StateQueue.h revision 399930859a75d806ce0ef124ac22025ae4ef0549 
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#ifndef ANDROID_AUDIO_STATE_QUEUE_H
18#define ANDROID_AUDIO_STATE_QUEUE_H
19
20namespace android {
21
22#ifdef STATE_QUEUE_DUMP
23// The StateQueueObserverDump and StateQueueMutatorDump keep
24// a cache of StateQueue statistics that can be logged by dumpsys.
25// Each individual native word-sized field is accessed atomically.  But the
26// overall structure is non-atomic, that is there may be an inconsistency between fields.
27// No barriers or locks are used for either writing or reading.
28// Only POD types are permitted, and the contents shouldn't be trusted (i.e. do range checks).
29// It has a different lifetime than the StateQueue, and so it can't be a member of StateQueue.
30
31struct StateQueueObserverDump {
32    StateQueueObserverDump() : mStateChanges(0) { }
33    /*virtual*/ ~StateQueueObserverDump() { }
34    unsigned    mStateChanges;    // incremented each time poll() detects a state change
35    void        dump(int fd);
36};
37
38struct StateQueueMutatorDump {
39    StateQueueMutatorDump() : mPushDirty(0), mPushAck(0), mBlockedSequence(0) { }
40    /*virtual*/ ~StateQueueMutatorDump() { }
41    unsigned    mPushDirty;       // incremented each time push() is called with a dirty state
42    unsigned    mPushAck;         // incremented each time push(BLOCK_UNTIL_ACKED) is called
43    unsigned    mBlockedSequence; // incremented before and after each time that push()
44                                  // blocks for more than one PUSH_BLOCK_ACK_NS;
45                                  // if odd, then mutator is currently blocked inside push()
46    void        dump(int fd);
47};
48#endif
49
50// manages a FIFO queue of states
51template<typename T> class StateQueue {
52
53public:
54            StateQueue();
55    virtual ~StateQueue();
56
57    // Observer APIs
58
59    // Poll for a state change.  Returns a pointer to a read-only state,
60    // or NULL if the state has not been initialized yet.
61    // If a new state has not pushed by mutator since the previous poll,
62    // then the returned pointer will be unchanged.
63    // The previous state pointer is guaranteed to still be valid;
64    // this allows the observer to diff the previous and new states.
65    const T* poll();
66
67    // Mutator APIs
68
69    // Begin a mutation.  Returns a pointer to a read/write state, except the
70    // first time it is called the state is write-only and _must_ be initialized.
71    // Mutations cannot be nested.
72    // If the state is dirty and has not been pushed onto the state queue yet, then
73    // this new mutation will be squashed together with the previous one.
74    T*      begin();
75
76    // End the current mutation and indicate whether caller modified the state.
77    // If didModify is true, then the state is marked dirty (in need of pushing).
78    // There is no rollback option because modifications are done in place.
79    // Does not automatically push the new state onto the state queue.
80    void    end(bool didModify = true);
81
82    // Push a new state, if any, out to the observer via the state queue.
83    // For BLOCK_NEVER, returns:
84    //      true if not dirty, or dirty and pushed successfully
85    //      false if dirty and not pushed because that would block; remains dirty
86    // For BLOCK_UNTIL_PUSHED and BLOCK_UNTIL_ACKED, always returns true.
87    // No-op if there are no pending modifications (not dirty), except
88    //      for BLOCK_UNTIL_ACKED it will wait until a prior push has been acknowledged.
89    // Must not be called in the middle of a mutation.
90    enum block_t {
91        BLOCK_NEVER,        // do not block
92        BLOCK_UNTIL_PUSHED, // block until there's a slot available for the push
93        BLOCK_UNTIL_ACKED,  // also block until the push is acknowledged by the observer
94    };
95    bool    push(block_t block = BLOCK_NEVER);
96
97    // Return whether the current state is dirty (modified and not pushed).
98    bool    isDirty() const { return mIsDirty; }
99
100#ifdef STATE_QUEUE_DUMP
101    // Register location of observer dump area
102    void    setObserverDump(StateQueueObserverDump *dump)
103            { mObserverDump = dump != NULL ? dump : &mObserverDummyDump; }
104
105    // Register location of mutator dump area
106    void    setMutatorDump(StateQueueMutatorDump *dump)
107            { mMutatorDump = dump != NULL ? dump : &mMutatorDummyDump; }
108#endif
109
110private:
111    static const unsigned kN = 4;       // values != 4 are not supported by this code
112    T                 mStates[kN];      // written by mutator, read by observer
113
114    // "volatile" is meaningless with SMP, but here it indicates that we're using atomic ops
115    volatile const T* mNext; // written by mutator to advance next, read by observer
116    volatile const T* mAck;  // written by observer to acknowledge advance of next, read by mutator
117
118    // only used by observer
119    const T*          mCurrent;         // most recent value returned by poll()
120
121    // only used by mutator
122    T*                mMutating;        // where updates by mutator are done in place
123    const T*          mExpecting;       // what the mutator expects mAck to be set to
124    bool              mInMutation;      // whether we're currently in the middle of a mutation
125    bool              mIsDirty;         // whether mutating state has been modified since last push
126    bool              mIsInitialized;   // whether mutating state has been initialized yet
127
128#ifdef STATE_QUEUE_DUMP
129    StateQueueObserverDump  mObserverDummyDump; // default area for observer dump if not set
130    StateQueueObserverDump* mObserverDump;      // pointer to active observer dump, always non-NULL
131    StateQueueMutatorDump   mMutatorDummyDump;  // default area for mutator dump if not set
132    StateQueueMutatorDump*  mMutatorDump;       // pointer to active mutator dump, always non-NULL
133#endif
134
135};  // class StateQueue
136
137}   // namespace android
138
139#endif  // ANDROID_AUDIO_STATE_QUEUE_H
140