Define/update structure and gamepad messages between renderer and browser.

content/common/gamepad_hardware_buffer.h

Index: content/common/gamepad_hardware_buffer.h
diff --git a/content/common/gamepad_hardware_buffer.h b/content/common/gamepad_hardware_buffer.h
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+// Copyright (c) 2011 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 CONTENT_COMMON_GAMEPAD_HARDWARE_BUFFER_H_
+#define CONTENT_COMMON_GAMEPAD_HARDWARE_BUFFER_H_
+
+#include "base/atomicops.h"
+#include "third_party/WebKit/Source/WebKit/chromium/public/WebGamepads.h"
+
+namespace gamepad {
+
+/*
+
+This structure is stored in shared memory that's shared between the browser
+which does the hardware polling, and the various consumers of the gamepad
+state (renderers and NaCl plugins). The performance characteristics are that
+we want low latency (so would like to avoid explicit communication via IPC
+between producer and consumer) and relatively large data size.
+
+Writer and reader operate on the same buffer assuming contention is low, and
+start_marker and end_marker are used to detect inconsistent reads.
+
+The writer atomically increments the start_marker counter before starting,
+then fills in the gamepad data, then increments end_marker to the same value
+as start_marker. The readers first reads end_marker, then the the data and
+then start_marker, and if the reader finds that the start and end markers were
+different, then it must retry as the buffer was updated while being read.
+
+There is a requirement for memory barriers between the accesses to the markers
+and the main data to ensure that both the reader and write see a consistent
+view of those values. In the current implementation, the writer uses an
+Barrier_AtomicIncrement for the counter, and the reader uses an Acquire_Load.
+
+*/
+
+struct GamepadHardwareBuffer {
+  base::subtle::Atomic32 start_marker;
+  WebKit::WebGamepads buffer;
+  base::subtle::Atomic32 end_marker;
+};
+
+}
+
+#endif // CONTENT_COMMON_GAMEPAD_HARDWARE_BUFFER_H_