MonoPipe.h revision 767094dd98b01baf21de2ad09c27b3c98776cf73 
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
24namespace android {
25
26// MonoPipe is similar to Pipe except:
27//  - supports only a single reader, called MonoPipeReader
28//  - write() cannot overrun; instead it will return a short actual count if insufficient space
29//  - write() can optionally block if the pipe is full
30// Like Pipe, it is not multi-thread safe for either writer or reader
31// but writer and reader can be different threads.
32class MonoPipe : public NBAIO_Sink {
33
34    friend class MonoPipeReader;
35
36public:
37    // reqFrames will be rounded up to a power of 2, and all slots are available. Must be >= 2.
38    // Note: whatever shares this object with another thread needs to do so in an SMP-safe way (like
39    // creating it the object before creating the other thread, or storing the object with a
40    // release_store). Otherwise the other thread could see a partially-constructed object.
41    MonoPipe(size_t reqFrames, NBAIO_Format format, bool writeCanBlock = false);
42    virtual ~MonoPipe();
43
44    // NBAIO_Port interface
45
46    //virtual ssize_t negotiate(const NBAIO_Format offers[], size_t numOffers,
47    //                          NBAIO_Format counterOffers[], size_t& numCounterOffers);
48    //virtual NBAIO_Format format() const;
49
50    // NBAIO_Sink interface
51
52    //virtual size_t framesWritten() const;
53    //virtual size_t framesUnderrun() const;
54    //virtual size_t underruns() const;
55
56    virtual ssize_t availableToWrite() const;
57    virtual ssize_t write(const void *buffer, size_t count);
58    //virtual ssize_t writeVia(writeVia_t via, size_t total, void *user, size_t block);
59
60    // MonoPipe's implementation of getNextWriteTimestamp works in conjunction
61    // with MonoPipeReader.  Every time a MonoPipeReader reads from the pipe, it
62    // receives a "readPTS" indicating the point in time for which the reader
63    // would like to read data.  This "last read PTS" is offset by the amt of
64    // data the reader is currently mixing and then cached cached along with the
65    // updated read pointer.  This cached value is the local time for which the
66    // reader is going to request data next time it reads data (assuming we are
67    // in steady state and operating with no underflows).  Writers to the
68    // MonoPipe who would like to know when their next write operation will hit
69    // the speakers can call getNextWriteTimestamp which will return the value
70    // of the last read PTS plus the duration of the amt of data waiting to be
71    // read in the MonoPipe.
72    virtual status_t getNextWriteTimestamp(int64_t *timestamp);
73
74            // average number of frames present in the pipe under normal conditions.
75            // See throttling mechanism in MonoPipe::write()
76            size_t  getAvgFrames() const { return mSetpoint; }
77            void    setAvgFrames(size_t setpoint);
78            size_t  maxFrames() const { return mMaxFrames; }
79
80            // Set the shutdown state for the write side of a pipe.
81            // This may be called by an unrelated thread.  When shutdown state is 'true',
82            // a write that would otherwise block instead returns a short transfer count.
83            // There is no guarantee how long it will take for the shutdown to be recognized,
84            // but it will not be an unbounded amount of time.
85            // The state can be restored to normal by calling shutdown(false).
86            void    shutdown(bool newState = true);
87
88            // Return true if the write side of a pipe is currently shutdown.
89            bool    isShutdown();
90
91            // Return NO_ERROR if there is a timestamp available
92            status_t getTimestamp(AudioTimestamp& timestamp);
93
94private:
95    // A pair of methods and a helper variable which allows the reader and the
96    // writer to update and observe the values of mFront and mNextRdPTS in an
97    // atomic lock-less fashion.
98    //
99    // :: Important ::
100    // Two assumptions must be true in order for this lock-less approach to
101    // function properly on all systems.  First, there may only be one updater
102    // thread in the system.  Second, the updater thread must be running at a
103    // strictly higher priority than the observer threads.  Currently, both of
104    // these assumptions are true.  The only updater is always a single
105    // FastMixer thread (which runs with SCHED_FIFO/RT priority while the only
106    // observer is always an AudioFlinger::PlaybackThread running with
107    // traditional (non-RT) audio priority.
108    void updateFrontAndNRPTS(int32_t newFront, int64_t newNextRdPTS);
109    void observeFrontAndNRPTS(int32_t *outFront, int64_t *outNextRdPTS);
110    volatile int32_t mUpdateSeq;
111
112    const size_t    mReqFrames;     // as requested in constructor, unrounded
113    const size_t    mMaxFrames;     // always a power of 2
114    void * const    mBuffer;
115    // mFront and mRear will never be separated by more than mMaxFrames.
116    // 32-bit overflow is possible if the pipe is active for a long time, but if that happens it's
117    // safe because we "&" with (mMaxFrames-1) at end of computations to calculate a buffer index.
118    volatile int32_t mFront;        // written by the reader with updateFrontAndNRPTS, observed by
119                                    // the writer with observeFrontAndNRPTS
120    volatile int32_t mRear;         // written by writer with android_atomic_release_store,
121                                    // read by reader with android_atomic_acquire_load
122    volatile int64_t mNextRdPTS;    // written by the reader with updateFrontAndNRPTS, observed by
123                                    // the writer with observeFrontAndNRPTS
124    bool            mWriteTsValid;  // whether mWriteTs is valid
125    struct timespec mWriteTs;       // time that the previous write() completed
126    size_t          mSetpoint;      // target value for pipe fill depth
127    const bool      mWriteCanBlock; // whether write() should block if the pipe is full
128
129    int64_t offsetTimestampByAudioFrames(int64_t ts, size_t audFrames);
130    LinearTransform mSamplesToLocalTime;
131
132    bool            mIsShutdown;    // whether shutdown(true) was called, no barriers are needed
133};
134
135}   // namespace android
136
137#endif  // ANDROID_AUDIO_MONO_PIPE_H
138