformAction(Action::RESET);
         return;
       }
 
       samples_.insert(samples_.end(), frame.begin(), frame.end());
 
       num_read_attempts_ = 1;
       PerformAction(Action::READ_DATA_FRAME);
       return;
     }
 
@@ skipping 109 matching lines @@
     case Action::SEND_GIT_HASH_REQUEST:
       connection_->Flush();
       SendControlMessage(
           BATTOR_CONTROL_MESSAGE_TYPE_GET_FIRMWARE_GIT_HASH, 0, 0);
       return;
 
     case Action::READ_GIT_HASH:
       connection_->ReadMessage(BATTOR_MESSAGE_TYPE_CONTROL_ACK);
       return;
 
+    case Action::RESET:
+      SendControlMessage(BATTOR_CONTROL_MESSAGE_TYPE_RESET, 0, 0);
+      return;
+
     case Action::INVALID:
       NOTREACHED();
   }
 }
 
 void BattOrAgent::PerformDelayedAction(Action action, base::TimeDelta delay) {
   base::ThreadTaskRunnerHandle::Get()->PostDelayedTask(
       FROM_HERE, base::Bind(&BattOrAgent::PerformAction, AsWeakPtr(), action),
       delay);
 }
@@ skipping 112 matching lines @@
     if (clock_sync_marker != clock_sync_markers_.end())
       trace_stream << " <" << clock_sync_marker->second << ">";
 
     trace_stream << std::endl;
   }
 
   return trace_stream.str();
 }
 
 }  // namespace battor