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216 lines
11 KiB
216 lines
11 KiB
/*
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* Copyright (C) 2012 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#ifndef ANDROID_AUDIO_STATE_QUEUE_H
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#define ANDROID_AUDIO_STATE_QUEUE_H
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#include <stdatomic.h>
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// The state queue template class was originally driven by this use case / requirements:
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// There are two threads: a fast mixer, and a normal mixer, and they share state.
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// The interesting part of the shared state is a set of active fast tracks,
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// and the output HAL configuration (buffer size in frames, sample rate, etc.).
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// Fast mixer thread:
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// periodic with typical period < 10 ms
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// FIFO/RR scheduling policy and a low fixed priority
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// ok to block for bounded time using nanosleep() to achieve desired period
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// must not block on condition wait, mutex lock, atomic operation spin, I/O, etc.
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// under typical operations of mixing, writing, or adding/removing tracks
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// ok to block for unbounded time when the output HAL configuration changes,
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// and this may result in an audible artifact
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// needs read-only access to a recent stable state,
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// but not necessarily the most current one
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// only allocate and free memory when configuration changes
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// avoid conventional logging, as this is a form of I/O and could block
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// defer computation to other threads when feasible; for example
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// cycle times are collected by fast mixer thread but the floating-point
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// statistical calculations on these cycle times are computed by normal mixer
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// these requirements also apply to callouts such as AudioBufferProvider and VolumeProvider
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// Normal mixer thread:
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// periodic with typical period ~20 ms
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// SCHED_OTHER scheduling policy and nice priority == urgent audio
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// ok to block, but prefer to avoid as much as possible
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// needs read/write access to state
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// The normal mixer may need to temporarily suspend the fast mixer thread during mode changes.
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// It will do this using the state -- one of the fields tells the fast mixer to idle.
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// Additional requirements:
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// - observer must always be able to poll for and view the latest pushed state; it must never be
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// blocked from seeing that state
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// - observer does not need to see every state in sequence; it is OK for it to skip states
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// [see below for more on this]
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// - mutator must always be able to read/modify a state, it must never be blocked from reading or
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// modifying state
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// - reduce memcpy where possible
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// - work well if the observer runs more frequently than the mutator,
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// as is the case with fast mixer/normal mixer.
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// It is not a requirement to work well if the roles were reversed,
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// and the mutator were to run more frequently than the observer.
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// In this case, the mutator could get blocked waiting for a slot to fill up for
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// it to work with. This could be solved somewhat by increasing the depth of the queue, but it would
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// still limit the mutator to a finite number of changes before it would block. A future
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// possibility, not implemented here, would be to allow the mutator to safely overwrite an already
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// pushed state. This could be done by the mutator overwriting mNext, but then being prepared to
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// read an mAck which is actually for the earlier mNext (since there is a race).
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// Solution:
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// Let's call the fast mixer thread the "observer" and normal mixer thread the "mutator".
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// We assume there is only a single observer and a single mutator; this is critical.
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// Each state is of type <T>, and should contain only POD (Plain Old Data) and raw pointers, as
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// memcpy() may be used to copy state, and the destructors are run in unpredictable order.
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// The states in chronological order are: previous, current, next, and mutating:
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// previous read-only, observer can compare vs. current to see the subset that changed
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// current read-only, this is the primary state for observer
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// next read-only, when observer is ready to accept a new state it will shift it in:
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// previous = current
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// current = next
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// and the slot formerly used by previous is now available to the mutator.
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// mutating invisible to observer, read/write to mutator
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// Initialization is tricky, especially for the observer. If the observer starts execution
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// before the mutator, there are no previous, current, or next states. And even if the observer
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// starts execution after the mutator, there is a next state but no previous or current states.
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// To solve this, we'll have the observer idle until there is a next state,
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// and it will have to deal with the case where there is no previous state.
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// The states are stored in a shared FIFO queue represented using a circular array.
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// The observer polls for mutations, and receives a new state pointer after a
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// a mutation is pushed onto the queue. To the observer, the state pointers are
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// effectively in random order, that is the observer should not do address
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// arithmetic on the state pointers. However to the mutator, the state pointers
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// are in a definite circular order.
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#include "Configuration.h"
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namespace android {
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#ifdef STATE_QUEUE_DUMP
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// The StateQueueObserverDump and StateQueueMutatorDump keep
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// a cache of StateQueue statistics that can be logged by dumpsys.
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// Each individual native word-sized field is accessed atomically. But the
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// overall structure is non-atomic, that is there may be an inconsistency between fields.
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// No barriers or locks are used for either writing or reading.
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// Only POD types are permitted, and the contents shouldn't be trusted (i.e. do range checks).
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// It has a different lifetime than the StateQueue, and so it can't be a member of StateQueue.
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struct StateQueueObserverDump {
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StateQueueObserverDump() : mStateChanges(0) { }
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/*virtual*/ ~StateQueueObserverDump() { }
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unsigned mStateChanges; // incremented each time poll() detects a state change
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void dump(int fd);
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};
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struct StateQueueMutatorDump {
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StateQueueMutatorDump() : mPushDirty(0), mPushAck(0), mBlockedSequence(0) { }
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/*virtual*/ ~StateQueueMutatorDump() { }
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unsigned mPushDirty; // incremented each time push() is called with a dirty state
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unsigned mPushAck; // incremented each time push(BLOCK_UNTIL_ACKED) is called
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unsigned mBlockedSequence; // incremented before and after each time that push()
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// blocks for more than one PUSH_BLOCK_ACK_NS;
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// if odd, then mutator is currently blocked inside push()
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void dump(int fd);
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};
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#endif
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// manages a FIFO queue of states
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template<typename T> class StateQueue {
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public:
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StateQueue();
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virtual ~StateQueue();
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// Observer APIs
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// Poll for a state change. Returns a pointer to a read-only state,
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// or NULL if the state has not been initialized yet.
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// If a new state has not pushed by mutator since the previous poll,
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// then the returned pointer will be unchanged.
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// The previous state pointer is guaranteed to still be valid;
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// this allows the observer to diff the previous and new states.
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const T* poll();
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// Mutator APIs
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// Begin a mutation. Returns a pointer to a read/write state, except the
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// first time it is called the state is write-only and _must_ be initialized.
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// Mutations cannot be nested.
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// If the state is dirty and has not been pushed onto the state queue yet, then
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// this new mutation will be squashed together with the previous one.
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T* begin();
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// End the current mutation and indicate whether caller modified the state.
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// If didModify is true, then the state is marked dirty (in need of pushing).
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// There is no rollback option because modifications are done in place.
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// Does not automatically push the new state onto the state queue.
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void end(bool didModify = true);
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// Push a new state, if any, out to the observer via the state queue.
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// For BLOCK_NEVER, returns:
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// true if not dirty, or dirty and pushed successfully
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// false if dirty and not pushed because that would block; remains dirty
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// For BLOCK_UNTIL_PUSHED and BLOCK_UNTIL_ACKED, always returns true.
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// No-op if there are no pending modifications (not dirty), except
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// for BLOCK_UNTIL_ACKED it will wait until a prior push has been acknowledged.
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// Must not be called in the middle of a mutation.
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enum block_t {
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BLOCK_NEVER, // do not block
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BLOCK_UNTIL_PUSHED, // block until there's a slot available for the push
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BLOCK_UNTIL_ACKED, // also block until the push is acknowledged by the observer
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};
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bool push(block_t block = BLOCK_NEVER);
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// Return whether the current state is dirty (modified and not pushed).
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bool isDirty() const { return mIsDirty; }
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#ifdef STATE_QUEUE_DUMP
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// Register location of observer dump area
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void setObserverDump(StateQueueObserverDump *dump)
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{ mObserverDump = dump != NULL ? dump : &mObserverDummyDump; }
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// Register location of mutator dump area
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void setMutatorDump(StateQueueMutatorDump *dump)
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{ mMutatorDump = dump != NULL ? dump : &mMutatorDummyDump; }
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#endif
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private:
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static const unsigned kN = 4; // values < 4 are not supported by this code
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T mStates[kN]; // written by mutator, read by observer
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// "volatile" is meaningless with SMP, but here it indicates that we're using atomic ops
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atomic_uintptr_t mNext; // written by mutator to advance next, read by observer
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volatile const T* mAck; // written by observer to acknowledge advance of next, read by mutator
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// only used by observer
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const T* mCurrent; // most recent value returned by poll()
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// only used by mutator
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T* mMutating; // where updates by mutator are done in place
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const T* mExpecting; // what the mutator expects mAck to be set to
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bool mInMutation; // whether we're currently in the middle of a mutation
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bool mIsDirty; // whether mutating state has been modified since last push
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bool mIsInitialized; // whether mutating state has been initialized yet
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#ifdef STATE_QUEUE_DUMP
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StateQueueObserverDump mObserverDummyDump; // default area for observer dump if not set
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StateQueueObserverDump* mObserverDump; // pointer to active observer dump, always non-NULL
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StateQueueMutatorDump mMutatorDummyDump; // default area for mutator dump if not set
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StateQueueMutatorDump* mMutatorDump; // pointer to active mutator dump, always non-NULL
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#endif
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}; // class StateQueue
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} // namespace android
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#endif // ANDROID_AUDIO_STATE_QUEUE_H
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