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 20 // The state queue template class was originally driven by this use case / requirements: 21 // There are two threads: a fast mixer, and a normal mixer, and they share state. 22 // The interesting part of the shared state is a set of active fast tracks, 23 // and the output HAL configuration (buffer size in frames, sample rate, etc.). 24 // Fast mixer thread: 25 // periodic with typical period < 10 ms 26 // FIFO/RR scheduling policy and a low fixed priority 27 // ok to block for bounded time using nanosleep() to achieve desired period 28 // must not block on condition wait, mutex lock, atomic operation spin, I/O, etc. 29 // under typical operations of mixing, writing, or adding/removing tracks 30 // ok to block for unbounded time when the output HAL configuration changes, 31 // and this may result in an audible artifact 32 // needs read-only access to a recent stable state, 33 // but not necessarily the most current one 34 // only allocate and free memory when configuration changes 35 // avoid conventional logging, as this is a form of I/O and could block 36 // defer computation to other threads when feasible; for example 37 // cycle times are collected by fast mixer thread but the floating-point 38 // statistical calculations on these cycle times are computed by normal mixer 39 // these requirements also apply to callouts such as AudioBufferProvider and VolumeProvider 40 // Normal mixer thread: 41 // periodic with typical period ~20 ms 42 // SCHED_OTHER scheduling policy and nice priority == urgent audio 43 // ok to block, but prefer to avoid as much as possible 44 // needs read/write access to state 45 // The normal mixer may need to temporarily suspend the fast mixer thread during mode changes. 46 // It will do this using the state -- one of the fields tells the fast mixer to idle. 47 48 // Additional requirements: 49 // - observer must always be able to poll for and view the latest pushed state; it must never be 50 // blocked from seeing that state 51 // - observer does not need to see every state in sequence; it is OK for it to skip states 52 // [see below for more on this] 53 // - mutator must always be able to read/modify a state, it must never be blocked from reading or 54 // modifying state 55 // - reduce memcpy where possible 56 // - work well if the observer runs more frequently than the mutator, 57 // as is the case with fast mixer/normal mixer. 58 // It is not a requirement to work well if the roles were reversed, 59 // and the mutator were to run more frequently than the observer. 60 // In this case, the mutator could get blocked waiting for a slot to fill up for 61 // it to work with. This could be solved somewhat by increasing the depth of the queue, but it would 62 // still limit the mutator to a finite number of changes before it would block. A future 63 // possibility, not implemented here, would be to allow the mutator to safely overwrite an already 64 // pushed state. This could be done by the mutator overwriting mNext, but then being prepared to 65 // read an mAck which is actually for the earlier mNext (since there is a race). 66 67 // Solution: 68 // Let's call the fast mixer thread the "observer" and normal mixer thread the "mutator". 69 // We assume there is only a single observer and a single mutator; this is critical. 70 // Each state is of type <T>, and should contain only POD (Plain Old Data) and raw pointers, as 71 // memcpy() may be used to copy state, and the destructors are run in unpredictable order. 72 // The states in chronological order are: previous, current, next, and mutating: 73 // previous read-only, observer can compare vs. current to see the subset that changed 74 // current read-only, this is the primary state for observer 75 // next read-only, when observer is ready to accept a new state it will shift it in: 76 // previous = current 77 // current = next 78 // and the slot formerly used by previous is now available to the mutator. 79 // mutating invisible to observer, read/write to mutator 80 // Initialization is tricky, especially for the observer. If the observer starts execution 81 // before the mutator, there are no previous, current, or next states. And even if the observer 82 // starts execution after the mutator, there is a next state but no previous or current states. 83 // To solve this, we'll have the observer idle until there is a next state, 84 // and it will have to deal with the case where there is no previous state. 85 // The states are stored in a shared FIFO queue represented using a circular array. 86 // The observer polls for mutations, and receives a new state pointer after a 87 // a mutation is pushed onto the queue. To the observer, the state pointers are 88 // effectively in random order, that is the observer should not do address 89 // arithmetic on the state pointers. However to the mutator, the state pointers 90 // are in a definite circular order. 91 92 namespace android { 93 94 #ifdef STATE_QUEUE_DUMP 95 // The StateQueueObserverDump and StateQueueMutatorDump keep 96 // a cache of StateQueue statistics that can be logged by dumpsys. 97 // Each individual native word-sized field is accessed atomically. But the 98 // overall structure is non-atomic, that is there may be an inconsistency between fields. 99 // No barriers or locks are used for either writing or reading. 100 // Only POD types are permitted, and the contents shouldn't be trusted (i.e. do range checks). 101 // It has a different lifetime than the StateQueue, and so it can't be a member of StateQueue. 102 103 struct StateQueueObserverDump { 104 StateQueueObserverDump() : mStateChanges(0) { } 105 /*virtual*/ ~StateQueueObserverDump() { } 106 unsigned mStateChanges; // incremented each time poll() detects a state change 107 void dump(int fd); 108 }; 109 110 struct StateQueueMutatorDump { 111 StateQueueMutatorDump() : mPushDirty(0), mPushAck(0), mBlockedSequence(0) { } 112 /*virtual*/ ~StateQueueMutatorDump() { } 113 unsigned mPushDirty; // incremented each time push() is called with a dirty state 114 unsigned mPushAck; // incremented each time push(BLOCK_UNTIL_ACKED) is called 115 unsigned mBlockedSequence; // incremented before and after each time that push() 116 // blocks for more than one PUSH_BLOCK_ACK_NS; 117 // if odd, then mutator is currently blocked inside push() 118 void dump(int fd); 119 }; 120 #endif 121 122 // manages a FIFO queue of states 123 template<typename T> class StateQueue { 124 125 public: 126 StateQueue(); 127 virtual ~StateQueue(); 128 129 // Observer APIs 130 131 // Poll for a state change. Returns a pointer to a read-only state, 132 // or NULL if the state has not been initialized yet. 133 // If a new state has not pushed by mutator since the previous poll, 134 // then the returned pointer will be unchanged. 135 // The previous state pointer is guaranteed to still be valid; 136 // this allows the observer to diff the previous and new states. 137 const T* poll(); 138 139 // Mutator APIs 140 141 // Begin a mutation. Returns a pointer to a read/write state, except the 142 // first time it is called the state is write-only and _must_ be initialized. 143 // Mutations cannot be nested. 144 // If the state is dirty and has not been pushed onto the state queue yet, then 145 // this new mutation will be squashed together with the previous one. 146 T* begin(); 147 148 // End the current mutation and indicate whether caller modified the state. 149 // If didModify is true, then the state is marked dirty (in need of pushing). 150 // There is no rollback option because modifications are done in place. 151 // Does not automatically push the new state onto the state queue. 152 void end(bool didModify = true); 153 154 // Push a new state, if any, out to the observer via the state queue. 155 // For BLOCK_NEVER, returns: 156 // true if not dirty, or dirty and pushed successfully 157 // false if dirty and not pushed because that would block; remains dirty 158 // For BLOCK_UNTIL_PUSHED and BLOCK_UNTIL_ACKED, always returns true. 159 // No-op if there are no pending modifications (not dirty), except 160 // for BLOCK_UNTIL_ACKED it will wait until a prior push has been acknowledged. 161 // Must not be called in the middle of a mutation. 162 enum block_t { 163 BLOCK_NEVER, // do not block 164 BLOCK_UNTIL_PUSHED, // block until there's a slot available for the push 165 BLOCK_UNTIL_ACKED, // also block until the push is acknowledged by the observer 166 }; 167 bool push(block_t block = BLOCK_NEVER); 168 169 // Return whether the current state is dirty (modified and not pushed). 170 bool isDirty() const { return mIsDirty; } 171 172 #ifdef STATE_QUEUE_DUMP 173 // Register location of observer dump area 174 void setObserverDump(StateQueueObserverDump *dump) 175 { mObserverDump = dump != NULL ? dump : &mObserverDummyDump; } 176 177 // Register location of mutator dump area 178 void setMutatorDump(StateQueueMutatorDump *dump) 179 { mMutatorDump = dump != NULL ? dump : &mMutatorDummyDump; } 180 #endif 181 182 private: 183 static const unsigned kN = 4; // values < 4 are not supported by this code 184 T mStates[kN]; // written by mutator, read by observer 185 186 // "volatile" is meaningless with SMP, but here it indicates that we're using atomic ops 187 volatile const T* mNext; // written by mutator to advance next, read by observer 188 volatile const T* mAck; // written by observer to acknowledge advance of next, read by mutator 189 190 // only used by observer 191 const T* mCurrent; // most recent value returned by poll() 192 193 // only used by mutator 194 T* mMutating; // where updates by mutator are done in place 195 const T* mExpecting; // what the mutator expects mAck to be set to 196 bool mInMutation; // whether we're currently in the middle of a mutation 197 bool mIsDirty; // whether mutating state has been modified since last push 198 bool mIsInitialized; // whether mutating state has been initialized yet 199 200 #ifdef STATE_QUEUE_DUMP 201 StateQueueObserverDump mObserverDummyDump; // default area for observer dump if not set 202 StateQueueObserverDump* mObserverDump; // pointer to active observer dump, always non-NULL 203 StateQueueMutatorDump mMutatorDummyDump; // default area for mutator dump if not set 204 StateQueueMutatorDump* mMutatorDump; // pointer to active mutator dump, always non-NULL 205 #endif 206 207 }; // class StateQueue 208 209 } // namespace android 210 211 #endif // ANDROID_AUDIO_STATE_QUEUE_H 212
