Android系统分析之Audio音频流, 音频策略, 输出设备之间的关系

音频流, 音频策略, 输出设备之间的关系

只针对 AudioManager.STREAM_VOICE_CALL 音频流类型进行分析

涉及到的类:

hardware/libhardware_legacy/audio/AudioPolicyManagerBase.cpp
frameworks/base/media/java/android/media/AudioManager.java
frameworks/base/media/java/android/media/AudioService.java
frameworks/base/media/java/android/media/AudioSystem.java

涉及到的方法及执行顺序:

AudioPolicyManagerBase.getStrategy(AudioSystem::stream_type stream);
AudioPolicyManagerBase.getDeviceForStrategy(routing_strategy strategy, bool fromCache);

一. 通过音频流的类型获取对应的音频策略

AudioPolicyManagerBase.cpp:

AudioPolicyManagerBase::routing_strategy AudioPolicyManagerBase::getStrategy(AudioSystem::stream_type stream) {
    // stream to strategy mapping
    switch (stream) {
    case AudioSystem::VOICE_CALL:
    case AudioSystem::BLUETOOTH_SCO:
        return STRATEGY_PHONE;
    ...
    }
}

二. 通过音频策略获取到对应的输出设备

AudioPolicyManagerBase.cpp:

audio_devices_t AudioPolicyManagerBase::getDeviceForStrategy(routing_strategy strategy, bool fromCache) {
    uint32_t device = AUDIO_DEVICE_NONE;
    ... // 省略
    switch (strategy) {
    ... // 省略
    case STRATEGY_PHONE:
        // for phone strategy, we first consider the forced use and then the available devices by order
        // of priority
        switch (mForceUse[AudioSystem::FOR_COMMUNICATION]) { // 判断是否有设置在通讯过程中, 强制使用某种输出设备
        case AudioSystem::FORCE_BT_SCO: // 强制使用蓝牙, 作为输出设备
            if (!isInCall() || strategy != STRATEGY_DTMF) {
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_BLUETOOTH_SCO_CARKIT;
                if (device) break;
            }
            device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_BLUETOOTH_SCO_HEADSET;
            if (device) break;
            device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_BLUETOOTH_SCO;
            if (device) break;
            // if SCO device is requested but no SCO device is available, fall back to default case
            // FALL THROUGH

        default:    // FORCE_NONE 没有设置通讯过程中, 强制使用某种输出设备
            // when not in a phone call, phone strategy should route STREAM_VOICE_CALL to A2DP
            if (mHasA2dp && !isInCall() &&
                    (mForceUse[AudioSystem::FOR_MEDIA] != AudioSystem::FORCE_NO_BT_A2DP) &&
                    (getA2dpOutput() != 0) && !mA2dpSuspended) {
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_BLUETOOTH_A2DP;
                if (device) break;
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_BLUETOOTH_A2DP_HEADPHONES;
                if (device) break;
            }
            device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_WIRED_HEADPHONE;
            if (device) break;
            device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_WIRED_HEADSET;
            if (device) break;
            if (mPhoneState != AudioSystem::MODE_IN_CALL) {
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_USB_ACCESSORY;
                if (device) break;
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_USB_DEVICE;
                if (device) break;
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_DGTL_DOCK_HEADSET;
                if (device) break;
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_AUX_DIGITAL;
                if (device) break;
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_ANLG_DOCK_HEADSET;
                if (device) break;
            }
            // 没有设置通讯过程中, 音频的输出设备, 则默认使用听筒作为输出设备
            device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_EARPIECE;
            if (device) break;
            device = mDefaultOutputDevice;
            if (device == AUDIO_DEVICE_NONE) {
                ALOGE("getDeviceForStrategy() no device found for STRATEGY_PHONE");
            }
            break;

        case AudioSystem::FORCE_SPEAKER: // 强制使用扬声器, 作为输出设备
            // when not in a phone call, phone strategy should route STREAM_VOICE_CALL to
            // A2DP speaker when forcing to speaker output
            if (mHasA2dp && !isInCall() &&
                    (mForceUse[AudioSystem::FOR_MEDIA] != AudioSystem::FORCE_NO_BT_A2DP) &&
                    (getA2dpOutput() != 0) && !mA2dpSuspended) {
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_BLUETOOTH_A2DP_SPEAKER;
                if (device) break;
            }
            if (mPhoneState != AudioSystem::MODE_IN_CALL) {
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_USB_ACCESSORY;
                if (device) break;
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_USB_DEVICE;
                if (device) break;
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_DGTL_DOCK_HEADSET;
                if (device) break;
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_AUX_DIGITAL;
                if (device) break;
                device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_ANLG_DOCK_HEADSET;
                if (device) break;
            }
            device = mAvailableOutputDevices & AUDIO_DEVICE_OUT_SPEAKER;
            if (device) break;
            device = mDefaultOutputDevice;
            if (device == AUDIO_DEVICE_NONE) {
                ALOGE("getDeviceForStrategy() no device found for STRATEGY_PHONE, FORCE_SPEAKER");
            }
            break;
        }
    break;
    // 关于上面default位置的问题, 如果每个case选择(包括default)最终都有break语句, 则default的位置不影响执行顺序
    // 当每个case选择(包括default)不一定有break语句时, 如果输入不满足其他选择, 最终执行default. 程序会从default处从上向下执行, 直到遇到break语句停止
    // 此处的default位置并不影响执行顺序
    ... // 省略
    }
    return device;
}

三. 音频流, 音频策略, 输出设备之间的关系

音频的流类型决定音频的策略, 音频的策略决定输出设备. 但是最终的输出设备的确定, 受强制设置输出设备影响.

四. 分析AudioManager.setSpeakerphoneOn(boolean on)方法, 查看强制设置对音频输出设备选择的影响

涉及到的类及方法:

1. AudioManager.setSpeakerphoneOn(boolean on);
2. AudioService.setSpeakerphoneOn(boolean on);
3. AudioService.AudioHandler.setForceUse(int usage, int config);
4. AudioSystem.setForceUse(int usage, int config);
5. AudioPolicyManagerBase.setForceUse(AudioSystem::force_use usage, AudioSystem::forced_config config);

4.1. AudioManager.java:
public void setSpeakerphoneOn(boolean on){
    IAudioService service = getService();
    service.setSpeakerphoneOn(on);
}

4.2. AudioService.java:
public void setSpeakerphoneOn(boolean on){
    if (on) {
        if (mForcedUseForComm == AudioSystem.FORCE_BT_SCO) {
                sendMsg(mAudioHandler, MSG_SET_FORCE_USE, SENDMSG_QUEUE,
                        AudioSystem.FOR_RECORD, AudioSystem.FORCE_NONE, null, 0);
        }
        mForcedUseForComm = AudioSystem.FORCE_SPEAKER;
    } else if (mForcedUseForComm == AudioSystem.FORCE_SPEAKER){
        mForcedUseForComm = AudioSystem.FORCE_NONE;
    }
    // 从此处可以看到 setSpeakerphoneOn(boolean on) 只针对 AudioSystem.FOR_COMMUNICATION 有效
    // 而 AudioSystem.FOR_COMMUNICATION 最终影响到的是输出设备的选择
    sendMsg(mAudioHandler, MSG_SET_FORCE_USE, SENDMSG_QUEUE,
            AudioSystem.FOR_COMMUNICATION, mForcedUseForComm, null, 0);
}

private class AudioHandler extends Handler {
    @Override
    public void handleMessage(Message msg) {
        switch (msg.what) {
        case MSG_SET_FORCE_USE:
        case MSG_SET_FORCE_BT_A2DP_USE:
            setForceUse(msg.arg1, msg.arg2);
            break;
        }
    }
    private void setForceUse(int usage, int config) {
        AudioSystem.setForceUse(usage, config);
    }       
}

4.3. AudioSystem.java:
public static native int setForceUse(int usage, int config);

4.4. AudioPolicyManagerBase.cpp:
void AudioPolicyManagerBase::setForceUse(AudioSystem::force_use usage, AudioSystem::forced_config config) {
    bool forceVolumeReeval = false;
    switch(usage) {
    case AudioSystem::FOR_COMMUNICATION:
        if (config != AudioSystem::FORCE_SPEAKER && config != AudioSystem::FORCE_BT_SCO &&
            config != AudioSystem::FORCE_NONE) {
            ALOGW("setForceUse() invalid config %d for FOR_COMMUNICATION", config);
            return;
        }
        forceVolumeReeval = true;
        // 保存设置到数组中, 最终在 getDeviceForStrategy(routing_strategy strategy, bool fromCache) 中被调用
        mForceUse[usage] = config;
        break;
    ... // 省略
    }
    ... // 省略
}