Renderer reading side of gamepad

content/renderer/gamepad_shared_memory_reader.cc

+// 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.
+
+#include "content/renderer/gamepad_shared_memory_reader.h"
+
+#include "base/debug/trace_event.h"
+#include "base/metrics/histogram.h"
+#include "content/common/gamepad_messages.h"
+#include "content/public/renderer/render_thread.h"
+#include "content/common/gamepad_hardware_buffer.h"
+#include "ipc/ipc_sync_message_filter.h"
+
+namespace content {
+
+GamepadSharedMemoryReader::GamepadSharedMemoryReader() {
+  memset(ever_interacted_with_, 0, sizeof(ever_interacted_with_));
+  CHECK(RenderThread::Get()->Send(new GamepadHostMsg_StartPolling(
+      &renderer_shared_memory_handle_)));
+  renderer_shared_memory_.reset(
+      new base::SharedMemory(renderer_shared_memory_handle_, true));
+  CHECK(renderer_shared_memory_->Map(sizeof(GamepadHardwareBuffer)));
+  void *memory = renderer_shared_memory_->memory();
+  CHECK(memory);
+  gamepad_hardware_buffer_ =
+      static_cast<GamepadHardwareBuffer*>(memory);
+}
+
+void GamepadSharedMemoryReader::SampleGamepads(WebKit::WebGamepads& gamepads) {
+  WebKit::WebGamepads read_into;
+  TRACE_EVENT0("GAMEPAD", "SampleGamepads");
+
+  // Only try to read this many times before failing to avoid waiting here
+  // very long in case of contention with the writer. TODO(scottmg) Tune this
+  // number (as low as 1?) if histogram shows distribution as mostly
+  // 0-and-maximum.
+  const int kMaximumContentionCount = 10;
+  int contention_count = -1;
+  base::subtle::Atomic32 version;
+  do {
+    version = gamepad_hardware_buffer_->sequence.ReadBegin();
+    memcpy(&read_into, &gamepad_hardware_buffer_->buffer, sizeof(read_into));
+    ++contention_count;
+    if (contention_count == kMaximumContentionCount)
+      break;
+  } while (gamepad_hardware_buffer_->sequence.ReadRetry(version));
+  HISTOGRAM_COUNTS("Gamepad.ReadContentionCount", contention_count);
+
+  if (contention_count >= kMaximumContentionCount) {
+      // We failed to successfully read, presumably because the hardware
+      // thread was taking unusually long. Don't copy the data to the output
+      // buffer, and simply leave what was there before.
+      return;
+  }
+
+  // New data was read successfully, copy it into the output buffer.
+  memcpy(&gamepads, &read_into, sizeof(gamepads));
+
+  // Override the "connected" with false until the user has interacted
+  // with the gamepad. This is to prevent fingerprinting on drive-by pages.
+  for (unsigned i = 0; i < WebKit::WebGamepads::itemsLengthCap; ++i) {
+    WebKit::WebGamepad& pad = gamepads.items[i];
+    // If the device is physically connected, then check if we should
+    // keep it disabled. We track if any of the primary 4 buttons have been
+    // pressed to determine a reasonable intentional interaction from the user.
+    if (pad.connected) {
+      if (ever_interacted_with_[i])
+        continue;
+      const unsigned kPrimaryInteractionButtons = 4;
+      for (unsigned j = 0; j < kPrimaryInteractionButtons; ++j)
+        ever_interacted_with_[i] |= pad.buttons[j] > 0.5f;
+      // If we've not previously set, and the user still hasn't touched
+      // these buttons, then don't pass the data on to the Chromium port.
+      if (!ever_interacted_with_[i])
+        pad.connected = false;
+    }
+  }
+}
+
+GamepadSharedMemoryReader::~GamepadSharedMemoryReader() {
+  RenderThread::Get()->Send(new GamepadHostMsg_StopPolling());
+}
+
+} // namespace content