xref: /frameworks/av/media/libaudioclient/AudioTrackShared.cpp

/*
 * Copyright (C) 2007 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#define LOG_TAG "AudioTrackShared"
//#define LOG_NDEBUG 0

#include <private/media/AudioTrackShared.h>
#include <utils/Log.h>
extern "C" {
#include "../private/bionic_futex.h"
}

namespace android {

audio_track_cblk_t::audio_track_cblk_t()
    : server(0), frameCount_(0), mFutex(0), mMinimum(0),
    mVolumeLR(0x10001000), mSampleRate(0), mSendLevel(0), mName(0), flags(0)
{
    memset(&u, 0, sizeof(u));
}

// ---------------------------------------------------------------------------

Proxy::Proxy(audio_track_cblk_t* cblk, void *buffers, size_t frameCount, size_t frameSize,
        bool isOut, bool clientInServer)
    : mCblk(cblk), mBuffers(buffers), mFrameCount(frameCount), mFrameSize(frameSize),
      mFrameCountP2(roundup(frameCount)), mIsOut(isOut), mClientInServer(clientInServer),
      mIsShutdown(false), mUnreleased(0)
{
}

// ---------------------------------------------------------------------------

ClientProxy::ClientProxy(audio_track_cblk_t* cblk, void *buffers, size_t frameCount,
        size_t frameSize, bool isOut, bool clientInServer)
    : Proxy(cblk, buffers, frameCount, frameSize, isOut, clientInServer), mEpoch(0)
{
}

const struct timespec ClientProxy::kForever = {INT_MAX /*tv_sec*/, 0 /*tv_nsec*/};
const struct timespec ClientProxy::kNonBlocking = {0 /*tv_sec*/, 0 /*tv_nsec*/};

#define MEASURE_NS 10000000 // attempt to provide accurate timeouts if requested >= MEASURE_NS

// To facilitate quicker recovery from server failure, this value limits the timeout per each futex
// wait.  However it does not protect infinite timeouts.  If defined to be zero, there is no limit.
// FIXME May not be compatible with audio tunneling requirements where timeout should be in the
// order of minutes.
#define MAX_SEC    5

status_t ClientProxy::obtainBuffer(Buffer* buffer, const struct timespec *requested,
        struct timespec *elapsed)
{
    LOG_ALWAYS_FATAL_IF(buffer == NULL || buffer->mFrameCount == 0);
    struct timespec total;          // total elapsed time spent waiting
    total.tv_sec = 0;
    total.tv_nsec = 0;
    bool measure = elapsed != NULL; // whether to measure total elapsed time spent waiting

    status_t status;
    enum {
        TIMEOUT_ZERO,       // requested == NULL || *requested == 0
        TIMEOUT_INFINITE,   // *requested == infinity
        TIMEOUT_FINITE,     // 0 < *requested < infinity
        TIMEOUT_CONTINUE,   // additional chances after TIMEOUT_FINITE
    } timeout;
    if (requested == NULL) {
        timeout = TIMEOUT_ZERO;
    } else if (requested->tv_sec == 0 && requested->tv_nsec == 0) {
        timeout = TIMEOUT_ZERO;
    } else if (requested->tv_sec == INT_MAX) {
        timeout = TIMEOUT_INFINITE;
    } else {
        timeout = TIMEOUT_FINITE;
        if (requested->tv_sec > 0 || requested->tv_nsec >= MEASURE_NS) {
            measure = true;
        }
    }
    struct timespec before;
    bool beforeIsValid = false;
    audio_track_cblk_t* cblk = mCblk;
    bool ignoreInitialPendingInterrupt = true;
    // check for shared memory corruption
    if (mIsShutdown) {
        status = NO_INIT;
        goto end;
    }
    for (;;) {
        int32_t flags = android_atomic_and(~CBLK_INTERRUPT, &cblk->flags);
        // check for track invalidation by server, or server death detection
        if (flags & CBLK_INVALID) {
            ALOGV("Track invalidated");
            status = DEAD_OBJECT;
            goto end;
        }
        // check for obtainBuffer interrupted by client
        if (!ignoreInitialPendingInterrupt && (flags & CBLK_INTERRUPT)) {
            ALOGV("obtainBuffer() interrupted by client");
            status = -EINTR;
            goto end;
        }
        ignoreInitialPendingInterrupt = false;
        // compute number of frames available to write (AudioTrack) or read (AudioRecord)
        int32_t front;
        int32_t rear;
        if (mIsOut) {
            // The barrier following the read of mFront is probably redundant.
            // We're about to perform a conditional branch based on 'filled',
            // which will force the processor to observe the read of mFront
            // prior to allowing data writes starting at mRaw.
            // However, the processor may support speculative execution,
            // and be unable to undo speculative writes into shared memory.
            // The barrier will prevent such speculative execution.
            front = android_atomic_acquire_load(&cblk->u.mStreaming.mFront);
            rear = cblk->u.mStreaming.mRear;
        } else {
            // On the other hand, this barrier is required.
            rear = android_atomic_acquire_load(&cblk->u.mStreaming.mRear);
            front = cblk->u.mStreaming.mFront;
        }
        ssize_t filled = rear - front;
        // pipe should not be overfull
        if (!(0 <= filled && (size_t) filled <= mFrameCount)) {
            ALOGE("Shared memory control block is corrupt (filled=%d); shutting down", filled);
            mIsShutdown = true;
            status = NO_INIT;
            goto end;
        }
        // don't allow filling pipe beyond the nominal size
        size_t avail = mIsOut ? mFrameCount - filled : filled;
        if (avail > 0) {
            // 'avail' may be non-contiguous, so return only the first contiguous chunk
            size_t part1;
            if (mIsOut) {
                rear &= mFrameCountP2 - 1;
                part1 = mFrameCountP2 - rear;
            } else {
                front &= mFrameCountP2 - 1;
                part1 = mFrameCountP2 - front;
            }
            if (part1 > avail) {
                part1 = avail;
            }
            if (part1 > buffer->mFrameCount) {
                part1 = buffer->mFrameCount;
            }
            buffer->mFrameCount = part1;
            buffer->mRaw = part1 > 0 ?
                    &((char *) mBuffers)[(mIsOut ? rear : front) * mFrameSize] : NULL;
            buffer->mNonContig = avail - part1;
            mUnreleased = part1;
            status = NO_ERROR;
            break;
        }
        struct timespec remaining;
        const struct timespec *ts;
        switch (timeout) {
        case TIMEOUT_ZERO:
            status = WOULD_BLOCK;
            goto end;
        case TIMEOUT_INFINITE:
            ts = NULL;
            break;
        case TIMEOUT_FINITE:
            timeout = TIMEOUT_CONTINUE;
            if (MAX_SEC == 0) {
                ts = requested;
                break;
            }
            // fall through
        case TIMEOUT_CONTINUE:
            // FIXME we do not retry if requested < 10ms? needs documentation on this state machine
            if (!measure || requested->tv_sec < total.tv_sec ||
                    (requested->tv_sec == total.tv_sec && requested->tv_nsec <= total.tv_nsec)) {
                status = TIMED_OUT;
                goto end;
            }
            remaining.tv_sec = requested->tv_sec - total.tv_sec;
            if ((remaining.tv_nsec = requested->tv_nsec - total.tv_nsec) < 0) {
                remaining.tv_nsec += 1000000000;
                remaining.tv_sec++;
            }
            if (0 < MAX_SEC && MAX_SEC < remaining.tv_sec) {
                remaining.tv_sec = MAX_SEC;
                remaining.tv_nsec = 0;
            }
            ts = &remaining;
            break;
        default:
            LOG_FATAL("%s timeout=%d", timeout);
            ts = NULL;
            break;
        }
        int32_t old = android_atomic_and(~CBLK_FUTEX_WAKE, &cblk->mFutex);
        if (!(old & CBLK_FUTEX_WAKE)) {
            int rc;
            if (measure && !beforeIsValid) {
                clock_gettime(CLOCK_MONOTONIC, &before);
                beforeIsValid = true;
            }
            int ret = __futex_syscall4(&cblk->mFutex,
                    mClientInServer ? FUTEX_WAIT_PRIVATE : FUTEX_WAIT, old & ~CBLK_FUTEX_WAKE, ts);
            // update total elapsed time spent waiting
            if (measure) {
                struct timespec after;
                clock_gettime(CLOCK_MONOTONIC, &after);
                total.tv_sec += after.tv_sec - before.tv_sec;
                long deltaNs = after.tv_nsec - before.tv_nsec;
                if (deltaNs < 0) {
                    deltaNs += 1000000000;
                    total.tv_sec--;
                }
                if ((total.tv_nsec += deltaNs) >= 1000000000) {
                    total.tv_nsec -= 1000000000;
                    total.tv_sec++;
                }
                before = after;
                beforeIsValid = true;
            }
            switch (ret) {
            case 0:             // normal wakeup by server, or by binderDied()
            case -EWOULDBLOCK:  // benign race condition with server
            case -EINTR:        // wait was interrupted by signal or other spurious wakeup
            case -ETIMEDOUT:    // time-out expired
                // FIXME these error/non-0 status are being dropped
                break;
            default:
                ALOGE("%s unexpected error %d", __func__, ret);
                status = -ret;
                goto end;
            }
        }
    }

end:
    if (status != NO_ERROR) {
        buffer->mFrameCount = 0;
        buffer->mRaw = NULL;
        buffer->mNonContig = 0;
        mUnreleased = 0;
    }
    if (elapsed != NULL) {
        *elapsed = total;
    }
    if (requested == NULL) {
        requested = &kNonBlocking;
    }
    if (measure) {
        ALOGV("requested %d.%03d elapsed %d.%03d", requested->tv_sec, requested->tv_nsec / 1000000,
                total.tv_sec, total.tv_nsec / 1000000);
    }
    return status;
}

void ClientProxy::releaseBuffer(Buffer* buffer)
{
    LOG_ALWAYS_FATAL_IF(buffer == NULL);
    size_t stepCount = buffer->mFrameCount;
    if (stepCount == 0 || mIsShutdown) {
        // prevent accidental re-use of buffer
        buffer->mFrameCount = 0;
        buffer->mRaw = NULL;
        buffer->mNonContig = 0;
        return;
    }
    LOG_ALWAYS_FATAL_IF(!(stepCount <= mUnreleased && mUnreleased <= mFrameCount));
    mUnreleased -= stepCount;
    audio_track_cblk_t* cblk = mCblk;
    // Both of these barriers are required
    if (mIsOut) {
        int32_t rear = cblk->u.mStreaming.mRear;
        android_atomic_release_store(stepCount + rear, &cblk->u.mStreaming.mRear);
    } else {
        int32_t front = cblk->u.mStreaming.mFront;
        android_atomic_release_store(stepCount + front, &cblk->u.mStreaming.mFront);
    }
}

void ClientProxy::binderDied()
{
    audio_track_cblk_t* cblk = mCblk;
    if (!(android_atomic_or(CBLK_INVALID, &cblk->flags) & CBLK_INVALID)) {
        // it seems that a FUTEX_WAKE_PRIVATE will not wake a FUTEX_WAIT, even within same process
        (void) __futex_syscall3(&cblk->mFutex, mClientInServer ? FUTEX_WAKE_PRIVATE : FUTEX_WAKE,
                1);
    }
}

void ClientProxy::interrupt()
{
    audio_track_cblk_t* cblk = mCblk;
    if (!(android_atomic_or(CBLK_INTERRUPT, &cblk->flags) & CBLK_INTERRUPT)) {
        (void) __futex_syscall3(&cblk->mFutex, mClientInServer ? FUTEX_WAKE_PRIVATE : FUTEX_WAKE,
                1);
    }
}

size_t ClientProxy::getMisalignment()
{
    audio_track_cblk_t* cblk = mCblk;
    return (mFrameCountP2 - (mIsOut ? cblk->u.mStreaming.mRear : cblk->u.mStreaming.mFront)) &
            (mFrameCountP2 - 1);
}

// ---------------------------------------------------------------------------

void AudioTrackClientProxy::flush()
{
    mCblk->u.mStreaming.mFlush++;
}

bool AudioTrackClientProxy::clearStreamEndDone() {
    return android_atomic_and(~CBLK_STREAM_END_DONE, &mCblk->flags) & CBLK_STREAM_END_DONE;
}

bool AudioTrackClientProxy::getStreamEndDone() const {
    return (mCblk->flags & CBLK_STREAM_END_DONE) != 0;
}

// ---------------------------------------------------------------------------

StaticAudioTrackClientProxy::StaticAudioTrackClientProxy(audio_track_cblk_t* cblk, void *buffers,
        size_t frameCount, size_t frameSize)
    : AudioTrackClientProxy(cblk, buffers, frameCount, frameSize),
      mMutator(&cblk->u.mStatic.mSingleStateQueue), mBufferPosition(0)
{
}

void StaticAudioTrackClientProxy::flush()
{
    LOG_FATAL("static flush");
}

void StaticAudioTrackClientProxy::setLoop(size_t loopStart, size_t loopEnd, int loopCount)
{
    StaticAudioTrackState newState;
    newState.mLoopStart = loopStart;
    newState.mLoopEnd = loopEnd;
    newState.mLoopCount = loopCount;
    mBufferPosition = loopStart;
    (void) mMutator.push(newState);
}

size_t StaticAudioTrackClientProxy::getBufferPosition()
{
    size_t bufferPosition;
    if (mMutator.ack()) {
        bufferPosition = mCblk->u.mStatic.mBufferPosition;
        if (bufferPosition > mFrameCount) {
            bufferPosition = mFrameCount;
        }
    } else {
        bufferPosition = mBufferPosition;
    }
    return bufferPosition;
}

// ---------------------------------------------------------------------------

ServerProxy::ServerProxy(audio_track_cblk_t* cblk, void *buffers, size_t frameCount,
        size_t frameSize, bool isOut, bool clientInServer)
    : Proxy(cblk, buffers, frameCount, frameSize, isOut, clientInServer),
      mAvailToClient(0), mFlush(0), mDeferWake(false)
{
}

status_t ServerProxy::obtainBuffer(Buffer* buffer)
{
    LOG_ALWAYS_FATAL_IF(buffer == NULL || buffer->mFrameCount == 0);
    if (mIsShutdown) {
        goto no_init;
    }
    {
    audio_track_cblk_t* cblk = mCblk;
    // compute number of frames available to write (AudioTrack) or read (AudioRecord),
    // or use previous cached value from framesReady(), with added barrier if it omits.
    int32_t front;
    int32_t rear;
    // See notes on barriers at ClientProxy::obtainBuffer()
    if (mIsOut) {
        int32_t flush = cblk->u.mStreaming.mFlush;
        rear = android_atomic_acquire_load(&cblk->u.mStreaming.mRear);
        if (flush != mFlush) {
            front = rear;
            mFlush = flush;
            // effectively obtain then release whatever is in the buffer
            android_atomic_release_store(rear, &cblk->u.mStreaming.mFront);
        } else {
            front = cblk->u.mStreaming.mFront;
        }
    } else {
        front = android_atomic_acquire_load(&cblk->u.mStreaming.mFront);
        rear = cblk->u.mStreaming.mRear;
    }
    ssize_t filled = rear - front;
    // pipe should not already be overfull
    if (!(0 <= filled && (size_t) filled <= mFrameCount)) {
        ALOGE("Shared memory control block is corrupt (filled=%d); shutting down", filled);
        mIsShutdown = true;
    }
    if (mIsShutdown) {
        goto no_init;
    }
    // don't allow filling pipe beyond the nominal size
    size_t availToServer;
    if (mIsOut) {
        availToServer = filled;
        mAvailToClient = mFrameCount - filled;
    } else {
        availToServer = mFrameCount - filled;
        mAvailToClient = filled;
    }
    // 'availToServer' may be non-contiguous, so return only the first contiguous chunk
    size_t part1;
    if (mIsOut) {
        front &= mFrameCountP2 - 1;
        part1 = mFrameCountP2 - front;
    } else {
        rear &= mFrameCountP2 - 1;
        part1 = mFrameCountP2 - rear;
    }
    if (part1 > availToServer) {
        part1 = availToServer;
    }
    size_t ask = buffer->mFrameCount;
    if (part1 > ask) {
        part1 = ask;
    }
    // is assignment redundant in some cases?
    buffer->mFrameCount = part1;
    buffer->mRaw = part1 > 0 ?
            &((char *) mBuffers)[(mIsOut ? front : rear) * mFrameSize] : NULL;
    buffer->mNonContig = availToServer - part1;
    mUnreleased = part1;
    // optimization to avoid waking up the client too early
    // FIXME need to test for recording
    mDeferWake = part1 < ask && availToServer >= ask;
    return part1 > 0 ? NO_ERROR : WOULD_BLOCK;
    }
no_init:
    buffer->mFrameCount = 0;
    buffer->mRaw = NULL;
    buffer->mNonContig = 0;
    mUnreleased = 0;
    return NO_INIT;
}

void ServerProxy::releaseBuffer(Buffer* buffer)
{
    LOG_ALWAYS_FATAL_IF(buffer == NULL);
    size_t stepCount = buffer->mFrameCount;
    if (stepCount == 0 || mIsShutdown) {
        // prevent accidental re-use of buffer
        buffer->mFrameCount = 0;
        buffer->mRaw = NULL;
        buffer->mNonContig = 0;
        return;
    }
    LOG_ALWAYS_FATAL_IF(!(stepCount <= mUnreleased && mUnreleased <= mFrameCount));
    mUnreleased -= stepCount;
    audio_track_cblk_t* cblk = mCblk;
    if (mIsOut) {
        int32_t front = cblk->u.mStreaming.mFront;
        android_atomic_release_store(stepCount + front, &cblk->u.mStreaming.mFront);
    } else {
        int32_t rear = cblk->u.mStreaming.mRear;
        android_atomic_release_store(stepCount + rear, &cblk->u.mStreaming.mRear);
    }

    mCblk->server += stepCount;

    size_t half = mFrameCount / 2;
    if (half == 0) {
        half = 1;
    }
    size_t minimum = cblk->mMinimum;
    if (minimum == 0) {
        minimum = mIsOut ? half : 1;
    } else if (minimum > half) {
        minimum = half;
    }
    // FIXME AudioRecord wakeup needs to be optimized; it currently wakes up client every time
    if (!mIsOut || (!mDeferWake && mAvailToClient + stepCount >= minimum)) {
        ALOGV("mAvailToClient=%u stepCount=%u minimum=%u", mAvailToClient, stepCount, minimum);
        int32_t old = android_atomic_or(CBLK_FUTEX_WAKE, &cblk->mFutex);
        if (!(old & CBLK_FUTEX_WAKE)) {
            (void) __futex_syscall3(&cblk->mFutex,
                    mClientInServer ? FUTEX_WAKE_PRIVATE : FUTEX_WAKE, 1);
        }
    }

    buffer->mFrameCount = 0;
    buffer->mRaw = NULL;
    buffer->mNonContig = 0;
}

// ---------------------------------------------------------------------------

size_t AudioTrackServerProxy::framesReady()
{
    LOG_ALWAYS_FATAL_IF(!mIsOut);

    if (mIsShutdown) {
        return 0;
    }
    audio_track_cblk_t* cblk = mCblk;
    // the acquire might not be necessary since not doing a subsequent read
    int32_t rear = android_atomic_acquire_load(&cblk->u.mStreaming.mRear);
    ssize_t filled = rear - cblk->u.mStreaming.mFront;
    // pipe should not already be overfull
    if (!(0 <= filled && (size_t) filled <= mFrameCount)) {
        ALOGE("Shared memory control block is corrupt (filled=%d); shutting down", filled);
        mIsShutdown = true;
        return 0;
    }
    //  cache this value for later use by obtainBuffer(), with added barrier
    //  and racy if called by normal mixer thread
    // ignores flush(), so framesReady() may report a larger mFrameCount than obtainBuffer()
    return filled;
}

bool  AudioTrackServerProxy::setStreamEndDone() {
    bool old =
            (android_atomic_or(CBLK_STREAM_END_DONE, &mCblk->flags) & CBLK_STREAM_END_DONE) != 0;
    if (!old) {
        (void) __futex_syscall3(&mCblk->mFutex, mClientInServer ? FUTEX_WAKE_PRIVATE : FUTEX_WAKE,
                1);
    }
    return old;
}

// ---------------------------------------------------------------------------

StaticAudioTrackServerProxy::StaticAudioTrackServerProxy(audio_track_cblk_t* cblk, void *buffers,
        size_t frameCount, size_t frameSize)
    : AudioTrackServerProxy(cblk, buffers, frameCount, frameSize),
      mObserver(&cblk->u.mStatic.mSingleStateQueue), mPosition(0),
      mEnd(frameCount), mFramesReadyIsCalledByMultipleThreads(false)
{
    mState.mLoopStart = 0;
    mState.mLoopEnd = 0;
    mState.mLoopCount = 0;
}

void StaticAudioTrackServerProxy::framesReadyIsCalledByMultipleThreads()
{
    mFramesReadyIsCalledByMultipleThreads = true;
}

size_t StaticAudioTrackServerProxy::framesReady()
{
    // FIXME
    // This is racy if called by normal mixer thread,
    // as we're reading 2 independent variables without a lock.
    // Can't call mObserver.poll(), as we might be called from wrong thread.
    // If looping is enabled, should return a higher number (since includes non-contiguous).
    size_t position = mPosition;
    if (!mFramesReadyIsCalledByMultipleThreads) {
        ssize_t positionOrStatus = pollPosition();
        if (positionOrStatus >= 0) {
            position = (size_t) positionOrStatus;
        }
    }
    size_t end = mEnd;
    return position < end ? end - position : 0;
}

ssize_t StaticAudioTrackServerProxy::pollPosition()
{
    size_t position = mPosition;
    StaticAudioTrackState state;
    if (mObserver.poll(state)) {
        bool valid = false;
        size_t loopStart = state.mLoopStart;
        size_t loopEnd = state.mLoopEnd;
        if (state.mLoopCount == 0) {
            if (loopStart > mFrameCount) {
                loopStart = mFrameCount;
            }
            // ignore loopEnd
            mPosition = position = loopStart;
            mEnd = mFrameCount;
            mState.mLoopCount = 0;
            valid = true;
        } else {
            if (loopStart < loopEnd && loopEnd <= mFrameCount &&
                    loopEnd - loopStart >= MIN_LOOP) {
                if (!(loopStart <= position && position < loopEnd)) {
                    mPosition = position = loopStart;
                }
                mEnd = loopEnd;
                mState = state;
                valid = true;
            }
        }
        if (!valid) {
            ALOGE("%s client pushed an invalid state, shutting down", __func__);
            mIsShutdown = true;
            return (ssize_t) NO_INIT;
        }
        mCblk->u.mStatic.mBufferPosition = position;
    }
    return (ssize_t) position;
}

status_t StaticAudioTrackServerProxy::obtainBuffer(Buffer* buffer)
{
    if (mIsShutdown) {
        buffer->mFrameCount = 0;
        buffer->mRaw = NULL;
        buffer->mNonContig = 0;
        mUnreleased = 0;
        return NO_INIT;
    }
    ssize_t positionOrStatus = pollPosition();
    if (positionOrStatus < 0) {
        buffer->mFrameCount = 0;
        buffer->mRaw = NULL;
        buffer->mNonContig = 0;
        mUnreleased = 0;
        return (status_t) positionOrStatus;
    }
    size_t position = (size_t) positionOrStatus;
    size_t avail;
    if (position < mEnd) {
        avail = mEnd - position;
        size_t wanted = buffer->mFrameCount;
        if (avail < wanted) {
            buffer->mFrameCount = avail;
        } else {
            avail = wanted;
        }
        buffer->mRaw = &((char *) mBuffers)[position * mFrameSize];
    } else {
        avail = 0;
        buffer->mFrameCount = 0;
        buffer->mRaw = NULL;
    }
    buffer->mNonContig = 0;     // FIXME should be > 0 for looping
    mUnreleased = avail;
    return NO_ERROR;
}

void StaticAudioTrackServerProxy::releaseBuffer(Buffer* buffer)
{
    size_t stepCount = buffer->mFrameCount;
    LOG_ALWAYS_FATAL_IF(!(stepCount <= mUnreleased));
    if (stepCount == 0) {
        // prevent accidental re-use of buffer
        buffer->mRaw = NULL;
        buffer->mNonContig = 0;
        return;
    }
    mUnreleased -= stepCount;
    audio_track_cblk_t* cblk = mCblk;
    size_t position = mPosition;
    size_t newPosition = position + stepCount;
    int32_t setFlags = 0;
    if (!(position <= newPosition && newPosition <= mFrameCount)) {
        ALOGW("%s newPosition %u outside [%u, %u]", __func__, newPosition, position, mFrameCount);
        newPosition = mFrameCount;
    } else if (mState.mLoopCount != 0 && newPosition == mState.mLoopEnd) {
        if (mState.mLoopCount == -1 || --mState.mLoopCount != 0) {
            newPosition = mState.mLoopStart;
            setFlags = CBLK_LOOP_CYCLE;
        } else {
            mEnd = mFrameCount;     // this is what allows playback to continue after the loop
            setFlags = CBLK_LOOP_FINAL;
        }
    }
    if (newPosition == mFrameCount) {
        setFlags |= CBLK_BUFFER_END;
    }
    mPosition = newPosition;

    cblk->server += stepCount;
    cblk->u.mStatic.mBufferPosition = newPosition;
    if (setFlags != 0) {
        (void) android_atomic_or(setFlags, &cblk->flags);
        // this would be a good place to wake a futex
    }

    buffer->mFrameCount = 0;
    buffer->mRaw = NULL;
    buffer->mNonContig = 0;
}

// ---------------------------------------------------------------------------

}   // namespace android