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 
25 namespace android {
26 
27 typedef 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.
35 class MonoPipe : public NBAIO_Sink {
36 
37     friend class MonoPipeReader;
38 
39 public:
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, const 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()
getAvgFrames()79             size_t  getAvgFrames() const { return mSetpoint; }
80             void    setAvgFrames(size_t setpoint);
maxFrames()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 
97 private:
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