MonoPipe.h revision a07a1c2c91dc7ee6ded319262499f20cd01edcf7 
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_MONO_PIPE_H
18#define ANDROID_AUDIO_MONO_PIPE_H
19
20#include <time.h>
21#include <utils/LinearTransform.h>
22#include "NBAIO.h"
23#include <media/SingleStateQueue.h>
24
25namespace android {
26
27typedef SingleStateQueue<AudioTimestamp> AudioTimestampSingleStateQueue;
28
29// MonoPipe is similar to Pipe except:
30//  - supports only a single reader, called MonoPipeReader
31//  - write() cannot overrun; instead it will return a short actual count if insufficient space
32//  - write() can optionally block if the pipe is full
33// Like Pipe, it is not multi-thread safe for either writer or reader
34// but writer and reader can be different threads.
35class MonoPipe : public NBAIO_Sink {
36
37    friend class MonoPipeReader;
38
39public:
40    // reqFrames will be rounded up to a power of 2, and all slots are available. Must be >= 2.
41    // Note: whatever shares this object with another thread needs to do so in an SMP-safe way (like
42    // creating it the object before creating the other thread, or storing the object with a
43    // release_store). Otherwise the other thread could see a partially-constructed object.
44    MonoPipe(size_t reqFrames, NBAIO_Format format, bool writeCanBlock = false);
45    virtual ~MonoPipe();
46
47    // NBAIO_Port interface
48
49    //virtual ssize_t negotiate(const NBAIO_Format offers[], size_t numOffers,
50    //                          NBAIO_Format counterOffers[], size_t& numCounterOffers);
51    //virtual NBAIO_Format format() const;
52
53    // NBAIO_Sink interface
54
55    //virtual size_t framesWritten() const;
56    //virtual size_t framesUnderrun() const;
57    //virtual size_t underruns() const;
58
59    virtual ssize_t availableToWrite() const;
60    virtual ssize_t write(const void *buffer, size_t count);
61    //virtual ssize_t writeVia(writeVia_t via, size_t total, void *user, size_t block);
62
63    // MonoPipe's implementation of getNextWriteTimestamp works in conjunction
64    // with MonoPipeReader.  Every time a MonoPipeReader reads from the pipe, it
65    // receives a "readPTS" indicating the point in time for which the reader
66    // would like to read data.  This "last read PTS" is offset by the amt of
67    // data the reader is currently mixing and then cached cached along with the
68    // updated read pointer.  This cached value is the local time for which the
69    // reader is going to request data next time it reads data (assuming we are
70    // in steady state and operating with no underflows).  Writers to the
71    // MonoPipe who would like to know when their next write operation will hit
72    // the speakers can call getNextWriteTimestamp which will return the value
73    // of the last read PTS plus the duration of the amt of data waiting to be
74    // read in the MonoPipe.
75    virtual status_t getNextWriteTimestamp(int64_t *timestamp);
76
77            // average number of frames present in the pipe under normal conditions.
78            // See throttling mechanism in MonoPipe::write()
79            size_t  getAvgFrames() const { return mSetpoint; }
80            void    setAvgFrames(size_t setpoint);
81            size_t  maxFrames() const { return mMaxFrames; }
82
83            // Set the shutdown state for the write side of a pipe.
84            // This may be called by an unrelated thread.  When shutdown state is 'true',
85            // a write that would otherwise block instead returns a short transfer count.
86            // There is no guarantee how long it will take for the shutdown to be recognized,
87            // but it will not be an unbounded amount of time.
88            // The state can be restored to normal by calling shutdown(false).
89            void    shutdown(bool newState = true);
90
91            // Return true if the write side of a pipe is currently shutdown.
92            bool    isShutdown();
93
94            // Return NO_ERROR if there is a timestamp available
95            status_t getTimestamp(AudioTimestamp& timestamp);
96
97private:
98    // A pair of methods and a helper variable which allows the reader and the
99    // writer to update and observe the values of mFront and mNextRdPTS in an
100    // atomic lock-less fashion.
101    //
102    // :: Important ::
103    // Two assumptions must be true in order for this lock-less approach to
104    // function properly on all systems.  First, there may only be one updater
105    // thread in the system.  Second, the updater thread must be running at a
106    // strictly higher priority than the observer threads.  Currently, both of
107    // these assumptions are true.  The only updater is always a single
108    // FastMixer thread (which runs with SCHED_FIFO/RT priority while the only
109    // observer is always an AudioFlinger::PlaybackThread running with
110    // traditional (non-RT) audio priority.
111    void updateFrontAndNRPTS(int32_t newFront, int64_t newNextRdPTS);
112    void observeFrontAndNRPTS(int32_t *outFront, int64_t *outNextRdPTS);
113    volatile int32_t mUpdateSeq;
114
115    const size_t    mReqFrames;     // as requested in constructor, unrounded
116    const size_t    mMaxFrames;     // always a power of 2
117    void * const    mBuffer;
118    // mFront and mRear will never be separated by more than mMaxFrames.
119    // 32-bit overflow is possible if the pipe is active for a long time, but if that happens it's
120    // safe because we "&" with (mMaxFrames-1) at end of computations to calculate a buffer index.
121    volatile int32_t mFront;        // written by the reader with updateFrontAndNRPTS, observed by
122                                    // the writer with observeFrontAndNRPTS
123    volatile int32_t mRear;         // written by writer with android_atomic_release_store,
124                                    // read by reader with android_atomic_acquire_load
125    volatile int64_t mNextRdPTS;    // written by the reader with updateFrontAndNRPTS, observed by
126                                    // the writer with observeFrontAndNRPTS
127    bool            mWriteTsValid;  // whether mWriteTs is valid
128    struct timespec mWriteTs;       // time that the previous write() completed
129    size_t          mSetpoint;      // target value for pipe fill depth
130    const bool      mWriteCanBlock; // whether write() should block if the pipe is full
131
132    int64_t offsetTimestampByAudioFrames(int64_t ts, size_t audFrames);
133    LinearTransform mSamplesToLocalTime;
134
135    bool            mIsShutdown;    // whether shutdown(true) was called, no barriers are needed
136
137    AudioTimestampSingleStateQueue::Shared      mTimestampShared;
138    AudioTimestampSingleStateQueue::Mutator     mTimestampMutator;
139    AudioTimestampSingleStateQueue::Observer    mTimestampObserver;
140};
141
142}   // namespace android
143
144#endif  // ANDROID_AUDIO_MONO_PIPE_H
145