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第一种情况，如果是多路mix的情况，就是MixerThread进行软件mixer，然后在mixer计算的时候来缩放PCM data，

首先，JNI层调用了AudioFlinger::setStreamVolume。

status_t AudioFlinger::setStreamVolume(audio_stream_type_t stream, float value,
        audio_io_handle_t output)
{
    AutoMutex lock(mLock);
    PlaybackThread *thread = NULL;
    if (output) {
        thread = checkPlaybackThread_l(output);  //获得对应的PlaybackThread
        if (thread == NULL) {
            return BAD_VALUE;
        }
    }
    if (thread == NULL) {
        for (size_t i = 0; i < mPlaybackThreads.size(); i++) {
            mPlaybackThreads.valueAt(i)->setStreamVolume(stream, value);
        }
    } else {
        thread->setStreamVolume(stream, value);  //继续向下层设置
    }
    return NO_ERROR;
}

可以看到，最终是调用了PlaybackThread：：setStreamVolume来继续设置音量

void AudioFlinger::PlaybackThread::setStreamVolume(audio_stream_type_t stream, float value)
{
    Mutex::Autolock _l(mLock);
    mStreamTypes[stream].volume = value;//把音量数据存起来
    broadcast_l();
}

AudioFlinger::PlaybackThread::mixer_state AudioFlinger::MixerThread::prepareTracks_l()
{
                float typeVolume = mStreamTypes[track->streamType()].volume;  //取出暂存的音量数据
                float v = masterVolume * typeVolume;
                AudioTrackServerProxy *proxy = track->mAudioTrackServerProxy;
                uint32_t vlr = proxy->getVolumeLR();
                vl = vlr & 0xFFFF;
                vr = vlr >> 16;
                vl = (uint32_t)(v * vl) << 12;
                vr = (uint32_t)(v * vr) << 12;
            mAudioMixer->setParameter(name, param, AudioMixer::VOLUME0, (void *)vl);  //设置给audioMixer
            mAudioMixer->setParameter(name, param, AudioMixer::VOLUME1, (void *)vr);
}

可以看到上述函数把参数设置到了audioMixer里面，在audioMixer章节里面，

我们介绍过在audioMixer的实际操作函数是track__16BitsStereo这种函数，

对track__16BitsStereo的分析中，我们可以看到根据音量对PCM data进行实际的缩放

不再继续赘述了。

对于第二种方式，控制DAC硬件的增益，主要用在了DirectOutputThread，中，因为DirectOutputThread只有一路音频，直接写入HAL层，直接写入硬件的，

所以需要直接调用硬件DAC芯片的控制接口来调整音量。

其主要流程如下：

和mixerThread的流程一样，上层在调用了AudioFlinger::setStreamVolume之后，会调用prepareTracks_l函数

AudioFlinger::PlaybackThread::mixer_state AudioFlinger::DirectOutputThread::prepareTracks_l(
    Vector< sp<Track> > *tracksToRemove
)
{
            // compute volume for this track
            processVolume_l(track, last);
}

AT章节中，我们提到过prepareTracks_l函数，其中会调用processVolume_l来处理音量

void AudioFlinger::DirectOutputThread::processVolume_l(Track *track, bool lastTrack)
{
    audio_track_cblk_t* cblk = track->cblk();
    float left, right;
        float typeVolume = mStreamTypes[track->streamType()].volume;  //和mixerThread一样，也是从mStreamTypes里面取出音量数据
        float v = mMasterVolume * typeVolume;
        AudioTrackServerProxy *proxy = track->mAudioTrackServerProxy;
        uint32_t vlr = proxy->getVolumeLR();
        float v_clamped = v * (vlr & 0xFFFF);
        if (v_clamped > MAX_GAIN) v_clamped = MAX_GAIN;
        left = v_clamped/MAX_GAIN;
        v_clamped = v * (vlr >> 16);
        if (v_clamped > MAX_GAIN) v_clamped = MAX_GAIN;
        right = v_clamped/MAX_GAIN;
    if (lastTrack) {
        if (left != mLeftVolFloat || right != mRightVolFloat) {
            mLeftVolFloat = left;
            mRightVolFloat = right;
            uint32_t vl = (uint32_t)(left * (1 << 24));
            uint32_t vr = (uint32_t)(right * (1 << 24));
            if (mOutput->stream->set_volume) {
                mOutput->stream->set_volume(mOutput->stream, left, right);  //向下层设置音量
            }
        }
    }
}

mOutput->stream->set_volume实际上调用的是 libhardware_legacy中的函数

static int out_set_volume(struct audio_stream_out *stream, float left,  //调用libhardware_legacy中的函数
                          float right)
{
    struct legacy_stream_out *out =
        reinterpret_cast<struct legacy_stream_out *>(stream);
    return out->legacy_out->setVolume(left, right);
}

然后就进入了HAL层代码，HAL层代码中最终调用了 ioctl(ctl->mixer->fd, SNDRV_CTL_IOCTL_ELEM_WRITE, &ev);

写入驱动

由于商业机密，HAL层代码不能贴出来。

到了内核中，则是按照以下调用序列，最终通过IIC总线，将音量控制命令写入了DAC芯片中

ioctl(ctl->mixer->fd, SNDRV_CTL_IOCTL_ELEM_WRITE, &ev);
snd_ctl_elem_write_user
snd_ctl_elem_write_user
snd_ctl_elem_write
wm8523_controls
snd_soc_put_volsw
snd_soc_update_bits_locked

IIC总线写入命令

关于驱动的流程我们在这里不展开讲，后续会单独讲解