Implement the content/renderer and content/browser part of the Device Motion API.

content/renderer/device_orientation/device_motion_shared_memory_reader.cc

+// Copyright (c) 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.
+
+#include "device_motion_shared_memory_reader.h"
+
+#include "base/debug/trace_event.h"
+#include "base/metrics/histogram.h"
+#include "content/common/device_motion_hardware_buffer.h"
+#include "content/common/device_motion_messages.h"
+#include "content/public/renderer/render_thread.h"
+#include "ipc/ipc_sync_message_filter.h"
+#include "third_party/WebKit/Source/Platform/chromium/public/WebDeviceMotionData.h"
+
+namespace content {
+
+bool DeviceMotionSharedMemoryReader::latestDeviceMotionData(
+    WebKit::WebDeviceMotionData& data) {
+
+  WebKit::WebDeviceMotionData read_into;
+  TRACE_EVENT0("DEVICEMOTION", "lastDeviceMotionData");
+
+  if (!base::SharedMemory::IsHandleValid(renderer_shared_memory_handle_))
+    return false;
+
+  // Only try to read this many times before failing to avoid waiting here
+  // very long in case of contention with the writer.
+  bool is_ready_for_read = false;
+  const int kMaximumContentionCount = 10;
+  int contention_count = -1;
+  base::subtle::Atomic32 version;
+  do {
+    version = device_motion_hardware_buffer_->sequence.ReadBegin();
+    is_ready_for_read = device_motion_hardware_buffer_->is_ready_for_read;
+    memcpy(&read_into, &device_motion_hardware_buffer_->buffer,
+        sizeof(read_into));
+    ++contention_count;
+    if (contention_count == kMaximumContentionCount)
+      break;
+  } while (device_motion_hardware_buffer_->sequence.ReadRetry(version));
+  UMA_HISTOGRAM_COUNTS("DeviceMotion.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 false;
+  }
+
+  // New data was read successfully, copy it into the output buffer.
+  memcpy(&data, &read_into, sizeof(data));
+
+  return is_ready_for_read;
+}
+
+DeviceMotionSharedMemoryReader::DeviceMotionSharedMemoryReader()
+    : device_motion_hardware_buffer_(NULL) {
+}
+
+void DeviceMotionSharedMemoryReader::startUpdating() {
+  CHECK(RenderThread::Get()->Send(

[darin (slow to review), 2013/05/29 06:41:53]

are you sure you need to CHECK here?

[timvolodine, 2013/05/29 18:59:06]

hmm, I thought the reasoning for this was that the sync call should not fail.
but maybe I should make it more flexible and just return silently if this
happens.

+      new DeviceMotionHostMsg_StartPolling(&renderer_shared_memory_handle_)));

[darin (slow to review), 2013/05/29 06:41:53]

Can you do this using an asynchronous IPC instead?  This is just adding jank to
the main thread for no apparent benefit other than perhaps making this code
simpler.  Instead, it seems like you could pair this StartPolling IPC with a
DeviceMotionMsg_DidStartPolling IPC that gets sent back to you.

[timvolodine, 2013/05/29 18:59:06]

I was basing this IPC messaging on how the gamepad is implemented. The idea here
is to obtain a shared memory handle from the browser only (not to wait until
sensors spin up). I did a couple of measurements and it appears the sync call
takes approx 1.5 milliseconds. Is this worth an async call ? If you think it is
I'll add a somewhat redesigned messaging structure, but will need to adapt the
code to passing a renderer process id, because now the messaging is
renderer-agnostic so to speak.

+
+  bool valid_handle = base::SharedMemory::IsHandleValid(
+      renderer_shared_memory_handle_);
+  if (!valid_handle)
+    return;
+  renderer_shared_memory_.reset(new base::SharedMemory(
+      renderer_shared_memory_handle_, true));
+  CHECK(renderer_shared_memory_->Map(sizeof(DeviceMotionHardwareBuffer)));
+  void *memory = renderer_shared_memory_->memory();
+  CHECK(memory);

[darin (slow to review), 2013/05/29 06:41:53]

i have seen cases where it is difficult to map a large segment of shared memory
into the renderer.  maybe that'll come up rarely in this case.

[timvolodine, 2013/05/29 18:59:06]

I actually borrowed that code from the gamepad implementation..
maybe I should make it DCHECK and add an if to exit silently.

+  device_motion_hardware_buffer_ =
+      static_cast<DeviceMotionHardwareBuffer*>(memory);
+

[darin (slow to review), 2013/05/29 06:41:53]

nit: kill gratuitous new line

[timvolodine, 2013/05/29 18:59:06]

Done.

+}
+
+void DeviceMotionSharedMemoryReader::stopUpdating() {
+  CHECK(RenderThread::Get()->Send(new DeviceMotionHostMsg_StopPolling()));

[darin (slow to review), 2013/05/29 06:41:53]

why do you need to CHECK here?  this could fail during renderer / browser
shutdown.  are you sure you want that to be regarded as a renderer crash?

[timvolodine, 2013/05/29 18:59:06]

Done. since the StopPolling message is async now (it used to be sync).

+}
+
+} // namespace content