1// Copyright (c) 2012 The Chromium Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5#ifndef GPU_COMMAND_BUFFER_COMMON_COMMAND_BUFFER_SHARED_H_
6#define GPU_COMMAND_BUFFER_COMMON_COMMAND_BUFFER_SHARED_H_
7
8#include "command_buffer.h"
9#include "base/atomicops.h"
10
11namespace gpu {
12
13// This is a standard 4-slot asynchronous communication mechanism, used to
14// ensure that the reader gets a consistent copy of what the writer wrote.
15template<typename T>
16class SharedState {
17  T states_[2][2];
18  base::subtle::Atomic32 reading_;
19  base::subtle::Atomic32 latest_;
20  base::subtle::Atomic32 slots_[2];
21
22public:
23
24  void Initialize() {
25    for (int i = 0; i < 2; ++i) {
26      for (int j = 0; j < 2; ++j) {
27        states_[i][j] = T();
28      }
29    }
30    base::subtle::NoBarrier_Store(&reading_, 0);
31    base::subtle::NoBarrier_Store(&latest_, 0);
32    base::subtle::NoBarrier_Store(&slots_[0], 0);
33    base::subtle::Release_Store(&slots_[1], 0);
34    base::subtle::MemoryBarrier();
35  }
36
37  void Write(const T& state) {
38    int towrite = !base::subtle::Acquire_Load(&reading_);
39    int index = !base::subtle::Acquire_Load(&slots_[towrite]);
40    states_[towrite][index] = state;
41    base::subtle::Release_Store(&slots_[towrite], index);
42    base::subtle::Release_Store(&latest_, towrite);
43    base::subtle::MemoryBarrier();
44  }
45
46  // Attempt to update the state, updating only if the generation counter is
47  // newer.
48  void Read(T* state) {
49    base::subtle::MemoryBarrier();
50    int toread = !!base::subtle::Acquire_Load(&latest_);
51    base::subtle::Release_Store(&reading_, toread);
52    base::subtle::MemoryBarrier();
53    int index = !!base::subtle::Acquire_Load(&slots_[toread]);
54    if (states_[toread][index].generation - state->generation < 0x80000000U)
55      *state = states_[toread][index];
56  }
57};
58
59typedef SharedState<CommandBuffer::State> CommandBufferSharedState;
60
61}  // namespace gpu
62
63#endif  // GPU_COMMAND_BUFFER_COMMON_COMMAND_BUFFER_SHARED_H_
64