Android 音频源码分析——AudioFlinger

Posted 2023-02-28 VNanyesheshou

Android 音频源码分析——AndroidRecord录音（一）
Android 音频源码分析——AndroidRecord录音（二）
Android 音频源码分析——AndroidRecord音频数据传输流程
Android 音频源码分析——audioserver启动
Android 音频源码分析——AudioFlinger
Android 音频源码分析——AudioTrack设备选择

源码基于Android9.0

混音：把多种来源的声音，整合至一个立体音轨或单音轨中。

AudioFlinger和AudioPolicyService是音频中的两大服务，这里先整理下AudioFlinger。

AudioPolicyService时策略制定者，不和AudioHal直接交互；而AudioFlinger是策略执行者，与AudioHal层进行交互。

简介

AudioFlinger运行在audioserver进程中，是一个音频服务。主要职责：

1. 音频数据接收和转发，录音时从HAL层获取音频数据，转给客户端；播放时从客户端获取数据，并发送给HAL层。
2. 音频数据处理，包括数据格式转换、声道数转换、重采样、混音、音效、变速、音量等。
3. 执行AudioFlinger的策略。

与HAL层通信

framework层hidl相关代码目录： frameworks/av/media/libaudiohal

hal层hidl相关代码： hardware/interfaces/audio

 AudioFlinger通过hidl接口的方式调用hal层服务。有上一篇文章可知，启动AudioPolicyService时，会加载音频配置文件，然后根据配置加载HAL module。后续可以调用hal接口进行其他交互（设备选择、通路、读写数据等）。

AudioPolicyService并不直接于Hal层交互，而是由AudioFlinger和Hal通信，这里看看AudioFlinger加载音频模块。

1 加载Module

audio_module_handle_t AudioFlinger::loadHwModule(const char *name)

    if (name == NULL) 
        return AUDIO_MODULE_HANDLE_NONE;
    
    if (!settingsAllowed()) 
        return AUDIO_MODULE_HANDLE_NONE;
    
    Mutex::Autolock _l(mLock);
    return loadHwModule_l(name);


// loadHwModule_l() must be called with AudioFlinger::mLock held
audio_module_handle_t AudioFlinger::loadHwModule_l(const char *name)

	sp<DeviceHalInterface> dev;

    int rc = mDevicesFactoryHal->openDevice(name, &dev);
    //......
    audio_module_handle_t handle = (audio_module_handle_t) nextUniqueId(AUDIO_UNIQUE_ID_USE_MODULE);
    mAudioHwDevs.add(handle, new AudioHwDevice(handle, name, dev, flags));
    return handle;

加载hal层module,mAudioHwDevs保存handle 和AudioHwDevice.

接着看mDevicesFactoryHal->openDevice流程

mDevicesFactoryHal->openDevice(name, &dev);
>>DevicesFactoryHalHybrid::openDevice
>>
status_t DevicesFactoryHalHidl::openDevice(const char *name, sp<DeviceHalInterface> *device) 
    if (mDevicesFactory == 0) return NO_INIT;
    IDevicesFactory::Device hidlDevice;
    status_t status = nameFromHal(name, &hidlDevice);
    if (status != OK) return status;
    Result retval = Result::NOT_INITIALIZED;
    Return<void> ret = mDevicesFactory->openDevice(
            hidlDevice,
            [&](Result r, const sp<IDevice>& result) 
                retval = r;
                if (retval == Result::OK) 
                    *device = new DeviceHalHidl(result);
                
            );
    if (ret.isOk()) 
        if (retval == Result::OK) return OK;
        else if (retval == Result::INVALID_ARGUMENTS) return BAD_VALUE;
        else return NO_INIT;
    
    return FAILED_TRANSACTION;

1. nameFromHal通过name获取Device类型：IDevicesFactory::Device::PRIMARY、IDevicesFactory::Device::A2DP、IDevicesFactory::Device::USB、IDevicesFactory::Device::R_SUBMIX等；
2. mDevicesFactory->openDevice通过hidl跨进程调用接口

hardware/interfaces/audio/core/all-versions/default/include/core/all-version/DevicesFactory.impl.h

Return<void> DevicesFactory::openDevice(IDevicesFactory::Device device, openDevice_cb _hidl_cb) 
    switch (device) 
        case IDevicesFactory::Device::PRIMARY:
            return openDevice<PrimaryDevice>(AUDIO_HARDWARE_MODULE_ID_PRIMARY, _hidl_cb);
        case IDevicesFactory::Device::A2DP:
            return openDevice(AUDIO_HARDWARE_MODULE_ID_A2DP, _hidl_cb);
        case IDevicesFactory::Device::USB:
            return openDevice(AUDIO_HARDWARE_MODULE_ID_USB, _hidl_cb);
        case IDevicesFactory::Device::R_SUBMIX:
            return openDevice(AUDIO_HARDWARE_MODULE_ID_REMOTE_SUBMIX, _hidl_cb);
        case IDevicesFactory::Device::STUB:
            return openDevice(AUDIO_HARDWARE_MODULE_ID_STUB, _hidl_cb);
    
    _hidl_cb(Result::INVALID_ARGUMENTS, nullptr);
    return Void();


int DevicesFactory::loadAudioInterface(const char* if_name, audio_hw_device_t** dev) 
    const hw_module_t* mod;
    int rc;

    rc = hw_get_module_by_class(AUDIO_HARDWARE_MODULE_ID, if_name, &mod);
    if (rc) 
        ALOGE("%s couldn't load audio hw module %s.%s (%s)", __func__, AUDIO_HARDWARE_MODULE_ID,
              if_name, strerror(-rc));
        goto out;
    
    rc = audio_hw_device_open(mod, dev);
    if (rc) 
        ALOGE("%s couldn't open audio hw device in %s.%s (%s)", __func__, AUDIO_HARDWARE_MODULE_ID,
              if_name, strerror(-rc));
        goto out;
    
    if ((*dev)->common.version < AUDIO_DEVICE_API_VERSION_MIN) 
        ALOGE("%s wrong audio hw device version %04x", __func__, (*dev)->common.version);
        rc = -EINVAL;
        audio_hw_device_close(*dev);
        goto out;
    
    return OK;

out:
    *dev = NULL;
    return rc;

1. hw_get_module_by_class动态链接 hal 库，如：audio.primary.default.so、audio.a2dp.default.so、audio.usb.default.so
2. audio_hw_device_open对应module method->open函数

2 open stream

这里看看加载后的openOutput流程

status_t AudioFlinger::openOutput(audio_module_handle_t module,
                                  audio_io_handle_t *output,
                                  audio_config_t *config,
                                  audio_devices_t *devices,
                                  const String8& address,
                                  uint32_t *latencyMs,
                                  audio_output_flags_t flags)

    if (devices == NULL || *devices == AUDIO_DEVICE_NONE) 
        return BAD_VALUE;
    
    Mutex::Autolock _l(mLock);

    sp<ThreadBase> thread = openOutput_l(module, output, config, *devices, address, flags);
    //......

    return NO_INIT;


sp<AudioFlinger::ThreadBase> AudioFlinger::openOutput_l(audio_module_handle_t module,
                                                            audio_io_handle_t *output,
                                                            audio_config_t *config,
                                                            audio_devices_t devices,
                                                            const String8& address,
                                                            audio_output_flags_t flags)

    AudioHwDevice *outHwDev = findSuitableHwDev_l(module, devices);
    //......
  	AudioStreamOut *outputStream = NULL;
    status_t status = outHwDev->openOutputStream(
            &outputStream,
            *output,
            devices,
            flags,
            config,
            address.string());

    mHardwareStatus = AUDIO_HW_IDLE;
    if (status == NO_ERROR) 
        if (flags & AUDIO_OUTPUT_FLAG_MMAP_NOIRQ) 
            sp<MmapPlaybackThread> thread =
                    new MmapPlaybackThread(this, *output, outHwDev, outputStream,
                                          devices, AUDIO_DEVICE_NONE, mSystemReady);
            mMmapThreads.add(*output, thread);
            return thread;
         else 
            sp<PlaybackThread> thread;
            if (flags & AUDIO_OUTPUT_FLAG_COMPRESS_OFFLOAD) 
                thread = new OffloadThread(this, outputStream, *output, devices, mSystemReady);
             else if ((flags & AUDIO_OUTPUT_FLAG_DIRECT)
                    || !isValidPcmSinkFormat(config->format)
                    || !isValidPcmSinkChannelMask(config->channel_mask)) 
                thread = new DirectOutputThread(this, outputStream, *output, devices, mSystemReady);
             else 
                thread = new MixerThread(this, outputStream, *output, devices, mSystemReady);
            
            mPlaybackThreads.add(*output, thread);
            return thread;
        
    
    return 0;

1. findSuitableHwDev_l找寻对应的AudioHwDevice；
2. openOutputStream打开输出Stream，这里会调用到hal层 adev_open_output_stream中，这里就不分析了；
3. 根据flags创建不同的通路线程；
4. 将线程添加到mPlaybackThreads；

openinput 流程就先不分析了。

与APP通信

App和AudioFlinger间音频数据则通过共享内存的方式进行传输。

如下图AudioRecord的数据传输：

如下图AudioTrack的数据传输：

AudioFlinger是一个binder服务，app中的native层可以通过获取其binder服务从而进行交互。

如下示例：

const sp<IAudioFlinger>& audioFlinger = AudioSystem::get_audio_flinger();
record = audioFlinger->createRecord(input,
                                                              output,
                                                              &status);