深入剖析Android音频之AudioTrack

播放声音能够用MediaPlayer和AudioTrack，两者都提供了java API供应用开发人员使用。尽管都能够播放声音。但两者还是有非常大的差别的。当中最大的差别是MediaPlayer能够播放多种格式的声音文件。比如MP3。AAC，WAV，OGG。MIDI等。

MediaPlayer会在framework层创建相应的音频解码器。而AudioTrack仅仅能播放已经解码的PCM流，假设是文件的话仅仅支持wav格式的音频文件，由于wav格式的音频文件大部分都是PCM流。AudioTrack不创建解码器。所以仅仅能播放不须要解码的wav文件。

当然两者之间还是有紧密的联系，MediaPlayer在framework层还是会创建AudioTrack，把解码后的PCM数流传递给AudioTrack。AudioTrack再传递给AudioFlinger进行混音，然后才传递给硬件播放,所以是MediaPlayer包括了AudioTrack。使用AudioTrack播放音乐演示样例：

AudioTrack audio = new AudioTrack(
     AudioManager.STREAM_MUSIC, // 指定流的类型
     32000, // 设置音频数据的採样率 32k，假设是44.1k就是44100
     AudioFormat.CHANNEL_OUT_STEREO, // 设置输出声道为双声道立体声，而CHANNEL_OUT_MONO类型是单声道
     AudioFormat.ENCODING_PCM_16BIT, // 设置音频数据块是8位还是16位。这里设置为16位。

 

好像如今绝大多数的音频都是16位的了 AudioTrack.MODE_STREAM // 设置模式类型，在这里设置为流类型，第二种MODE_STATIC貌似没有什么效果 ); audio.play(); // 启动音频设备。以下就能够真正開始音频数据的播放了 // 打开mp3文件，读取数据，解码等操作省略 ... byte[] buffer = new buffer[4096]; int count; while(true) { // 最关键的是将解码后的数据，从缓冲区写入到AudioTrack对象中 audio.write(buffer, 0, 4096); if(文件结束) break; } //关闭并释放资源 audio.stop(); audio.release();

AudioTrack构造过程

 

每个音频流相应着一个AudioTrack类的一个实例，每个AudioTrack会在创建时注冊到 AudioFlinger中。由AudioFlinger把全部的AudioTrack进行混合（Mixer）。然后输送到AudioHardware中进行播放。眼下Android同一时候最多能够创建32个音频流，也就是说。Mixer最多会同一时候处理32个AudioTrack的数据流。

 

frameworks\base\media\java\android\media\AudioTrack.java

/**
 * streamType:音频流类型
 * sampleRateInHz:採样率
 * channelConfig:音频声道
 * audioFormat:音频格式
 * bufferSizeInBytes缓冲区大小:
 * mode:音频数据载入模式
 * sessionId:会话id
 */
public AudioTrack(int streamType, int sampleRateInHz, int channelConfig, int audioFormat,
        int bufferSizeInBytes, int mode, int sessionId)
throws IllegalArgumentException {
    // mState already == STATE_UNINITIALIZED

    // remember which looper is associated with the AudioTrack instantiation
    Looper looper;
    if ((looper = Looper.myLooper()) == null) {
        looper = Looper.getMainLooper();
    }
    mInitializationLooper = looper;
    /**
     * 參数检查
     * 1.检查streamType是否为：STREAM_ALARM、STREAM_MUSIC、STREAM_RING、STREAM_SYSTEM、STREAM_VOICE_CALL、
     *  STREAM_NOTIFICATION、STREAM_BLUETOOTH_SCO、STREAM_BLUETOOTH_SCO，并赋值给mStreamType
     * 2.检查sampleRateInHz是否在4000到48000之间。并赋值给mSampleRate
     * 3.设置mChannels： 
     *      CHANNEL_OUT_DEFAULT、CHANNEL_OUT_MONO、CHANNEL_CONFIGURATION_MONO ---> CHANNEL_OUT_MONO
     *      CHANNEL_OUT_STEREO、CHANNEL_CONFIGURATION_STEREO                  ---> CHANNEL_OUT_STEREO
     * 4.设置mAudioFormat： 
     *      ENCODING_PCM_16BIT、ENCODING_DEFAULT ---> ENCODING_PCM_16BIT
     *      ENCODING_PCM_8BIT ---> ENCODING_PCM_8BIT
     * 5.设置mDataLoadMode：
     *      MODE_STREAM
     *      MODE_STATIC
     */
    audioParamCheck(streamType, sampleRateInHz, channelConfig, audioFormat, mode);
    /**
     * buffer大小检查,计算每帧字节大小，假设是ENCODING_PCM_16BIT，则为mChannelCount * 2
     * mNativeBufferSizeInFrames为帧数
     */
    audioBuffSizeCheck(bufferSizeInBytes);
    if (sessionId < 0) {
        throw new IllegalArgumentException("Invalid audio session ID: "+sessionId);
    }
    //进入native层初始化
    int[] session = new int[1];
    session[0] = sessionId;
    // native initialization
    int initResult = native_setup(new WeakReference(this),
            mStreamType, mSampleRate, mChannels, mAudioFormat,
            mNativeBufferSizeInBytes, mDataLoadMode, session);
    if (initResult != SUCCESS) {
        loge("Error code "+initResult+" when initializing AudioTrack.");
        return; // with mState == STATE_UNINITIALIZED
    }
    mSessionId = session[0];
    if (mDataLoadMode == MODE_STATIC) {
        mState = STATE_NO_STATIC_DATA;
    } else {
        mState = STATE_INITIALIZED;
    }
}

with audio session. Use this constructor when the AudioTrack must be attached to a particular audio session. The primary use of the audio session ID is to associate audio effects to a particular instance of AudioTrack: if an audio session ID is provided when creating an AudioEffect, this effect will be applied only to audio tracks and media players in the same session and not to the output mix. When an AudioTrack is created without specifying a session, it will create its own session which can be retreived by calling the getAudioSessionId() method. If a non-zero session ID is provided, this AudioTrack will share effects attached to this session with all other media players or audio tracks in the same session, otherwise a new session will be created for this track if none is supplied.

streamType	the type of the audio stream. See STREAM_VOICE_CALL,STREAM_SYSTEM,STREAM_RING,STREAM_MUSIC,STREAM_ALARM, andSTREAM_NOTIFICATION.
sampleRateInHz	the sample rate expressed in Hertz.
channelConfig	describes the configuration of the audio channels. SeeCHANNEL_OUT_MONO andCHANNEL_OUT_STEREO
audioFormat	the format in which the audio data is represented. SeeENCODING_PCM_16BIT andENCODING_PCM_8BIT
bufferSizeInBytes	the total size (in bytes) of the buffer where audio data is read from for playback. If using the AudioTrack in streaming mode, you can write data into this buffer in smaller chunks than this size. If using the AudioTrack in static mode, this is the maximum size of the sound that will be played for this instance. SeegetMinBufferSize(int, int, int) to determine the minimum required buffer size for the successful creation of an AudioTrack instance in streaming mode. Using values smaller than getMinBufferSize() will result in an initialization failure.
mode	streaming or static buffer. See MODE_STATIC andMODE_STREAM
sessionId	Id of audio session the AudioTrack must be attached to

AudioTrack有两种数据载入模式：

1. MODE_STREAM

在这样的模式下，应用程序持续地write音频数据流到AudioTrack中，而且write动作将堵塞直到数据流从Java层传输到native层，同一时候增加到播放队列中。这样的模式适用于播放大音频数据，但该模式也造成了一定的延时；

1. MODE_STATIC

在播放之前，先把全部数据一次性write到AudioTrack的内部缓冲区中。适用于播放内存占用小、延时要求较高的音频数据。

 

frameworks\base\core\jni\android_media_AudioTrack.cpp

static int android_media_AudioTrack_native_setup(JNIEnv *env, jobject thiz, jobject weak_this,jint streamType, jint sampleRateInHertz, jint javaChannelMask,
        jint audioFormat, jint buffSizeInBytes, jint memoryMode, jintArray jSession)
{
    ALOGV("sampleRate=%d, audioFormat(from Java)=%d, channel mask=%x, buffSize=%d",
        sampleRateInHertz, audioFormat, javaChannelMask, buffSizeInBytes);
    int afSampleRate;//採样率
    int afFrameCount;//帧数
	//通过AudioSystem从AudioPolicyService中读取相应音频流类型的帧数
    if (AudioSystem::getOutputFrameCount(&afFrameCount, (audio_stream_type_t) streamType) != NO_ERROR) {
        ALOGE("Error creating AudioTrack: Could not get AudioSystem frame count.");
        return AUDIOTRACK_ERROR_SETUP_AUDIOSYSTEM;
    }
    //通过AudioSystem从AudioPolicyService中读取相应音频流类型的採样率
    if (AudioSystem::getOutputSamplingRate(&afSampleRate, (audio_stream_type_t) streamType) != NO_ERROR) {
        ALOGE("Error creating AudioTrack: Could not get AudioSystem sampling rate.");
        return AUDIOTRACK_ERROR_SETUP_AUDIOSYSTEM;
    }
    // Java channel masks don't map directly to the native definition, but it's a simple shift
    // to skip the two deprecated channel configurations "default" and "mono".
    uint32_t nativeChannelMask = ((uint32_t)javaChannelMask) >> 2;
	//推断是否为输出通道
    if (!audio_is_output_channel(nativeChannelMask)) {
        ALOGE("Error creating AudioTrack: invalid channel mask.");
        return AUDIOTRACK_ERROR_SETUP_INVALIDCHANNELMASK;
    }
    //得到通道个数，popcount函数用于统计一个整数中有多少位为1
    int nbChannels = popcount(nativeChannelMask);
    // check the stream type
    audio_stream_type_t atStreamType;
    switch (streamType) {
    case AUDIO_STREAM_VOICE_CALL:
    case AUDIO_STREAM_SYSTEM:
    case AUDIO_STREAM_RING:
    case AUDIO_STREAM_MUSIC:
    case AUDIO_STREAM_ALARM:
    case AUDIO_STREAM_NOTIFICATION:
    case AUDIO_STREAM_BLUETOOTH_SCO:
    case AUDIO_STREAM_DTMF:
        atStreamType = (audio_stream_type_t) streamType;
        break;
    default:
        ALOGE("Error creating AudioTrack: unknown stream type.");
        return AUDIOTRACK_ERROR_SETUP_INVALIDSTREAMTYPE;
    }
    // This function was called from Java, so we compare the format against the Java constants
    if ((audioFormat != javaAudioTrackFields.PCM16) && (audioFormat != javaAudioTrackFields.PCM8)) {
        ALOGE("Error creating AudioTrack: unsupported audio format.");
        return AUDIOTRACK_ERROR_SETUP_INVALIDFORMAT;
    }
    // for the moment 8bitPCM in MODE_STATIC is not supported natively in the AudioTrack C++ class so we declare everything as 16bitPCM, the 8->16bit conversion for MODE_STATIC will be handled in android_media_AudioTrack_native_write_byte()
    if ((audioFormat == javaAudioTrackFields.PCM8)
        && (memoryMode == javaAudioTrackFields.MODE_STATIC)) {
        ALOGV("android_media_AudioTrack_native_setup(): requesting MODE_STATIC for 8bit \
            buff size of %dbytes, switching to 16bit, buff size of %dbytes",
            buffSizeInBytes, 2*buffSizeInBytes);
        audioFormat = javaAudioTrackFields.PCM16;
        // we will need twice the memory to store the data
        buffSizeInBytes *= 2;
    }
    //依据不同的採样方式得到一个採样点的字节数
    int bytesPerSample = audioFormat == javaAudioTrackFields.PCM16 ? 2 : 1;
    audio_format_t format = audioFormat == javaAudioTrackFields.PCM16 ?
            AUDIO_FORMAT_PCM_16_BIT : AUDIO_FORMAT_PCM_8_BIT;
    //依据buffer大小反向计算帧数  。 一帧大小=一个採样点字节数 * 声道数
    int frameCount = buffSizeInBytes / (nbChannels * bytesPerSample);
    //推断參数的合法性
    jclass clazz = env->GetObjectClass(thiz);
    if (clazz == NULL) {
        ALOGE("Can't find %s when setting up callback.", kClassPathName);
        return AUDIOTRACK_ERROR_SETUP_NATIVEINITFAILED;
    }
    if (jSession == NULL) {
        ALOGE("Error creating AudioTrack: invalid session ID pointer");
        return AUDIOTRACK_ERROR;
    }
    jint* nSession = (jint *) env->GetPrimitiveArrayCritical(jSession, NULL);
    if (nSession == NULL) {
        ALOGE("Error creating AudioTrack: Error retrieving session id pointer");
        return AUDIOTRACK_ERROR;
    }
    int sessionId = nSession[0];
    env->ReleasePrimitiveArrayCritical(jSession, nSession, 0);
    nSession = NULL;
    // create the native AudioTrack object
    sp lpTrack = new AudioTrack();
    if (lpTrack == NULL) {
        ALOGE("Error creating uninitialized AudioTrack");
        return AUDIOTRACK_ERROR_SETUP_NATIVEINITFAILED;
    }
    // 创建存储音频数据的容器
    AudioTrackJniStorage* lpJniStorage = new AudioTrackJniStorage();
    lpJniStorage->mStreamType = atStreamType;
    //将Java层的AudioTrack引用保存到AudioTrackJniStorage中
    lpJniStorage->mCallbackData.audioTrack_class = (jclass)env->NewGlobalRef(clazz);
    // we use a weak reference so the AudioTrack object can be garbage collected.
    lpJniStorage->mCallbackData.audioTrack_ref = env->NewGlobalRef(weak_this);
    lpJniStorage->mCallbackData.busy = false;
    //初始化不同模式下的native AudioTrack对象 
    if (memoryMode == javaAudioTrackFields.MODE_STREAM) { //stream模式
        lpTrack->set( 
            atStreamType,// stream type
            sampleRateInHertz,
            format,// word length, PCM
            nativeChannelMask,
            frameCount,
            AUDIO_OUTPUT_FLAG_NONE,
            audioCallback, 
            &(lpJniStorage->mCallbackData),//callback, callback data (user)
            0,// notificationFrames == 0 since not using EVENT_MORE_DATA to feed the AudioTrack
            0,//stream模式下的共享内存在AudioFlinger中创建
            true,// thread can call Java
            sessionId);// audio session ID
    } else if (memoryMode == javaAudioTrackFields.MODE_STATIC) {//static模式
        // 为AudioTrack分配共享内存区域 
        if (!lpJniStorage->allocSharedMem(buffSizeInBytes)) {
            ALOGE("Error creating AudioTrack in static mode: error creating mem heap base");
            goto native_init_failure;
        }
        lpTrack->set(
            atStreamType,// stream type
            sampleRateInHertz,
            format,// word length, PCM
            nativeChannelMask,
            frameCount,
            AUDIO_OUTPUT_FLAG_NONE,
            audioCallback, &(lpJniStorage->mCallbackData),//callback, callback data (user));
            0,// notificationFrames == 0 since not using EVENT_MORE_DATA to feed the AudioTrack
            lpJniStorage->mMemBase,// shared mem
            true,// thread can call Java
            sessionId);// audio session ID
    }
    if (lpTrack->initCheck() != NO_ERROR) {
        ALOGE("Error initializing AudioTrack");
        goto native_init_failure;
    }
    nSession = (jint *) env->GetPrimitiveArrayCritical(jSession, NULL);
    if (nSession == NULL) {
        ALOGE("Error creating AudioTrack: Error retrieving session id pointer");
        goto native_init_failure;
    }
    // read the audio session ID back from AudioTrack in case we create a new session
    nSession[0] = lpTrack->getSessionId();
    env->ReleasePrimitiveArrayCritical(jSession, nSession, 0);
    nSession = NULL;
    {   // scope for the lock
        Mutex::Autolock l(sLock);
        sAudioTrackCallBackCookies.add(&lpJniStorage->mCallbackData);
    }
    // save our newly created C++ AudioTrack in the "nativeTrackInJavaObj" field
    // of the Java object (in mNativeTrackInJavaObj)
    setAudioTrack(env, thiz, lpTrack);
    // save the JNI resources so we can free them later
    //ALOGV("storing lpJniStorage: %x\n", (int)lpJniStorage);
    env->SetIntField(thiz, javaAudioTrackFields.jniData, (int)lpJniStorage);
    return AUDIOTRACK_SUCCESS;
    // failures:
native_init_failure:
    if (nSession != NULL) {
        env->ReleasePrimitiveArrayCritical(jSession, nSession, 0);
    }
    env->DeleteGlobalRef(lpJniStorage->mCallbackData.audioTrack_class);
    env->DeleteGlobalRef(lpJniStorage->mCallbackData.audioTrack_ref);
    delete lpJniStorage;
    env->SetIntField(thiz, javaAudioTrackFields.jniData, 0);
    return AUDIOTRACK_ERROR_SETUP_NATIVEINITFAILED;
}

1.       检查音频參数；

2.       创建一个AudioTrack(native)对象；

3.       创建一个AudioTrackJniStorage对象。

4.       调用set函数初始化AudioTrack。

buffersize = frameCount * 每帧数据量 = frameCount * (Channel数 * 每一个Channel数据量)

构造native AudioTrack

frameworks\av\media\libmedia\AudioTrack.cpp

AudioTrack::AudioTrack(): mStatus(NO_INIT),
  mIsTimed(false),
  mPreviousPriority(ANDROID_PRIORITY_NORMAL),
  mPreviousSchedulingGroup(SP_DEFAULT),
  mCblk(NULL)
{
}

构造AudioTrackJniStorage

AudioTrackJniStorage是音频数据存储的容器，是对匿名共享内存的封装。

struct audiotrack_callback_cookie {
    jclass      audioTrack_class;
    jobject     audioTrack_ref;//Java层AudioTrack对象引用
    bool        busy;//忙推断
    Condition   cond;//相互排斥量
};

class AudioTrackJniStorage {
    public:
        sp         mMemHeap;
        sp             mMemBase;
        audiotrack_callback_cookie mCallbackData;
        audio_stream_type_t        mStreamType;

    AudioTrackJniStorage() {
        mCallbackData.audioTrack_class = 0;
        mCallbackData.audioTrack_ref = 0;
        mStreamType = AUDIO_STREAM_DEFAULT;
    }

    ~AudioTrackJniStorage() {
        mMemBase.clear();
        mMemHeap.clear();
    }
    /**
     * 分配一块指定大小的匿名共享内存
     * @param sizeInBytes：匿名共享内存大小
     * @return
     */
    bool allocSharedMem(int sizeInBytes) {
    	 //创建一个匿名共享内存
        mMemHeap = new MemoryHeapBase(sizeInBytes, 0, "AudioTrack Heap Base");
        if (mMemHeap->getHeapID() < 0) {
            return false;
        }
        mMemBase = new MemoryBase(mMemHeap, 0, sizeInBytes);
        return true;
    }
};

/**
 * 创建匿名共享内存区域
 * @param size：匿名共享内存大小
 * @param flags：创建标志位
 * @param name：匿名共享内存名称
 */
MemoryHeapBase::MemoryHeapBase(size_t size, uint32_t flags, char const * name)
: mFD(-1), mSize(0), mBase(MAP_FAILED), mFlags(flags),
  mDevice(0), mNeedUnmap(false), mOffset(0)
{
	//获取内存页大小
	const size_t pagesize = getpagesize();
	//字节对齐
	size = ((size + pagesize-1) & ~(pagesize-1));
	/* 创建共享内存。打开/dev/ashmem设备。得到一个文件描写叙述符 */
	int fd = ashmem_create_region(name == NULL ? "MemoryHeapBase" : name, size);
	ALOGE_IF(fd<0, "error creating ashmem region: %s", strerror(errno));
	if (fd >= 0) {
		//通过mmap将匿名共享内存映射到当前进程地址空间
	    if (mapfd(fd, size) == NO_ERROR) {
	        if (flags & READ_ONLY) {
	            ashmem_set_prot_region(fd, PROT_READ);
	        }
	    }
	}
}

初始化AudioTrack

为AudioTrack设置音频參数信息,在Android4.4中，添加了一个參数transfer_type用于指定音频数据的传输方式，Android4.4定义了4种音频传输数据方式：

enum transfer_type {

            TRANSFER_DEFAULT,   // not specified explicitly; determine from the other parameters

            TRANSFER_CALLBACK,  // callback EVENT_MORE_DATA

            TRANSFER_OBTAIN,    // FIXME deprecated: call obtainBuffer() and releaseBuffer()

            TRANSFER_SYNC,      // synchronous write()

            TRANSFER_SHARED,    // shared memory

};

/**
 * 初始化AudioTrack
 * @param streamType  音频流类型
 * @param sampleRate  採样率
 * @param format      音频格式
 * @param channelMask 输出声道
 * @param frameCount  帧数
 * @param flags       输出标志位
 * @param cbf   Callback function. If not null, this function is called periodically
 *   to provide new data and inform of marker, position updates, etc.
 * @param user   Context for use by the callback receiver.
 * @param notificationFrames   The callback function is called each time notificationFrames          *  PCM frames have been consumed from track input buffer.
 * @param sharedBuffer 共享内存 
 * @param threadCanCallJava 
 * @param sessionId  	            
 * @return
 */
status_t AudioTrack::set(
        audio_stream_type_t streamType,
        uint32_t sampleRate,
        audio_format_t format,
        audio_channel_mask_t channelMask,
        int frameCountInt,
        audio_output_flags_t flags,
        callback_t cbf,
        void* user,
        int notificationFrames,
        const sp& sharedBuffer,
        bool threadCanCallJava,
        int sessionId,
        transfer_type transferType,
        const audio_offload_info_t *offloadInfo,
        int uid)
{
	//设置音频传输数据类型
    switch (transferType) {
    case TRANSFER_DEFAULT:
        if (sharedBuffer != 0) {
            transferType = TRANSFER_SHARED;
        } else if (cbf == NULL || threadCanCallJava) {
            transferType = TRANSFER_SYNC;
        } else {
            transferType = TRANSFER_CALLBACK;
        }
        break;
    case TRANSFER_CALLBACK:
        if (cbf == NULL || sharedBuffer != 0) {
            ALOGE("Transfer type TRANSFER_CALLBACK but cbf == NULL || sharedBuffer != 0");
            return BAD_VALUE;
        }
        break;
    case TRANSFER_OBTAIN:
    case TRANSFER_SYNC:
        if (sharedBuffer != 0) {
            ALOGE("Transfer type TRANSFER_OBTAIN but sharedBuffer != 0");
            return BAD_VALUE;
        }
        break;
    case TRANSFER_SHARED:
        if (sharedBuffer == 0) {
            ALOGE("Transfer type TRANSFER_SHARED but sharedBuffer == 0");
            return BAD_VALUE;
        }
        break;
    default:
        ALOGE("Invalid transfer type %d", transferType);
        return BAD_VALUE;
    }
    mTransfer = transferType;
    // FIXME "int" here is legacy and will be replaced by size_t later
    if (frameCountInt < 0) {
        ALOGE("Invalid frame count %d", frameCountInt);
        return BAD_VALUE;
    }
    size_t frameCount = frameCountInt;
    ALOGV_IF(sharedBuffer != 0, "sharedBuffer: %p, size: %d", sharedBuffer->pointer(),
            sharedBuffer->size());
    ALOGV("set() streamType %d frameCount %u flags %04x", streamType, frameCount, flags);
    AutoMutex lock(mLock);
    // invariant that mAudioTrack != 0 is true only after set() returns successfully
    if (mAudioTrack != 0) {
        ALOGE("Track already in use");
        return INVALID_OPERATION;
    }
    mOutput = 0;
    // 音频流类型设置
    if (streamType == AUDIO_STREAM_DEFAULT) {
        streamType = AUDIO_STREAM_MUSIC;
    }
    //依据音频流类型从AudioPolicyService中得到相应的音频採样率
    if (sampleRate == 0) {
        uint32_t afSampleRate;
        if (AudioSystem::getOutputSamplingRate(&afSampleRate, streamType) != NO_ERROR) {
            return NO_INIT;
        }
        sampleRate = afSampleRate;
    }
    mSampleRate = sampleRate;
    //音频格式设置
    if (format == AUDIO_FORMAT_DEFAULT) {
        format = AUDIO_FORMAT_PCM_16_BIT;
    }
    //假设没有设置声道，则默认设置为立体声通道
    if (channelMask == 0) {
        channelMask = AUDIO_CHANNEL_OUT_STEREO;
    }
    // validate parameters
    if (!audio_is_valid_format(format)) {
        ALOGE("Invalid format %d", format);
        return BAD_VALUE;
    }
    // AudioFlinger does not currently support 8-bit data in shared memory
    if (format == AUDIO_FORMAT_PCM_8_BIT && sharedBuffer != 0) {
        ALOGE("8-bit data in shared memory is not supported");
        return BAD_VALUE;
    }
    // force direct flag if format is not linear PCM
    // or offload was requested
    if ((flags & AUDIO_OUTPUT_FLAG_COMPRESS_OFFLOAD)
            || !audio_is_linear_pcm(format)) {
        ALOGV( (flags & AUDIO_OUTPUT_FLAG_COMPRESS_OFFLOAD)
                    ?

 

"Offload request, forcing to Direct Output" : "Not linear PCM, forcing to Direct Output"); flags = (audio_output_flags_t) // FIXME why can't we allow direct AND fast? ((flags | AUDIO_OUTPUT_FLAG_DIRECT) & ~AUDIO_OUTPUT_FLAG_FAST); } // only allow deep buffering for music stream type if (streamType != AUDIO_STREAM_MUSIC) { flags = (audio_output_flags_t)(flags &~AUDIO_OUTPUT_FLAG_DEEP_BUFFER); } //输出声道合法性检查 if (!audio_is_output_channel(channelMask)) { ALOGE("Invalid channel mask %#x", channelMask); return BAD_VALUE; } mChannelMask = channelMask; //计算声道个数 uint32_t channelCount = popcount(channelMask); mChannelCount = channelCount; if (audio_is_linear_pcm(format)) { mFrameSize = channelCount * audio_bytes_per_sample(format); mFrameSizeAF = channelCount * sizeof(int16_t); } else { mFrameSize = sizeof(uint8_t); mFrameSizeAF = sizeof(uint8_t); } /** * audio_io_handle_t是一个整形值，用于标示音频播放线程，这里更加音频參数 * 从AudioFlinger中查找用于播放此音频的播放线程。并返回该播放线程的ID值 */ audio_io_handle_t output = AudioSystem::getOutput( streamType, sampleRate, format, channelMask, flags, offloadInfo); if (output == 0) { ALOGE("Could not get audio output for stream type %d", streamType); return BAD_VALUE; } //AudioTrack初始化 mVolume[LEFT] = 1.0f; mVolume[RIGHT] = 1.0f; mSendLevel = 0.0f; mFrameCount = frameCount; mReqFrameCount = frameCount; mNotificationFramesReq = notificationFrames; mNotificationFramesAct = 0; mSessionId = sessionId; if (uid == -1 || (IPCThreadState::self()->getCallingPid() != getpid())) { mClientUid = IPCThreadState::self()->getCallingUid(); } else { mClientUid = uid; } mAuxEffectId = 0; mFlags = flags; mCbf = cbf; //假设设置了提供音频数据的回调函数，则启动AudioTrackThread线程来提供音频数据 if (cbf != NULL) { mAudioTrackThread = new AudioTrackThread(*this, threadCanCallJava); mAudioTrackThread->run("AudioTrack", ANDROID_PRIORITY_AUDIO, 0 /*stack*/); } // create the IAudioTrack status_t status = createTrack_l(streamType, sampleRate, format, frameCount, flags, sharedBuffer, output, 0 /*epoch*/); if (status != NO_ERROR) { if (mAudioTrackThread != 0) { mAudioTrackThread->requestExit(); // see comment in AudioTrack.h mAudioTrackThread->requestExitAndWait(); mAudioTrackThread.clear(); } //Use of direct and offloaded output streams is ref counted by audio policy manager. // As getOutput was called above and resulted in an output stream to be opened, // we need to release it. AudioSystem::releaseOutput(output); return status; } mStatus = NO_ERROR; mStreamType = streamType; mFormat = format; mSharedBuffer = sharedBuffer; mState = STATE_STOPPED; mUserData = user; mLoopPeriod = 0; mMarkerPosition = 0; mMarkerReached = false; mNewPosition = 0; mUpdatePeriod = 0; AudioSystem::acquireAudioSessionId(mSessionId); mSequence = 1; mObservedSequence = mSequence; mInUnderrun = false; mOutput = output; return NO_ERROR; }

我们知道，AudioPolicyService启动时载入了系统支持的全部音频接口。而且打开了默认的音频输出。打开音频输出时，调用AudioFlinger::openOutput()函数为当前打开的音频输出接口创建一个PlaybackThread线程，同一时候为该线程分配一个全局唯一的audio_io_handle_t值，并以键值对的形式保存在AudioFlinger的成员变量mPlaybackThreads中。在这里首先依据音频參数通过调用AudioSystem::getOutput()函数得到当前音频输出接口的PlaybackThread线程id号。同一时候传递给createTrack函数用于创建Track。

AudioTrack在AudioFlinger中是以Track来管理的。不过由于它们之间是跨进程的关系，因此须要一个“桥梁”来维护，这个沟通的媒介是IAudioTrack。函数createTrack_l除了为AudioTrack在AudioFlinger中申请一个Track外。还会建立两者间IAudioTrack桥梁。

获取音频输出

获取音频输出就是依据音频參数如採样率、声道、格式等从已经打开的音频输出描写叙述符列表中查找合适的音频输出AudioOutputDescriptor，并返回该音频输出在AudioFlinger中创建的播放线程id号，假设没有合适当前音频输出參数的AudioOutputDescriptor。则请求AudioFlinger打开一个新的音频输出通道，并为当前音频输出创建相应的播放线程，返回该播放线程的id号。详细过程请參考Android AudioPolicyService服务启动过程中的打开输出小节。

创建AudioTrackThread线程

初始化AudioTrack时，假设audioCallback为Null，就会创建AudioTrackThread线程。

AudioTrack支持两种数据输入方式：

1）  Push方式：用户主动write，MediaPlayerService通常採用此方式；

2）  Pull方式： AudioTrackThread线程通过audioCallback回调函数主动从用户那里获取数据。ToneGenerator就是採用这样的方式。

bool AudioTrack::AudioTrackThread::threadLoop()
{
    {
        AutoMutex _l(mMyLock);
        if (mPaused) {
            mMyCond.wait(mMyLock);
            // caller will check for exitPending()
            return true;
        }
}
//调用创建当前AudioTrackThread线程的AudioTrack的processAudioBuffer函数
    if (!mReceiver.processAudioBuffer(this)) {
        pause();
    }
    return true;
}

申请Track

音频播放须要AudioTrack写入音频数据，同一时候须要AudioFlinger完毕混音，因此须要在AudioTrack与AudioFlinger之间建立数据通道，而AudioTrack与AudioFlinger又分属不同的进程空间，Android系统採用Binder通信方式来搭建它们之间的桥梁。

 

status_t AudioTrack::createTrack_l(
        audio_stream_type_t streamType,
        uint32_t sampleRate,
        audio_format_t format,
        size_t frameCount,
        audio_output_flags_t flags,
        const sp& sharedBuffer,
        audio_io_handle_t output,
        size_t epoch)
{
    status_t status;
    //得到AudioFlinger的代理对象
    const sp& audioFlinger = AudioSystem::get_audio_flinger();
    if (audioFlinger == 0) {
        ALOGE("Could not get audioflinger");
        return NO_INIT;
    }
    //得到输出时延
    uint32_t afLatency;
    status = AudioSystem::getLatency(output, streamType, &afLatency);
    if (status != NO_ERROR) {
        ALOGE("getLatency(%d) failed status %d", output, status);
        return NO_INIT;
    }
    //得到音频帧数
    size_t afFrameCount;
    status = AudioSystem::getFrameCount(output, streamType, &afFrameCount);
    if (status != NO_ERROR) {
        ALOGE("getFrameCount(output=%d, streamType=%d) status %d", output, streamType, status);
        return NO_INIT;
    }
    //得到採样率
    uint32_t afSampleRate;
    status = AudioSystem::getSamplingRate(output, streamType, &afSampleRate);
    if (status != NO_ERROR) {
        ALOGE("getSamplingRate(output=%d, streamType=%d) status %d", output, streamType, status);
        return NO_INIT;
    }
    // Client decides whether the track is TIMED (see below), but can only express a preference
    // for FAST.  Server will perform additional tests.
    if ((flags & AUDIO_OUTPUT_FLAG_FAST) && !(
            // either of these use cases:
            // use case 1: shared buffer
            (sharedBuffer != 0) ||
            // use case 2: callback handler
            (mCbf != NULL))) {
        ALOGW("AUDIO_OUTPUT_FLAG_FAST denied by client");
        // once denied, do not request again if IAudioTrack is re-created
        flags = (audio_output_flags_t) (flags & ~AUDIO_OUTPUT_FLAG_FAST);
        mFlags = flags;
    }
    ALOGV("createTrack_l() output %d afLatency %d", output, afLatency);
    // The client's AudioTrack buffer is divided into n parts for purpose of wakeup by server, where
    //  n = 1   fast track; nBuffering is ignored
    //  n = 2   normal track, no sample rate conversion
    //  n = 3   normal track, with sample rate conversion
    //          (pessimistic; some non-1:1 conversion ratios don't actually need triple-buffering)
    //  n > 3   very high latency or very small notification interval; nBuffering is ignored
    const uint32_t nBuffering = (sampleRate == afSampleRate) ? 2 : 3;
    mNotificationFramesAct = mNotificationFramesReq;
    if (!audio_is_linear_pcm(format)) {
        if (sharedBuffer != 0) {//static模式
            // Same comment as below about ignoring frameCount parameter for set()
            frameCount = sharedBuffer->size();
        } else if (frameCount == 0) {
            frameCount = afFrameCount;
        }
        if (mNotificationFramesAct != frameCount) {
            mNotificationFramesAct = frameCount;
        }
    } else if (sharedBuffer != 0) {// static模式
        // Ensure that buffer alignment matches channel count
        // 8-bit data in shared memory is not currently supported by AudioFlinger
        size_t alignment = /* format == AUDIO_FORMAT_PCM_8_BIT ? 1 : */ 2;
        if (mChannelCount > 1) {
            // More than 2 channels does not require stronger alignment than stereo
            alignment <<= 1;
        }
        if (((size_t)sharedBuffer->pointer() & (alignment - 1)) != 0) {
            ALOGE("Invalid buffer alignment: address %p, channel count %u",
                    sharedBuffer->pointer(), mChannelCount);
            return BAD_VALUE;
        }
        // When initializing a shared buffer AudioTrack via constructors,
        // there's no frameCount parameter.
        // But when initializing a shared buffer AudioTrack via set(),
        // there _is_ a frameCount parameter.  We silently ignore it.
        frameCount = sharedBuffer->size()/mChannelCount/sizeof(int16_t);
    } else if (!(flags & AUDIO_OUTPUT_FLAG_FAST)) {
        // FIXME move these calculations and associated checks to server
        // Ensure that buffer depth covers at least audio hardware latency
        uint32_t minBufCount = afLatency / ((1000 * afFrameCount)/afSampleRate);
        ALOGV("afFrameCount=%d, minBufCount=%d, afSampleRate=%u, afLatency=%d",
                afFrameCount, minBufCount, afSampleRate, afLatency);
        if (minBufCount <= nBuffering) {
            minBufCount = nBuffering;
        }
        size_t minFrameCount = (afFrameCount*sampleRate*minBufCount)/afSampleRate;
        ALOGV("minFrameCount: %u, afFrameCount=%d, minBufCount=%d, sampleRate=%u, afSampleRate=%u"", afLatency=%d",minFrameCount, afFrameCount, minBufCount, sampleRate, afSampleRate, afLatency);
        if (frameCount == 0) {
            frameCount = minFrameCount;
        } else if (frameCount < minFrameCount) {
            // not ALOGW because it happens all the time when playing key clicks over A2DP
            ALOGV("Minimum buffer size corrected from %d to %d",
                     frameCount, minFrameCount);
            frameCount = minFrameCount;
        }
        // Make sure that application is notified with sufficient margin before underrun
        if (mNotificationFramesAct == 0 || mNotificationFramesAct > frameCount/nBuffering) {
            mNotificationFramesAct = frameCount/nBuffering;
        }
    } else {
        // For fast tracks, the frame count calculations and checks are done by server
    }
    IAudioFlinger::track_flags_t trackFlags = IAudioFlinger::TRACK_DEFAULT;
    if (mIsTimed) {
        trackFlags |= IAudioFlinger::TRACK_TIMED;
    }
    pid_t tid = -1;
    if (flags & AUDIO_OUTPUT_FLAG_FAST) {
        trackFlags |= IAudioFlinger::TRACK_FAST;
        if (mAudioTrackThread != 0) {
            tid = mAudioTrackThread->getTid();
        }
    }
    if (flags & AUDIO_OUTPUT_FLAG_COMPRESS_OFFLOAD) {
        trackFlags |= IAudioFlinger::TRACK_OFFLOAD;
    }
    //向AudioFlinger发送createTrack请求，在stream模式下sharedBuffer为空，output为AudioFlinger中播放线程的id号
    sp track = audioFlinger->createTrack(streamType,
                                                      sampleRate,
                                                      // AudioFlinger only sees 16-bit PCM
                                                      format == AUDIO_FORMAT_PCM_8_BIT ?
                                                              AUDIO_FORMAT_PCM_16_BIT : format,
                                                      mChannelMask,
                                                      frameCount,
                                                      &trackFlags,
                                                      sharedBuffer,
                                                      output,
                                                      tid,
                                                      &mSessionId,
                                                      mName,
                                                      mClientUid,
                                                      &status);
    if (track == 0) {
        ALOGE("AudioFlinger could not create track, status: %d", status);
        return status;
    }
    //AudioFlinger创建Tack对象时会分配一块共享内存，这里得到这块共享内存的代理对象BpMemory
    sp iMem = track->getCblk();
    if (iMem == 0) {
        ALOGE("Could not get control block");
        return NO_INIT;
    }
    // invariant that mAudioTrack != 0 is true only after set() returns successfully
    if (mAudioTrack != 0) {
        mAudioTrack->asBinder()->unlinkToDeath(mDeathNotifier, this);
        mDeathNotifier.clear();
    }
    //将创建的Track代理对象、匿名共享内存代理对象保存到AudioTrack的成员变量中
    mAudioTrack = track;
    mCblkMemory = iMem;
    //保存匿名共享内存的首地址。在匿名共享内存的头部存放了一个audio_track_cblk_t对象
    audio_track_cblk_t* cblk = static_cast(iMem->pointer());
    mCblk = cblk;
    size_t temp = cblk->frameCount_;
    if (temp < frameCount || (frameCount == 0 && temp == 0)) {
        // In current design, AudioTrack client checks and ensures frame count validity before
        // passing it to AudioFlinger so AudioFlinger should not return a different value except
        // for fast track as it uses a special method of assigning frame count.
        ALOGW("Requested frameCount %u but received frameCount %u", frameCount, temp);
    }
    frameCount = temp;
    mAwaitBoost = false;
    if (flags & AUDIO_OUTPUT_FLAG_FAST) {
        if (trackFlags & IAudioFlinger::TRACK_FAST) {
            ALOGV("AUDIO_OUTPUT_FLAG_FAST successful; frameCount %u", frameCount);
            mAwaitBoost = true;
            if (sharedBuffer == 0) {
                // double-buffering is not required for fast tracks, due to tighter scheduling
                if (mNotificationFramesAct == 0 || mNotificationFramesAct > frameCount) {
                    mNotificationFramesAct = frameCount;
                }
            }
        } else {
            ALOGV("AUDIO_OUTPUT_FLAG_FAST denied by server; frameCount %u", frameCount);
            // once denied, do not request again if IAudioTrack is re-created
            flags = (audio_output_flags_t) (flags & ~AUDIO_OUTPUT_FLAG_FAST);
            mFlags = flags;
            if (sharedBuffer == 0) {//stream模式
                if (mNotificationFramesAct == 0 || mNotificationFramesAct > frameCount/nBuffering) {
                    mNotificationFramesAct = frameCount/nBuffering;
                }
            }
        }
    }
    if (flags & AUDIO_OUTPUT_FLAG_COMPRESS_OFFLOAD) {
        if (trackFlags & IAudioFlinger::TRACK_OFFLOAD) {
            ALOGV("AUDIO_OUTPUT_FLAG_OFFLOAD successful");
        } else {
            ALOGW("AUDIO_OUTPUT_FLAG_OFFLOAD denied by server");
            flags = (audio_output_flags_t) (flags & ~AUDIO_OUTPUT_FLAG_COMPRESS_OFFLOAD);
            mFlags = flags;
            return NO_INIT;
        }
    }
    mRefreshRemaining = true;
    // Starting address of buffers in shared memory.  If there is a shared buffer, buffers
    // is the value of pointer() for the shared buffer, otherwise buffers points
    // immediately after the control block.  This address is for the mapping within client
    // address space.  AudioFlinger::TrackBase::mBuffer is for the server address space.
    void* buffers;
    if (sharedBuffer == 0) {//stream模式
        buffers = (char*)cblk + sizeof(audio_track_cblk_t);
    } else {
        buffers = sharedBuffer->pointer();
    }
    mAudioTrack->attachAuxEffect(mAuxEffectId);
    // FIXME don't believe this lie
    mLatency = afLatency + (1000*frameCount) / sampleRate;
    mFrameCount = frameCount;
    // If IAudioTrack is re-created, don't let the requested frameCount
    // decrease.  This can confuse clients that cache frameCount().
    if (frameCount > mReqFrameCount) {
        mReqFrameCount = frameCount;
    }
    // update proxy
    if (sharedBuffer == 0) {
        mStaticProxy.clear();
        mProxy = new AudioTrackClientProxy(cblk, buffers, frameCount, mFrameSizeAF);
    } else {
        mStaticProxy = new StaticAudioTrackClientProxy(cblk, buffers, frameCount, mFrameSizeAF);
        mProxy = mStaticProxy;
    }
    mProxy->setVolumeLR((uint32_t(uint16_t(mVolume[RIGHT] * 0x1000)) << 16) |
            uint16_t(mVolume[LEFT] * 0x1000));
    mProxy->setSendLevel(mSendLevel);
    mProxy->setSampleRate(mSampleRate);
    mProxy->setEpoch(epoch);
    mProxy->setMinimum(mNotificationFramesAct);
    mDeathNotifier = new DeathNotifier(this);
    mAudioTrack->asBinder()->linkToDeath(mDeathNotifier, this);
    return NO_ERROR;
}

IAudioTrack建立了AudioTrack与AudioFlinger之间的关系，在static模式下，用于存放音频数据的匿名共享内存在AudioTrack这边创建。在stream播放模式下，匿名共享内存却是在AudioFlinger这边创建。这两种播放模式下创建的匿名共享内存是有差别的，stream模式下的匿名共享内存头部会创建一个audio_track_cblk_t对象，用于协调生产者AudioTrack和消费者AudioFlinger之间的步调。createTrack就是在AudioFlinger中创建一个Track对象。

 

frameworks\av\services\audioflinger\ AudioFlinger.cpp

sp AudioFlinger::createTrack(
        audio_stream_type_t streamType,
        uint32_t sampleRate,
        audio_format_t format,
        audio_channel_mask_t channelMask,
        size_t frameCount,
        IAudioFlinger::track_flags_t *flags,
        const sp& sharedBuffer,
        audio_io_handle_t output,
        pid_t tid,
        int *sessionId,
        String8& name,
        int clientUid,
        status_t *status)
{
    sp track;
    sp trackHandle;
    sp client;
    status_t lStatus;
    int lSessionId;
    // client AudioTrack::set already implements AUDIO_STREAM_DEFAULT => AUDIO_STREAM_MUSIC,
    // but if someone uses binder directly they could bypass that and cause us to crash
    if (uint32_t(streamType) >= AUDIO_STREAM_CNT) {
        ALOGE("createTrack() invalid stream type %d", streamType);
        lStatus = BAD_VALUE;
        goto Exit;
    }
    // client is responsible for conversion of 8-bit PCM to 16-bit PCM,
    // and we don't yet support 8.24 or 32-bit PCM
    if (audio_is_linear_pcm(format) && format != AUDIO_FORMAT_PCM_16_BIT) {
        ALOGE("createTrack() invalid format %d", format);
        lStatus = BAD_VALUE;
        goto Exit;
    }
    {
        Mutex::Autolock _l(mLock);
        //依据播放线程ID号查找出相应的PlaybackThread，在openout时，播放线程以key/value形式保存在AudioFlinger的mPlaybackThreads中
        PlaybackThread *thread = checkPlaybackThread_l(output);
        PlaybackThread *effectThread = NULL;
        if (thread == NULL) {
            ALOGE("no playback thread found for output handle %d", output);
            lStatus = BAD_VALUE;
            goto Exit;
        }
        pid_t pid = IPCThreadState::self()->getCallingPid();
        //依据客户端进程pid查找是否已经为该客户进程创建了Client对象。假设没有，则创建一个Client对象
        client = registerPid_l(pid);
        ALOGV("createTrack() sessionId: %d", (sessionId == NULL) ?

 

-2 : *sessionId); if (sessionId != NULL && *sessionId != AUDIO_SESSION_OUTPUT_MIX) { // check if an effect chain with the same session ID is present on another // output thread and move it here. //遍历全部的播放线程。不包含输出线程，假设该线程中Track的sessionId与当前同样，则取出该线程作为当前Track的effectThread。 for (size_t i = 0; i < mPlaybackThreads.size(); i++) { sp t = mPlaybackThreads.valueAt(i); if (mPlaybackThreads.keyAt(i) != output) { uint32_t sessions = t->hasAudioSession(*sessionId); if (sessions & PlaybackThread::EFFECT_SESSION) { effectThread = t.get(); break; } } } lSessionId = *sessionId; } else { // if no audio session id is provided, create one here lSessionId = nextUniqueId(); if (sessionId != NULL) { *sessionId = lSessionId; } } ALOGV("createTrack() lSessionId: %d", lSessionId); //在找到的PlaybackThread线程中创建Track track = thread->createTrack_l(client, streamType, sampleRate, format, channelMask, frameCount, sharedBuffer, lSessionId, flags, tid, clientUid, &lStatus); // move effect chain to this output thread if an effect on same session was waiting // for a track to be created if (lStatus == NO_ERROR && effectThread != NULL) { Mutex::Autolock _dl(thread->mLock); Mutex::Autolock _sl(effectThread->mLock); moveEffectChain_l(lSessionId, effectThread, thread, true); } // Look for sync events awaiting for a session to be used. for (int i = 0; i < (int)mPendingSyncEvents.size(); i++) { if (mPendingSyncEvents[i]->triggerSession() == lSessionId) { if (thread->isValidSyncEvent(mPendingSyncEvents[i])) { if (lStatus == NO_ERROR) { (void) track->setSyncEvent(mPendingSyncEvents[i]); } else { mPendingSyncEvents[i]->cancel(); } mPendingSyncEvents.removeAt(i); i--; } } } } //此时Track已成功创建，还须要为该Track创建代理对象TrackHandle if (lStatus == NO_ERROR) { // s for server's pid, n for normal mixer name, f for fast index name = String8::format("s:%d;n:%d;f:%d", getpid_cached, track->name() - AudioMixer::TRACK0,track->fastIndex()); trackHandle = new TrackHandle(track); } else { // remove local strong reference to Client before deleting the Track so that the Client destructor is called by the TrackBase destructor with mLock held client.clear(); track.clear(); } Exit: if (status != NULL) { *status = lStatus; } /** * 向客户进程返回IAudioTrack的代理对象，这样客户进程就能够跨进程訪问创建的Track了， * 訪问方式例如以下：BpAudioTrack --> BnAudioTrack --> TrackHandle --> Track */ return trackHandle; }

该函数首先以单例模式为应用程序进程创建一个Client对象，直接对话某个客户进程。然后依据播放线程ID找出相应的PlaybackThread。并将创建Track的任务转交给它，PlaybackThread完毕Track创建后，因为Track没有通信功能，因此还须要为其创建一个代理通信业务的TrackHandle对象。

构造Client对象

依据进程pid。为请求播放音频的client创建一个Client对象。

 

sp AudioFlinger::registerPid_l(pid_t pid)
{
    // If pid is already in the mClients wp<> map, then use that entry
    // (for which promote() is always != 0), otherwise create a new entry and Client.
    sp client = mClients.valueFor(pid).promote();
    if (client == 0) {
        client = new Client(this, pid);
        mClients.add(pid, client);
    }
    return client;
}

AudioFlinger的成员变量mClients以键值对的形式保存pid和Client对象。这里首先取出pid相应的Client对象，假设该对象为空。则为client进程创建一个新的Client对象。

 

AudioFlinger::Client::Client(const sp& audioFlinger, pid_t pid)
    : RefBase(),mAudioFlinger(audioFlinger),
        // FIXME should be a "k" constant not hard-coded, in .h or ro. property, see 4 lines below
        mMemoryDealer(new MemoryDealer(2*1024*1024, "AudioFlinger::Client")),
        mPid(pid),
        mTimedTrackCount(0)
{
    // 1 MB of address space is good for 32 tracks, 8 buffers each, 4 KB/buffer
}

构造Client对象时。创建了一个MemoryDealer对象，该对象用于分配共享内存。

frameworks\native\libs\binder\ MemoryDealer.cpp

MemoryDealer::MemoryDealer(size_t size, const char* name)
    : mHeap(new MemoryHeapBase(size, 0, name)),//创建指定大小的共享内存
    mAllocator(new SimpleBestFitAllocator(size))//创建内存分配器
{    
}

MemoryDealer是个工具类。用于分配共享内存。每个Client都拥有一个MemoryDealer对象，这就意味着每个client进程都是在自己独有的内存空间中分配共享内存。MemoryDealer构造时创建了一个大小为2*1024*1024的匿名共享内存，该客户进程全部的AudioTrack在AudioFlinger中创建的Track都是在这块共享内存中分配buffer。

SimpleBestFitAllocator::SimpleBestFitAllocator(size_t size)
{
    size_t pagesize = getpagesize();
    mHeapSize = ((size + pagesize-1) & ~(pagesize-1));//页对齐
    chunk_t* node = new chunk_t(0, mHeapSize / kMemoryAlign);
    mList.insertHead(node);
}

由此可知。当应用程序进程中的AudioTrack请求AudioFlinger在某个PlaybackThread中创建Track对象时，AudioFlinger首先会为应用程序进程创建一个Client对象，同一时候创建一块大小为2M的共享内存。在创建Track时，Track将在2M共享内存中分配buffer用于音频播放。

 

创建Track对象

sp AudioFlinger::PlaybackThread::createTrack_l(
        const sp& client,
        audio_stream_type_t streamType,
        uint32_t sampleRate,
        audio_format_t format,
        audio_channel_mask_t channelMask,
        size_t frameCount,
        const sp& sharedBuffer,
        int sessionId,
        IAudioFlinger::track_flags_t *flags,
        pid_t tid,
        int uid,
        status_t *status)
{
    sp track;
    status_t lStatus;
    bool isTimed = (*flags & IAudioFlinger::TRACK_TIMED) != 0;
    // client expresses a preference for FAST, but we get the final say
    if (*flags & IAudioFlinger::TRACK_FAST) {
      if (
            // not timed
            (!isTimed) &&
            // either of these use cases:
            (
              // use case 1: shared buffer with any frame count
              (
                (sharedBuffer != 0)
              ) ||
              // use case 2: callback handler and frame count is default or at least as large as HAL
              (
                (tid != -1) &&
                ((frameCount == 0) ||
                (frameCount >= (mFrameCount * kFastTrackMultiplier)))
              )
            ) &&
            // PCM data
            audio_is_linear_pcm(format) &&
            // mono or stereo
            ( (channelMask == AUDIO_CHANNEL_OUT_MONO) ||
              (channelMask == AUDIO_CHANNEL_OUT_STEREO) ) &&
#ifndef FAST_TRACKS_AT_NON_NATIVE_SAMPLE_RATE
            // hardware sample rate
            (sampleRate == mSampleRate) &&
#endif
            // normal mixer has an associated fast mixer
            hasFastMixer() &&
            // there are sufficient fast track slots available
            (mFastTrackAvailMask != 0)
            // FIXME test that MixerThread for this fast track has a capable output HAL
            // FIXME add a permission test also?

 

) { // if frameCount not specified, then it defaults to fast mixer (HAL) frame count if (frameCount == 0) { frameCount = mFrameCount * kFastTrackMultiplier; } ALOGV("AUDIO_OUTPUT_FLAG_FAST accepted: frameCount=%d mFrameCount=%d", frameCount, mFrameCount); } else { ALOGV("AUDIO_OUTPUT_FLAG_FAST denied: isTimed=%d sharedBuffer=%p frameCount=%d " "mFrameCount=%d format=%d isLinear=%d channelMask=%#x sampleRate=%u mSampleRate=%u ""hasFastMixer=%d tid=%d fastTrackAvailMask=%#x", isTimed, sharedBuffer.get(), frameCount, mFrameCount, format, audio_is_linear_pcm(format), channelMask, sampleRate, mSampleRate, hasFastMixer(), tid, mFastTrackAvailMask); *flags &= ~IAudioFlinger::TRACK_FAST; // For compatibility with AudioTrack calculation, buffer depth is forced // to be at least 2 x the normal mixer frame count and cover audio hardware latency. // This is probably too conservative, but legacy application code may depend on it. // If you change this calculation, also review the start threshold which is related. uint32_t latencyMs = mOutput->stream->get_latency(mOutput->stream); uint32_t minBufCount = latencyMs / ((1000 * mNormalFrameCount) / mSampleRate); if (minBufCount < 2) { minBufCount = 2; } size_t minFrameCount = mNormalFrameCount * minBufCount; if (frameCount < minFrameCount) { frameCount = minFrameCount; } } } if (mType == DIRECT) { if ((format & AUDIO_FORMAT_MAIN_MASK) == AUDIO_FORMAT_PCM) { if (sampleRate != mSampleRate || format != mFormat || channelMask != mChannelMask) { ALOGE("createTrack_l() Bad parameter: sampleRate %u format %d, channelMask 0x%08x ""for output %p with format %d",sampleRate, format, channelMask, mOutput, mFormat); lStatus = BAD_VALUE; goto Exit; } } } else if (mType == OFFLOAD) { if (sampleRate != mSampleRate || format != mFormat || channelMask != mChannelMask) { ALOGE("createTrack_l() Bad parameter: sampleRate %d format %d, channelMask 0x%08x \"""for output %p with format %d",sampleRate, format, channelMask, mOutput, mFormat); lStatus = BAD_VALUE; goto Exit; } } else { if ((format & AUDIO_FORMAT_MAIN_MASK) != AUDIO_FORMAT_PCM) { ALOGE("createTrack_l() Bad parameter: format %d \"" "for output %p with format %d",format, mOutput, mFormat); lStatus = BAD_VALUE; goto Exit; } // Resampler implementation limits input sampling rate to 2 x output sampling rate. if (sampleRate > mSampleRate*2) { ALOGE("Sample rate out of range: %u mSampleRate %u", sampleRate, mSampleRate); lStatus = BAD_VALUE; goto Exit; } } lStatus = initCheck(); if (lStatus != NO_ERROR) { ALOGE("Audio driver not initialized."); goto Exit; } { // scope for mLock Mutex::Autolock _l(mLock); ALOGD("ceateTrack_l() got lock"); // SPRD: Add some log // all tracks in same audio session must share the same routing strategy otherwise // conflicts will happen when tracks are moved from one output to another by audio policy // manager uint32_t strategy = AudioSystem::getStrategyForStream(streamType); for (size_t i = 0; i < mTracks.size(); ++i) { sp t = mTracks[i]; if (t != 0 && !t->isOutputTrack()) { uint32_t actual = AudioSystem::getStrategyForStream(t->streamType()); if (sessionId == t->sessionId() && strategy != actual) { ALOGE("createTrack_l() mismatched strategy; expected %u but found %u", strategy, actual); lStatus = BAD_VALUE; goto Exit; } } } if (!isTimed) { track = new Track(this, client, streamType, sampleRate, format, channelMask, frameCount, sharedBuffer, sessionId, uid, *flags); } else { track = TimedTrack::create(this, client, streamType, sampleRate, format, channelMask, frameCount, sharedBuffer, sessionId, uid); } if (track == 0 || track->getCblk() == NULL || track->name() < 0) { lStatus = NO_MEMORY; goto Exit; } mTracks.add(track); sp chain = getEffectChain_l(sessionId); if (chain != 0) { ALOGV("createTrack_l() setting main buffer %p", chain->inBuffer()); track->setMainBuffer(chain->inBuffer()); chain->setStrategy(AudioSystem::getStrategyForStream(track->streamType())); chain->incTrackCnt(); } if ((*flags & IAudioFlinger::TRACK_FAST) && (tid != -1)) { pid_t callingPid = IPCThreadState::self()->getCallingPid(); // we don't have CAP_SYS_NICE, nor do we want to have it as it's too powerful, // so ask activity manager to do this on our behalf sendPrioConfigEvent_l(callingPid, tid, kPriorityAudioApp); } } lStatus = NO_ERROR; Exit: if (status) { *status = lStatus; } return track; }

这里就为AudioTrack创建了一个Track对象。Track继承于TrackBase，因此构造Track时，首先运行TrackBase的构造函数。

 

AudioFlinger::ThreadBase::TrackBase::TrackBase(
            ThreadBase *thread,//所属的播放线程
            const sp& client,//所属的Client
            uint32_t sampleRate,//採样率
            audio_format_t format,//音频格式
            audio_channel_mask_t channelMask,//声道
            size_t frameCount,//音频帧个数
            const sp& sharedBuffer,//共享内存
            int sessionId,
            int clientUid,
            bool isOut)
    :   RefBase(),
        mThread(thread),
        mClient(client),
        mCblk(NULL),
        // mBuffer
        mState(IDLE),
        mSampleRate(sampleRate),
        mFormat(format),
        mChannelMask(channelMask),
        mChannelCount(popcount(channelMask)),
        mFrameSize(audio_is_linear_pcm(format) ?
                mChannelCount * audio_bytes_per_sample(format) : sizeof(int8_t)),
        mFrameCount(frameCount),
        mSessionId(sessionId),
        mIsOut(isOut),
        mServerProxy(NULL),
        mId(android_atomic_inc(&nextTrackId)),
        mTerminated(false)
{
    // if the caller is us, trust the specified uid
    if (IPCThreadState::self()->getCallingPid() != getpid_cached || clientUid == -1) {
        int newclientUid = IPCThreadState::self()->getCallingUid();
        if (clientUid != -1 && clientUid != newclientUid) {
            ALOGW("uid %d tried to pass itself off as %d", newclientUid, clientUid);
        }
        clientUid = newclientUid;
    }
    // clientUid contains the uid of the app that is responsible for this track, so we can blame
    //得到应用进程uid
    mUid = clientUid;
    // client == 0 implies sharedBuffer == 0
    ALOG_ASSERT(!(client == 0 && sharedBuffer != 0));
    ALOGV_IF(sharedBuffer != 0, "sharedBuffer: %p, size: %d", sharedBuffer->pointer(),
            sharedBuffer->size());
    //计算audio_track_cblk_t大小
    size_t size = sizeof(audio_track_cblk_t);
    //计算存放音频数据的buffer大小。= frameCount*mFrameSize
    size_t bufferSize = (sharedBuffer == 0 ? roundup(frameCount) : frameCount) * mFrameSize;
    /**
     * stream模式下，须要audio_track_cblk_t来协调生成者和消费者。计算共享内存大小   
     *  --------------------------------------------------------
     * | audio_track_cblk_t |               buffer                   |
     *  --------------------------------------------------------
     */
    if (sharedBuffer == 0) {//stream模式下
        size += bufferSize;
    }
    //假设Client不为空。就通过Client来分配buffer
    if (client != 0) {
    	//请求Client中的MemoryDealer工具类来分配buffer
        mCblkMemory = client->heap()->allocate(size);
        //分配成功
        if (mCblkMemory != 0) {
        	//将共享内存的指针强制转换为audio_track_cblk_t
            mCblk = static_cast(mCblkMemory->pointer());
            // can't assume mCblk != NULL
        } else {
            ALOGE("not enough memory for AudioTrack size=%u", size);
            client->heap()->dump("AudioTrack");
            return;
        }
    } else {//Client为空，使用数组方式分配内存空间
        // this syntax avoids calling the audio_track_cblk_t constructor twice
        mCblk = (audio_track_cblk_t *) new uint8_t[size];
        // assume mCblk != NULL
    }
    /**
     * 当为应用进程创建了Client对象。则通过Client来分配音频数据buffer，否则通过数组方式分配buffer。  
     * stream模式下，在分配好的buffer头部创建audio_track_cblk_t对象。而static模式下。创建单独的
     * audio_track_cblk_t对象。
     */
if (mCblk != NULL) {
	  // construct the shared structure in-place.
        new(mCblk) audio_track_cblk_t();
        // clear all buffers
        mCblk->frameCount_ = frameCount;
        if (sharedBuffer == 0) {//stream模式
        	//将mBuffer指向数据buffer的首地址
            mBuffer = (char*)mCblk + sizeof(audio_track_cblk_t);
            //清空数据buffer
            memset(mBuffer, 0, bufferSize);
         } else {//static模式
            mBuffer = sharedBuffer->pointer();
 #if 0
            mCblk->mFlags = CBLK_FORCEREADY;    // FIXME hack, need to fix the track ready logic
#endif
        }
#ifdef TEE_SINK
	…
#endif
        ALOGD("TrackBase constructed"); // SPRD: add some log
    }
}

TrackBase构造过程主要是为音频播放分配共享内存，在static模式下，共享内存由应用进程自身分配，但在stream模式，共享内存由AudioFlinger分配，static和stream模式下，都会创建audio_track_cblk_t对象，唯一的差别在于，在stream模式下。audio_track_cblk_t对象创建在共享内存的头部。

static模式：

stream模式：

接下来继续分析Track的构造过程：

AudioFlinger::PlaybackThread::Track::Track(
            PlaybackThread *thread, //所属的播放线程
            const sp& client, //所属的Client
            audio_stream_type_t streamType,//音频流类型
            uint32_t sampleRate, //採样率
            audio_format_t format, //音频格式
            audio_channel_mask_t channelMask, //声道
            size_t frameCount, //音频帧个数
            const sp& sharedBuffer, //共享内存
            int sessionId,
            int uid,
            IAudioFlinger::track_flags_t flags)
    :   TrackBase(thread, client, sampleRate, format, channelMask, frameCount, sharedBuffer,sessionId, uid, true /*isOut*/),
    mFillingUpStatus(FS_INVALID),
    // mRetryCount initialized later when needed
    mSharedBuffer(sharedBuffer),
    mStreamType(streamType),
    mName(-1),  // see note below
    mMainBuffer(thread->mixBuffer()),
    mAuxBuffer(NULL),
    mAuxEffectId(0), mHasVolumeController(false),
    mPresentationCompleteFrames(0),
    mFlags(flags),
    mFastIndex(-1),
    mCachedVolume(1.0),
    mIsInvalid(false),
    mAudioTrackServerProxy(NULL),
    mResumeToStopping(false)
{
    if (mCblk != NULL) {//audio_track_cblk_t对象不为空
        if (sharedBuffer == 0) {//stream模式
            mAudioTrackServerProxy = new AudioTrackServerProxy(mCblk, mBuffer, frameCount,
                    mFrameSize);
        } else {//static模式
            mAudioTrackServerProxy = new StaticAudioTrackServerProxy(mCblk, mBuffer, frameCount,mFrameSize);
        }
        mServerProxy = mAudioTrackServerProxy;
        // to avoid leaking a track name, do not allocate one unless there is an mCblk
        mName = thread->getTrackName_l(channelMask, sessionId);
        if (mName < 0) {
            ALOGE("no more track names available");
            return;
        }
        // only allocate a fast track index if we were able to allocate a normal track name
        if (flags & IAudioFlinger::TRACK_FAST) {
            mAudioTrackServerProxy->framesReadyIsCalledByMultipleThreads();
            ALOG_ASSERT(thread->mFastTrackAvailMask != 0);
            int i = __builtin_ctz(thread->mFastTrackAvailMask);
            ALOG_ASSERT(0 < i && i < (int)FastMixerState::kMaxFastTracks);
            // FIXME This is too eager.  We allocate a fast track index before the
            //       fast track becomes active.  Since fast tracks are a scarce resource,
            //       this means we are potentially denying other more important fast tracks 
            //       from being created.  It would be better to allocate the index dynamically.
            mFastIndex = i;
            // Read the initial underruns because this field is never cleared by the fast mixer
            mObservedUnderruns = thread->getFastTrackUnderruns(i);
            thread->mFastTrackAvailMask &= ~(1 << i);
        }
    }
    ALOGV("Track constructor name %d, calling pid %d", mName,
            IPCThreadState::self()->getCallingPid());
}

在TrackBase的构造过程中依据是否创建Client对象来採取不同方式分配audio_track_cblk_t对象内存空间，而且创建audio_track_cblk_t对象。在Track构造中。依据不同的播放模式，创建不同的代理对象：

1. Stream模式下。创建AudioTrackServerProxy代理对象；
2. Static模式下，创建StaticAudioTrackServerProxy代理对象；

 

在stream模式下，同一时候分配指定大小的音频数据buffer ，该buffer的结构例如以下所看到的：
 

我们知道在构造Client对象时，创建了一个内存分配工具对象MemoryDealer，同一时候创建了一块大小为2M的匿名共享内存。这里就是使用MemoryDealer对象在这块匿名共享内存上分配指定大小的buffer。

 

frameworks\native\libs\binder\MemoryDealer.cpp

sp MemoryDealer::allocate(size_t size)
{
sp memory;
    //分配size大小的共享内存，并返回该buffer的偏移量
    const ssize_t offset = allocator()->allocate(size);
if (offset >= 0) {
        //将分配的buffer封装为Allocation对象
        memory = new Allocation(this, heap(), offset, size);
    }
    return memory;
}

size_t SimpleBestFitAllocator::allocate(size_t size, uint32_t flags)
{
    Mutex::Autolock _l(mLock);
    ssize_t offset = alloc(size, flags);
    return offset;
}

 

ssize_t SimpleBestFitAllocator::alloc(size_t size, uint32_t flags)
{
    if (size == 0) {
        return 0;
    }
    size = (size + kMemoryAlign-1) / kMemoryAlign;
    chunk_t* free_chunk = 0;
    chunk_t* cur = mList.head();
    size_t pagesize = getpagesize();
    while (cur) {
        int extra = 0;
        if (flags & PAGE_ALIGNED)
            extra = ( -cur->start & ((pagesize/kMemoryAlign)-1) ) ;
        // best fit
        if (cur->free && (cur->size >= (size+extra))) {
            if ((!free_chunk) || (cur->size < free_chunk->size)) {
                free_chunk = cur;
            }
            if (cur->size == size) {
                break;
            }
        }
        cur = cur->next;
    }
    if (free_chunk) {
        const size_t free_size = free_chunk->size;
        free_chunk->free = 0;
        free_chunk->size = size;
        if (free_size > size) {
            int extra = 0;
            if (flags & PAGE_ALIGNED)
                extra = ( -free_chunk->start & ((pagesize/kMemoryAlign)-1) ) ;
            if (extra) {
                chunk_t* split = new chunk_t(free_chunk->start, extra);
                free_chunk->start += extra;
                mList.insertBefore(free_chunk, split);
            }
            ALOGE_IF((flags&PAGE_ALIGNED) && 
                    ((free_chunk->start*kMemoryAlign)&(pagesize-1)),
                    "PAGE_ALIGNED requested, but page is not aligned!!!");
            const ssize_t tail_free = free_size - (size+extra);
            if (tail_free > 0) {
                chunk_t* split = new chunk_t(
                        free_chunk->start + free_chunk->size, tail_free);
                mList.insertAfter(free_chunk, split);
            }
        }
        return (free_chunk->start)*kMemoryAlign;
    }
    return NO_MEMORY;
}

audio_track_cblk_t对象用于协调生产者AudioTrack和消费者AudioFlinger之间的步调。

在createTrack时由AudioFlinger申请对应的内存，然后通过IMemory接口返回AudioTrack，这样AudioTrack和AudioFlinger管理着同一个audio_track_cblk_t，通过它实现了环形FIFO。AudioTrack向FIFO中写入音频数据，AudioFlinger从FIFO中读取音频数据，经Mixer后送给AudioHardware进行播放。

1)      AudioTrack是FIFO的数据生产者；

2)      AudioFlinger是FIFO的数据消费者；

构造TrackHandle

Track对象仅仅负责音频相关业务，对外并没有提供夸进程的Binder调用接口，因此须要将通信业务托付给另外一个对象来完毕。这就是TrackHandle存在的意义，TrackHandle负责代理Track的通信业务。它是Track与AudioTrack之间的跨进程通道。

AudioFlinger::TrackHandle::TrackHandle(const sp& track): BnAudioTrack(),mTrack(track)
{
}

AudioFlinger拥有多个工作线程，每一个线程拥有多个Track。播放线程实际上是MixerThread的实例。MixerThread的threadLoop()中。会把该线程中的各个Track进行混合，必要时还要进行ReSample(重採样)的动作，转换为统一的採样率（44.1K）。然后通过音频系统的AudioHardware层输出音频数据。

Ø  Framework或者Java层通过JNI创建AudioTrack对象;

Ø  依据StreamType等參数，查找已打开的音频输出设备。假设查找不到匹配的音频输出设备。则请求AudioFlinger打开新的音频输出设备；

Ø  AudioFlinger为该输出设备创建混音线程MixerThread，并把该线程的id作为getOutput()的返回值返回给AudioTrack;

Ø  AudioTrack通过binder机制调用AudioFlinger的createTrack()创建Track。而且创建TrackHandle Binder本地对象，同一时候返回IAudioTrack的代理对象。

Ø  AudioFlinger注冊该Track到MixerThread中；

Ø  AudioTrack通过IAudioTrack接口，得到在AudioFlinger中创建的FIFO(audio_track_cblk_t); 

 

 

AudioTrack启动过程

 

AudioTrack数据写入过程

 

AudioTrack停止过程

 

 

https://www.cnblogs.com/mfmdaoyou/p/7348969.html