1 /*
2 * Copyright (C) 2012 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17 #include <new>
18 #include <cutils/atomic.h>
19 #include <cutils/atomic-inline.h> // for android_memory_barrier()
20 #include <media/SingleStateQueue.h>
21
22 namespace android {
23
Mutator(Shared * shared)24 template<typename T> SingleStateQueue<T>::Mutator::Mutator(Shared *shared)
25 : mSequence(0), mShared((Shared *) shared)
26 {
27 // exactly one of Mutator and Observer must initialize, currently it is Observer
28 //shared->init();
29 }
30
push(const T & value)31 template<typename T> int32_t SingleStateQueue<T>::Mutator::push(const T& value)
32 {
33 Shared *shared = mShared;
34 int32_t sequence = mSequence;
35 sequence++;
36 android_atomic_acquire_store(sequence, &shared->mSequence);
37 shared->mValue = value;
38 sequence++;
39 android_atomic_release_store(sequence, &shared->mSequence);
40 mSequence = sequence;
41 // consider signalling a futex here, if we know that observer is waiting
42 return sequence;
43 }
44
ack()45 template<typename T> bool SingleStateQueue<T>::Mutator::ack()
46 {
47 return mShared->mAck - mSequence == 0;
48 }
49
ack(int32_t sequence)50 template<typename T> bool SingleStateQueue<T>::Mutator::ack(int32_t sequence)
51 {
52 // this relies on 2's complement rollover to detect an ancient sequence number
53 return mShared->mAck - sequence >= 0;
54 }
55
Observer(Shared * shared)56 template<typename T> SingleStateQueue<T>::Observer::Observer(Shared *shared)
57 : mSequence(0), mSeed(1), mShared((Shared *) shared)
58 {
59 // exactly one of Mutator and Observer must initialize, currently it is Observer
60 shared->init();
61 }
62
poll(T & value)63 template<typename T> bool SingleStateQueue<T>::Observer::poll(T& value)
64 {
65 Shared *shared = mShared;
66 int32_t before = shared->mSequence;
67 if (before == mSequence) {
68 return false;
69 }
70 for (int tries = 0; ; ) {
71 const int MAX_TRIES = 5;
72 if (before & 1) {
73 if (++tries >= MAX_TRIES) {
74 return false;
75 }
76 before = shared->mSequence;
77 } else {
78 android_memory_barrier();
79 T temp = shared->mValue;
80 int32_t after = android_atomic_release_load(&shared->mSequence);
81 if (after == before) {
82 value = temp;
83 shared->mAck = before;
84 mSequence = before;
85 return true;
86 }
87 if (++tries >= MAX_TRIES) {
88 return false;
89 }
90 before = after;
91 }
92 }
93 }
94
95 #if 0
96 template<typename T> SingleStateQueue<T>::SingleStateQueue(void /*Shared*/ *shared)
97 {
98 ((Shared *) shared)->init();
99 }
100 #endif
101
102 } // namespace android
103
104 // hack for gcc
105 #ifdef SINGLE_STATE_QUEUE_INSTANTIATIONS
106 #include SINGLE_STATE_QUEUE_INSTANTIATIONS
107 #endif
108