Android Audio代码分析=Audio Strategy

在看示例代码 testPlaybackHeadPositionIncrease 的时候，我们对 play 函数进行了研究。
不过，当时对 Android 中声音竞争策略相关的内容，并没有详细分析。
今天就以 AudioTrack 的播放为引子，来仔细看看 Anroid 中各种声音是以什么样的策略来竞争的。

从 Java 侧类 AudioTrack 的 play 函数到函数 AudioFlinger::PlaybackThread::Track::start 之间的调用关系就不再叙述了。
在看示例代码 testPlaybackHeadPositionIncrease 的时候已经说明过了。
今天就从函数 AudioFlinger::PlaybackThread::Track::start 开始分析。

*****************************************源码*************************************************
status_t AudioFlinger::PlaybackThread::Track::start()
{
status_t status = NO_ERROR;
LOGV("start(%d), calling thread %d session %d",
mName, IPCThreadState::self()->getCallingPid(), mSessionId);
sp<ThreadBase> thread = mThread.promote();
if (thread != 0) {
Mutex::Autolock _l(thread->mLock);
int state = mState;
// here the track could be either new, or restarted
// in both cases "unstop" the track
if (mState == PAUSED) {
mState = TrackBase::RESUMING;
LOGV("PAUSED => RESUMING (%d) on thread %p", mName, this);
} else {
mState = TrackBase::ACTIVE;
LOGV("? => ACTIVE (%d) on thread %p", mName, this);
}

if (!isOutputTrack() && state != ACTIVE && state != RESUMING) {
thread->mLock.unlock();
status = AudioSystem::startOutput(thread->id(),
(AudioSystem::stream_type)mStreamType,
mSessionId);
thread->mLock.lock();
}
if (status == NO_ERROR) {
PlaybackThread *playbackThread = (PlaybackThread *)thread.get();
playbackThread->addTrack_l(this);
} else {
mState = state;
}
} else {
status = BAD_VALUE;
}
return status;
}
**********************************************************************************************
源码路径：
frameworks\base\services\AudioFlinger.cpp

#######################说明################################
status_t AudioFlinger::PlaybackThread::Track::start()
{
status_t status = NO_ERROR;
LOGV("start(%d), calling thread %d session %d",
mName, IPCThreadState::self()->getCallingPid(), mSessionId);
sp<ThreadBase> thread = mThread.promote();
if (thread != 0) {
Mutex::Autolock _l(thread->mLock);
int state = mState;
// here the track could be either new, or restarted
// in both cases "unstop" the track
if (mState == PAUSED) {
mState = TrackBase::RESUMING;
LOGV("PAUSED => RESUMING (%d) on thread %p", mName, this);
} else {
mState = TrackBase::ACTIVE;
LOGV("? => ACTIVE (%d) on thread %p", mName, this);
}

if (!isOutputTrack() && state != ACTIVE && state != RESUMING) {
thread->mLock.unlock();
status = AudioSystem::startOutput(thread->id(),
(AudioSystem::stream_type)mStreamType,
mSessionId);
++++++++++++++++++++++++++++AudioSystem::startOutput++++++++++++++++++++++++++++++++++++
从函数 AudioFlinger::PlaybackThread::Track::start 进入：
status_t AudioSystem::startOutput(audio_io_handle_t output,
AudioSystem::stream_type stream,
int session)
{
const sp<IAudioPolicyService>& aps = AudioSystem::get_audio_policy_service();
if (aps == 0) return PERMISSION_DENIED;
return aps->startOutput(output, stream, session);
+++++++++++++++++++++++++++++AudioPolicyService::startOutput+++++++++++++++++++++++++++++++++++
从函数 AudioSystem::startOutput 进入：
status_t AudioPolicyService::startOutput(audio_io_handle_t output,
AudioSystem::stream_type stream,
int session)
{
if (mpPolicyManager == NULL) {
return NO_INIT;
}
LOGV("startOutput() tid %d", gettid());
Mutex::Autolock _l(mLock);
return mpPolicyManager->startOutput(output, stream, session);
+++++++++++++++++++++++++++++AudioPolicyManagerBase::startOutput+++++++++++++++++++++++++++++++++++
从函数 AudioPolicyService::startOutput 进入：
// 主要的处理是从这儿开始的

status_t AudioPolicyManagerBase::startOutput(audio_io_handle_t output,
AudioSystem::stream_type stream,
int session)
{
LOGV("startOutput() output %d, stream %d, session %d", output, stream, session);
ssize_t index = mOutputs.indexOfKey(output);
if (index < 0) {
LOGW("startOutput() unknow output %d", output);
return BAD_VALUE;
}

AudioOutputDescriptor *outputDesc = mOutputs.valueAt(index);
// 函数 getStrategy 就是根据 stream type 返回特定的 strategy
routing_strategy strategy = getStrategy((AudioSystem::stream_type)stream);
++++++++++++++++++++++++++++AudioPolicyManagerBase::getStrategy++++++++++++++++++++++++++++++++++++
从函数 AudioPolicyManagerBase::startOutput 进入：

本来不打算列出该函数，不过，看到后面关于 strategy 优先级的时候，发现需要知道每种策略分别是用来干吗的。
所以首先要知道策略对应的 stream type ，
AudioPolicyManagerBase::routing_strategy AudioPolicyManagerBase::getStrategy(
AudioSystem::stream_type stream) {
// stream to strategy mapping
switch (stream) {
case AudioSystem::VOICE_CALL: // 电话来了
case AudioSystem::BLUETOOTH_SCO: // 蓝牙耳机接通了
return STRATEGY_PHONE;
case AudioSystem::RING: // 铃声响了
case AudioSystem::NOTIFICATION: // 通知，例如界面中最上面一栏中有消息了
case AudioSystem::ALARM: // 警告，电池没电时的警告？
case AudioSystem::ENFORCED_AUDIBLE:
return STRATEGY_SONIFICATION;
case AudioSystem::DTMF: // 可参考链接：http://baike.baidu.com/view/171916.htm
return STRATEGY_DTMF;
default:
LOGE("unknown stream type");
case AudioSystem::SYSTEM: // 系统声音采用 media strategy, 例如，如果正在播放音乐的时候按键，
// mute 掉音乐，并切换 output 的话，将导致 很差的用户体验。
// 其中可以得到以下信息：
// 1、按键声音属于 system stream type 。
// 2、策略的改变将会导致 output 的切换
// 3、优先级高的策略 start 时会 mute 掉优先级低的策略
// NOTE: SYSTEM stream uses MEDIA strategy because muting music and switching outputs
// while key clicks are played produces a poor result
case AudioSystem::TTS: // TTS 就是 Text To Speech
case AudioSystem::MUSIC:
return STRATEGY_MEDIA;
}
}
返回到函数 AudioPolicyManagerBase::startOutput
----------------------------AudioPolicyManagerBase::getStrategy------------------------------------

#ifdef WITH_A2DP
if (mA2dpOutput != 0 && !a2dpUsedForSonification() && strategy == STRATEGY_SONIFICATION) {
setStrategyMute(STRATEGY_MEDIA, true, mA2dpOutput);
}
#endif

// incremenent usage count for this stream on the requested output:
// NOTE that the usage count is the same for duplicated output and hardware output which is
// necassary for a correct control of hardware output routing by startOutput() and stopOutput()
// 增加请求的 output 上该 stream 的使用计数
// 注意： duplicated output 和 hardware output 中的使用计数是相同的。
// 因为这对通过 startOutput() 函数和 stopOutput() 函数正确地控制 hardware output routing 是必要的。
outputDesc->changeRefCount(stream, 1);
++++++++++++++++++++++++++++AudioPolicyManagerBase::AudioOutputDescriptor::changeRefCount++++++++++++++++++++++++++++++++++++
从函数 AudioPolicyManagerBase::startOutput 进入：
void AudioPolicyManagerBase::AudioOutputDescriptor::changeRefCount(AudioSystem::stream_type stream, int delta)
{
// forward usage count change to attached outputs
// 如果是 duplicated 的，需要改变它所包含的两个 output 中的使用计数。
if (isDuplicated()) {
mOutput1->changeRefCount(stream, delta);
mOutput2->changeRefCount(stream, delta);
}
if ((delta + (int)mRefCount[stream]) < 0) {
LOGW("changeRefCount() invalid delta %d for stream %d, refCount %d", delta, stream, mRefCount[stream]);
mRefCount[stream] = 0;
return;
}
mRefCount[stream] += delta;
LOGV("changeRefCount() stream %d, count %d", stream, mRefCount[stream]);
}
返回到函数 AudioPolicyManagerBase::startOutput
----------------------------AudioPolicyManagerBase::AudioOutputDescriptor::changeRefCount------------------------------------

setOutputDevice(output, getNewDevice(output));
++++++++++++++++++++++++++++AudioPolicyManagerBase::getNewDevice++++++++++++++++++++++++++++++++++++
从函数 AudioPolicyManagerBase::startOutput 进入：
uint32_t AudioPolicyManagerBase::getNewDevice(audio_io_handle_t output, bool fromCache)
{
uint32_t device = 0;

AudioOutputDescriptor *outputDesc = mOutputs.valueFor(output);
// check the following by order of priority to request a routing change if necessary:
// 1: we are in call or the strategy phone is active on the hardware output:
// use device for strategy phone
// 2: the strategy sonification is active on the hardware output:
// use device for strategy sonification
// 3: the strategy media is active on the hardware output:
// use device for strategy media
// 4: the strategy DTMF is active on the hardware output:
// use device for strategy DTMF
// 根据以下的优先级顺序，来检查必要的路线改变
// 1、如果来电话了，或者 hardware output 中的 strategy phone 是活动的，
// 则使用 phone strategy 。
// 有两种情况会使用到 strategy phone ，来电话了和蓝牙耳机接通了。
// 2、如果 hardware output 中的 strategy sonification 是活动的，
// 则使用 strategy sonification .
// 有四种情况会使用 stratety sonification ，来电铃声，通知，警告，
// 和是 ENFORCED_AUDIBLE stream 的时候。
// 3、如果 hardware output 中的 strategy media 是活动的，
// 则使用 strategy media .
// Media 播放， 系统声音和 TTS 会使用 media 策略。
// 4、如果 hardware output 中的 strategy DTMF 是活动的，
// 则使用 strategy DTMF .
// 在 stream type 是 DTMF 的时候会使用 DTMF 策略。
// 关于 DTMF 请参考 http://baike.baidu.com/view/171916.htm
if (isInCall() ||
outputDesc->isUsedByStrategy(STRATEGY_PHONE)) {
// bool isUsedByStrategy(routing_strategy strategy) { return (strategyRefCount(strategy) != 0);}
+++++++++++++++++++++++++++AudioPolicyManagerBase::AudioOutputDescriptor::strategyRefCount+++++++++++++++++++++++++++++++++++++
从函数 AudioPolicyManagerBase::getNewDevice 进入：
// 检查 output 中是否使用了指定的 strategy
uint32_t AudioPolicyManagerBase::AudioOutputDescriptor::strategyRefCount(routing_strategy strategy)
{
uint32_t refCount = 0;
for (int i = 0; i < (int)AudioSystem::NUM_STREAM_TYPES; i++) {
if (getStrategy((AudioSystem::stream_type)i) == strategy) {
refCount += mRefCount[i];
}
}
return refCount;
}
返回到函数 AudioPolicyManagerBase::getNewDevice
---------------------------AudioPolicyManagerBase::AudioOutputDescriptor::strategyRefCount-------------------------------------
device = getDeviceForStrategy(STRATEGY_PHONE, fromCache);
} else if (outputDesc->isUsedByStrategy(STRATEGY_SONIFICATION)) {
device = getDeviceForStrategy(STRATEGY_SONIFICATION, fromCache);
} else if (outputDesc->isUsedByStrategy(STRATEGY_MEDIA)) {
device = getDeviceForStrategy(STRATEGY_MEDIA, fromCache);
} else if (outputDesc->isUsedByStrategy(STRATEGY_DTMF)) {
device = getDeviceForStrategy(STRATEGY_DTMF, fromCache);
}
++++++++++++++++++++++++++++AudioPolicyManagerBase::isInCall++++++++++++++++++++++++++++++++++++
从函数 AudioPolicyManagerBase::getNewDevice 进入：
bool AudioPolicyManagerBase::isInCall()
{
// 函数 AudioPolicyManagerBase::setPhoneState 中会改变 mPhoneState 的值

// 调用关系：android_media_AudioSystem_setPhoneState
// 调用 AudioSystem::setPhoneState
// 调用 AudioPolicyService::setPhoneState
// 调用 AudioPolicyManagerBase::setPhoneState

return isStateInCall(mPhoneState);
++++++++++++++++++++++++++++AudioPolicyManagerBase::isStateInCall++++++++++++++++++++++++++++++++++++
从函数 AudioPolicyManagerBase::isInCall 进入：
bool AudioPolicyManagerBase::isStateInCall(int state) {
return ((state == AudioSystem::MODE_IN_CALL) ||
(state == AudioSystem::MODE_IN_COMMUNICATION));
+++++++++++++++++++++++++++++audio_mode+++++++++++++++++++++++++++++++++++
从函数 AudioPolicyManagerBase::isStateInCall 进入：
enum audio_mode {
MODE_INVALID = -2,
MODE_CURRENT = -1,
MODE_NORMAL = 0,
MODE_RINGTONE,
MODE_IN_CALL,
MODE_IN_COMMUNICATION,
NUM_MODES // not a valid entry, denotes end-of-list
};
返回到函数 AudioPolicyManagerBase::isStateInCall
-----------------------------audio_mode-----------------------------------
}
返回到函数 AudioPolicyManagerBase::isInCall
----------------------------AudioPolicyManagerBase::isStateInCall------------------------------------
}
返回到函数 AudioPolicyManagerBase::getNewDevice
----------------------------AudioPolicyManagerBase::isInCall------------------------------------

+++++++++++++++++++++++++++AudioPolicyManagerBase::getDeviceForStrategy+++++++++++++++++++++++++++++++++++++
从函数 AudioPolicyManagerBase::getNewDevice 进入：
uint32_t AudioPolicyManagerBase::getDeviceForStrategy(routing_strategy strategy, bool fromCache)
{
uint32_t device = 0;

if (fromCache) {
LOGV("getDeviceForStrategy() from cache strategy %d, device %x", strategy, mDeviceForStrategy[strategy]);
return mDeviceForStrategy[strategy];
}

switch (strategy) {
case STRATEGY_DTMF:
if (!isInCall()) {
// 不是打电话过来或者在通话中的话，DTMF strategy 和 MEDIA strategy 规则一样
// when off call, DTMF strategy follows the same rules as MEDIA strategy
device = getDeviceForStrategy(STRATEGY_MEDIA, false);
break;
}
// 有电话打过来，或者在通话中的话， DTMF strategy 和 PHONE strategy 规则一致
// when in call, DTMF and PHONE strategies follow the same rules
// FALL THROUGH

case STRATEGY_PHONE:
// 对于 PHONE strategy ， 首先判断是否强制要求使用了什么设备，然后再根据优先级顺序，寻找可用的 device
// for phone strategy, we first consider the forced use and then the available devices by order
// of priority
switch (mForceUse[AudioSystem::FOR_COMMUNICATION]) {
case AudioSystem::FORCE_BT_SCO:
if (!isInCall() || strategy != STRATEGY_DTMF) {
// 不是上述两种情况的话，肯定是outputDesc->isUsedByStrategy(STRATEGY_PHONE)作为条件走到这一步的
device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_BLUETOOTH_SCO_CARKIT;
if (device) break;
}
device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_BLUETOOTH_SCO_HEADSET;
if (device) break;
device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_BLUETOOTH_SCO;
if (device) break;
// 如果请求了 SCO device ，但是没有 SCO device 可用，则进入 default case
// if SCO device is requested but no SCO device is available, fall back to default case
// FALL THROUGH

default: // FORCE_NONE
device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_WIRED_HEADPHONE;
if (device) break;
device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_WIRED_HEADSET;
if (device) break;
#ifdef WITH_A2DP
// when not in a phone call, phone strategy should route STREAM_VOICE_CALL to A2DP
if (!isInCall()) {
device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_BLUETOOTH_A2DP;
if (device) break;
device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_BLUETOOTH_A2DP_HEADPHONES;
if (device) break;
}
#endif
device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_EARPIECE;
if (device == 0) {
LOGE("getDeviceForStrategy() earpiece device not found");
}
break;

case AudioSystem::FORCE_SPEAKER:
if (!isInCall() || strategy != STRATEGY_DTMF) {
device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_BLUETOOTH_SCO_CARKIT;
if (device) break;
}
#ifdef WITH_A2DP
// when not in a phone call, phone strategy should route STREAM_VOICE_CALL to
// A2DP speaker when forcing to speaker output
if (!isInCall()) {
device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_BLUETOOTH_A2DP_SPEAKER;
if (device) break;
}
#endif
device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_SPEAKER;
if (device == 0) {
LOGE("getDeviceForStrategy() speaker device not found");
}
break;
}
break;

case STRATEGY_SONIFICATION:

// If incall, just select the STRATEGY_PHONE device: The rest of the behavior is handled by
// handleIncallSonification().
if (isInCall()) {
device = getDeviceForStrategy(STRATEGY_PHONE, false);
break;
}
device = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_SPEAKER;
if (device == 0) {
LOGE("getDeviceForStrategy() speaker device not found");
}
// The second device used for sonification is the same as the device used by media strategy
// FALL THROUGH

case STRATEGY_MEDIA: {
uint32_t device2 = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_AUX_DIGITAL;
if (device2 == 0) {
device2 = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_WIRED_HDMI;
}
if (device2 == 0) {
device2 = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_WIRED_HEADPHONE;
}
if (device2 == 0) {
device2 = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_WIRED_HEADSET;
}
#ifdef WITH_A2DP
if (mA2dpOutput != 0) {
if (strategy == STRATEGY_SONIFICATION && !a2dpUsedForSonification()) {
break;
}
if (device2 == 0) {
device2 = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_BLUETOOTH_A2DP;
}
if (device2 == 0) {
device2 = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_BLUETOOTH_A2DP_HEADPHONES;
}
if (device2 == 0) {
device2 = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_BLUETOOTH_A2DP_SPEAKER;
}
}
#endif
if (device2 == 0) {
device2 = mAvailableOutputDevices & AudioSystem::DEVICE_OUT_SPEAKER;
}

// device is DEVICE_OUT_SPEAKER if we come from case STRATEGY_SONIFICATION, 0 otherwise
device |= device2;
if (device == 0) {
LOGE("getDeviceForStrategy() speaker device not found");
}
} break;

default:
LOGW("getDeviceForStrategy() unknown strategy: %d", strategy);
break;
}

LOGV("getDeviceForStrategy() strategy %d, device %x", strategy, device);
return device;
}
返回到函数 AudioPolicyManagerBase::getNewDevice
---------------------------AudioPolicyManagerBase::getDeviceForStrategy-------------------------------------

LOGV("getNewDevice() selected device %x", device);
return device;
}
返回到函数 AudioPolicyManagerBase::startOutput
----------------------------AudioPolicyManagerBase::getNewDevice------------------------------------
++++++++++++++++++++++++++++AudioPolicyManagerBase::setOutputDevice++++++++++++++++++++++++++++++++++++
从函数 AudioPolicyManagerBase::startOutput 进入
void AudioPolicyManagerBase::setOutputDevice(audio_io_handle_t output, uint32_t device, bool force, int delayMs)
{
LOGV("setOutputDevice() output %d device %x delayMs %d", output, device, delayMs);
AudioOutputDescriptor *outputDesc = mOutputs.valueFor(output);

// 如果 output 是 duplicated 的，直接去处理其包含的两个 output
if (outputDesc->isDuplicated()) {
setOutputDevice(outputDesc->mOutput1->mId, device, force, delayMs);
setOutputDevice(outputDesc->mOutput2->mId, device, force, delayMs);
return;
}
#ifdef WITH_A2DP
// filter devices according to output selected
if (output == m A2dpOutput) {
device &= AudioSystem::DEVICE_OUT_ALL_A2DP;
} else {
device &= ~AudioSystem::DEVICE_OUT_ALL_A2DP;
}
#endif

uint32_t prevDevice = (uint32_t)outputDesc->device();
// Do not change the routing if:
// - the requestede device is 0
// - the requested device is the same as current device and force is not specified.
// Doing this check here allows the caller to call setOutputDevice() without conditions
if ((device == 0 || device == prevDevice) && !force) {
LOGV("setOutputDevice() setting same device %x or null device for output %d", device, output);
return;
}

// 修改 output 中的 current device
outputDesc->mDevice = device;
// mute media streams if both speaker and headset are selected
if (output == mHardwareOutput && AudioSystem::popCount(device) == 2) {
setStrategyMute(STRATEGY_MEDIA, true, output);
++++++++++++++++++++++++++AudioPolicyManagerBase::setStrategyMute++++++++++++++++++++++++++++++++++++++
从函数 AudioPolicyManagerBase::setOutputDevice 进入
// mute 掉指定 strategy 对应的所有 stream
void AudioPolicyManagerBase::setStrategyMute(routing_strategy strategy, bool on, audio_io_handle_t output, int delayMs)
{
LOGV("setStrategyMute() strategy %d, mute %d, output %d", strategy, on, output);
for (int stream = 0; stream < AudioSystem::NUM_STREAM_TYPES; stream++) {
if (getStrategy((AudioSystem::stream_type)stream) == strategy) {
setStreamMute(stream, on, output, delayMs);
+++++++++++++++++++++++++++AudioPolicyManagerBase::setStreamMute+++++++++++++++++++++++++++++++++++++
从函数 AudioPolicyManagerBase::setStrategyMute 进入
void AudioPolicyManagerBase::setStreamMute(int stream, bool on, audio_io_handle_t output, int delayMs)
{
StreamDescriptor &streamDesc = mStreams[stream];
AudioOutputDescriptor *outputDesc = mOutputs.valueFor(output);

LOGV("setStreamMute() stream %d, mute %d, output %d, mMuteCount %d", stream, on, output, outputDesc->mMuteCount[stream]);

if (on) {
if (outputDesc->mMuteCount[stream] == 0) {
// 如果该 stream 还没有 mute 过，则对其进行 mute
if (streamDesc.mCanBeMuted) {
// mute 其实就是将 stream 的音量设置为 0
// checkAndSetVolume 函数在此处就先不看了，等到下面看音量设置的时候在分析
checkAndSetVolume(stream, 0, output, outputDesc->device(), delayMs);
}
}
// increment mMuteCount after calling checkAndSetVolume() so that volume change is not ignored
// 将 mute 计数加1，以防止声音改变被忽略
outputDesc->mMuteCount[stream]++;
} else {
if (outputDesc->mMuteCount[stream] == 0) {
// 如果 mute 计数为0，不进行任何操作
LOGW("setStreamMute() unmuting non muted stream!");
return;
}
if (--outputDesc->mMuteCount[stream] == 0) {
// mute 计数减1，若 mute 计数变为0，则将 stream 变为 mute off
// 也就是恢复 stream 的音量
checkAndSetVolume(stream, streamDesc.mIndexCur, output, outputDesc->device(), delayMs);
}
}
}
返回到函数 AudioPolicyManagerBase::setStrategyMute
---------------------------AudioPolicyManagerBase::setStreamMute-------------------------------------
}
}
}
返回到函数 AudioPolicyManagerBase::setOutputDevice
--------------------------AudioPolicyManagerBase::setStrategyMute--------------------------------------
// wait for the PCM output buffers to empty before proceeding with the rest of the command
usleep(outputDesc->mLatency*2*1000);
}

// do the routing
AudioParameter param = AudioParameter();
param.addInt(String8(AudioParameter::keyRouting), (int)device);
// 调用的其实是函数 AudioPolicyService::setParameters
// 会通过函数 AudioPolicyService::AudioCommandThread::parametersCommand 向 AudioCommandThread 的 command list
// 添加一个 command
// AudioPolicyService::AudioCommandThread::threadLoop 函数中会处理 command list 中的 command
// 对于 SET_PARAMETERS command ，最终调用了函数 AudioSystem::setParameters
// 调用了 AudioFlinger::setParameters 函数
// 调用了 AudioFlinger::ThreadBase::setParameters 函数添加成员到 mNewParameters
// 函数 AudioFlinger::MixerThread::checkForNewParameters_l 中会处理 mNewParameters 中的参数
// 函数 AudioFlinger::MixerThread::threadLoop 会调用函数 AudioFlinger::MixerThread::checkForNewParameters_l
mpClientInterface->setParameters(mHardwareOutput, param.toString(), delayMs);
// update stream volumes according to new device
applyStreamVolumes(output, device, delayMs);
+++++++++++++++++++++++++AudioPolicyManagerBase::applyStreamVolumes+++++++++++++++++++++++++++++++++++++++
从函数 AudioPolicyManagerBase::setOutputDevice 进入：
void AudioPolicyManagerBase::applyStreamVolumes(audio_io_handle_t output, uint32_t device, int delayMs)
{
LOGV("applyStreamVolumes() for output %d and device %x", output, device);

for (int stream = 0; stream < AudioSystem::NUM_STREAM_TYPES; stream++) {
checkAndSetVolume(stream, mStreams[stream].mIndexCur, output, device, delayMs);
++++++++++++++++++++++++++AudioPolicyManagerBase::checkAndSetVolume++++++++++++++++++++++++++++++++++++++
从函数 AudioPolicyManagerBase::applyStreamVolumes 进入：
status_t AudioPolicyManagerBase::checkAndSetVolume(int stream, int index, audio_io_handle_t output, uint32_t device, int delayMs, bool force /* = false */)
{

// do not change actual stream volume if the stream is muted
// 如果 stream 是 mute ，并不真正去改变其音量
if (mOutputs.valueFor(output)->mMuteCount[stream] != 0) {
LOGV("checkAndSetVolume() stream %d muted count %d", stream, mOutputs.valueFor(output)->mMuteCount[stream]);
return NO_ERROR;
}

// do not change in call volume if bluetooth is connected and vice versa
// 如果链接了蓝牙耳机，则不去改变 VOICE_CALL 的音量，反之亦然
if ((stream == AudioSystem::VOICE_CALL && mForceUse[AudioSystem::FOR_COMMUNICATION] == AudioSystem::FORCE_BT_SCO) ||
(stream == AudioSystem::BLUETOOTH_SCO && mForceUse[AudioSystem::FOR_COMMUNICATION] != AudioSystem::FORCE_BT_SCO)) {
LOGV("checkAndSetVolume() cannot set stream %d volume with force use = %d for comm",
stream, mForceUse[AudioSystem::FOR_COMMUNICATION]);
return INVALID_OPERATION;
}

float volume = computeVolume(stream, index, output, device);
+++++++++++++++++++++++++++AudioPolicyManagerBase::computeVolume+++++++++++++++++++++++++++++++++++++
从函数 AudioPolicyManagerBase::checkAndSetVolume 进入：
float AudioPolicyManagerBase::computeVolume(int stream, int index, audio_io_handle_t output, uint32_t device)
{
float volume = 1.0;
AudioOutputDescriptor *outputDesc = mOutputs.valueFor(output);
StreamDescriptor &streamDesc = mStreams[stream];

if (device == 0) {
device = outputDesc->device();
}

int volInt = (100 * (index - streamDesc.mIndexMin)) / (streamDesc.mIndexMax - streamDesc.mIndexMin);
volume = AudioSystem::linearToLog(volInt);
++++++++++++++++++++++++++AudioSystem::linearToLog++++++++++++++++++++++++++++++++++++++
从函数 AudioPolicyManagerBase::computeVolume 进入：
// convert volume steps to natural log scale

// change this value to change volume scaling
static const float dBPerStep = 0.5f;
// shouldn't need to touch these
static const float dBConvert = -dBPerStep * 2.302585093f / 20.0f;
static const float dBConvertInverse = 1.0f / dBConvert;

float AudioSystem::linearToLog(int volume)
{
// float v = volume ? exp(float(100 - volume) * dBConvert) : 0;
// LOGD("linearToLog(%d)=%f", volume, v);
// return v;
return volume ? exp(float(100 - volume) * dBConvert) : 0;
}
返回到函数 AudioPolicyManagerBase::computeVolume
--------------------------AudioSystem::linearToLog--------------------------------------

// if a headset is connected, apply the following rules to ring tones and notifications
// to avoid sound level bursts in user's ears:
// - always attenuate ring tones and notifications volume by 6dB
// - if music is playing, always limit the volume to current music volume,
// with a minimum threshold at -36dB so that notification is always perceived.
// 如果连接了耳机，为了防止在用户耳朵中产生爆音，对铃声和警告声需要使用以下规则：
// - 对铃声和警告声总是减弱 6dB。
// - 如果正在播放音乐，铃声和警告声不应该高于音乐声音，
// 不过，下限是 -36dB
if ((device &
(AudioSystem::DEVICE_OUT_BLUETOOTH_A2DP |
AudioSystem::DEVICE_OUT_BLUETOOTH_A2DP_HEADPHONES |
AudioSystem::DEVICE_OUT_WIRED_HEADSET |
AudioSystem::DEVICE_OUT_WIRED_HEADPHONE)) &&
((getStrategy((AudioSystem::stream_type)stream) == STRATEGY_SONIFICATION) ||
(stream == AudioSystem::SYSTEM)) &&
streamDesc.mCanBeMuted) {
// Attenuation applied to STRATEGY_SONIFICATION streams when a headset is connected: 6dB
// #define SONIFICATION_HEADSET_VOLUME_FACTOR 0.5
volume *= SONIFICATION_HEADSET_VOLUME_FACTOR;
// when the phone is ringing we must consider that music could have been paused just before
// by the music application and behave as if music was active if the last music track was
// just stopped
if (outputDesc->mRefCount[AudioSystem::MUSIC] || mLimitRingtoneVolume) {
float musicVol = computeVolume(AudioSystem::MUSIC, mStreams[AudioSystem::MUSIC].mIndexCur, output, device);
// Min volume for STRATEGY_SONIFICATION streams when limited by music volume: -36dB
// #define SONIFICATION_HEADSET_VOLUME_MIN 0.016
float minVol = (musicVol > SONIFICATION_HEADSET_VOLUME_MIN) ? musicVol : SONIFICATION_HEADSET_VOLUME_MIN;
if (volume > minVol) {
volume = minVol;
LOGV("computeVolume limiting volume to %f musicVol %f", minVol, musicVol);
}
}
}

return volume;
}
返回到函数 AudioPolicyManagerBase::checkAndSetVolume
---------------------------AudioPolicyManagerBase::computeVolume-------------------------------------
// We actually change the volume if:
// - the float value returned by computeVolume() changed
// - the force flag is set
// 只有当 compute 返回的音量与当前的音量不同，或者强制要求改音量时，才会去真正改变音量
if (volume != mOutputs.valueFor(output)->mCurVolume[stream] ||
force) {
mOutputs.valueFor(output)->mCurVolume[stream] = volume;
LOGV("setStreamVolume() for output %d stream %d, volume %f, delay %d", output, stream, volume, delayMs);
if (stream == AudioSystem::VOICE_CALL ||
stream == AudioSystem::DTMF ||
stream == AudioSystem::BLUETOOTH_SCO) {
// offset value to reflect actual hardware volume that never reaches 0
// 1% corresponds roughly to first step in VOICE_CALL stream volume setting (see AudioService.java)
volume = 0.01 + 0.99 * volume;
}
mpClientInterface->setStreamVolume((AudioSystem::stream_type)stream, volume, output, delayMs);
+++++++++++++++++++++++++++AudioPolicyService::setStreamVolume+++++++++++++++++++++++++++++++++++++
从函数 AudioPolicyManagerBase::checkAndSetVolume 进入：

status_t AudioPolicyService::setStreamVolume(AudioSystem::stream_type stream,
float volume,
audio_io_handle_t output,
int delayMs)
{
// 函数 AudioPolicyService::AudioCommandThread::volumeCommand 会调用函数 AudioPolicyService::AudioCommandThread::insertCommand_l
// 往 mAudioCommands 中添加成员。
// 函数 AudioPolicyService::AudioCommandThread::threadLoop 会处理 mAudioCommands 中的成员
// 对于 SET_VOLUME 命令，调用了函数 AudioSystem::setStreamVolume
return mAudioCommandThread->volumeCommand((int)stream, volume, (int)output, delayMs);
+++++++++++++++++++++++++++++AudioSystem::setStreamVolume+++++++++++++++++++++++++++++++++++
从函数 AudioPolicyService::setStreamVolume 进入：

status_t AudioSystem::setStreamVolume(int stream, float value, int output)
{
if (uint32_t(stream) >= NUM_STREAM_TYPES) return BAD_VALUE;
const sp<IAudioFlinger>& af = AudioSystem::get_audio_flinger();
if (af == 0) return PERMISSION_DENIED;
af->setStreamVolume(stream, value, output);
++++++++++++++++++++++++++++AudioFlinger::setStreamVolume++++++++++++++++++++++++++++++++++++
从函数 AudioSystem::setStreamVolume 进入：

status_t AudioFlinger::setStreamVolume(int stream, float value, int output)
{
// check calling permissions
if (!settingsAllowed()) {
return PERMISSION_DENIED;
}

if (stream < 0 || uint32_t(stream) >= AudioSystem::NUM_STREAM_TYPES) {
return BAD_VALUE;
}

AutoMutex lock(mLock);
PlaybackThread *thread = NULL;
if (output) {
thread = checkPlaybackThread_l(output);
if (thread == NULL) {
return BAD_VALUE;
}
}

mStreamTypes[stream].volume = value;

if (thread == NULL) {
for (uint32_t i = 0; i < mPlaybackThreads.size(); i++) {
mPlaybackThreads.valueAt(i)->setStreamVolume(stream, value);
}
} else {
thread->setStreamVolume(stream, value);
+++++++++++++++++++++++++++++AudioFlinger::PlaybackThread::setStreamVolume+++++++++++++++++++++++++++++++++++
从函数 AudioFlinger::setStreamVolume 进入：

status_t AudioFlinger::PlaybackThread::setStreamVolume(int stream, float value)
{
#ifdef LVMX
int audioOutputType = LifeVibes::getMixerType(mId, mType);
if (LifeVibes::audioOutputTypeIsLifeVibes(audioOutputType)) {
LifeVibes::setStreamVolume(audioOutputType, stream, value);
}
#endif
// 这一块在看 testPlaybackHeadPositionIncrease 代码的时候已经介绍过
mStreamTypes[stream].volume = value;
return NO_ERROR;
}
返回到函数 AudioFlinger::setStreamVolume
------------------------------AudioFlinger::PlaybackThread::setStreamVolume----------------------------------
}

return NO_ERROR;
}
返回到函数 AudioSystem::setStreamVolume
----------------------------AudioFlinger::setStreamVolume------------------------------------
return NO_ERROR;
}
返回到函数 AudioPolicyService::setStreamVolume
-----------------------------AudioSystem::setStreamVolume-----------------------------------
}
返回到函数 AudioPolicyManagerBase::checkAndSetVolume
---------------------------AudioPolicyService::setStreamVolume-------------------------------------
}

if (stream == AudioSystem::VOICE_CALL ||
stream == AudioSystem::BLUETOOTH_SCO) {
float voiceVolume;
// Force voice volume to max for bluetooth SCO as volume is managed by the headset
if (stream == AudioSystem::VOICE_CALL) {
voiceVolume = (float)index/(float)mStreams[stream].mIndexMax;
} else {
voiceVolume = 1.0;
}
if (voiceVolume != mLastVoiceVolume && output == mHardwareOutput) {
// 这儿与 setStreamVolume 函数类似，最终调用到了函数 AudioFlinger::setVoiceVolume
// 不过，setVoiceVolume最终会调用底层接口，改变硬件音量
// 这一块也在看 testPlaybackHeadPositionIncrease 代码的时候有看过
mpClientInterface->setVoiceVolume(voiceVolume, delayMs);
mLastVoiceVolume = voiceVolume;
}
}

return NO_ERROR;
}
返回到函数 AudioPolicyManagerBase::applyStreamVolumes
--------------------------AudioPolicyManagerBase::checkAndSetVolume--------------------------------------
}
}
返回到函数 AudioPolicyManagerBase::setOutputDevice
-------------------------AudioPolicyManagerBase::applyStreamVolumes---------------------------------------

// if changing from a combined headset + speaker route, unmute media streams
// 前面，如果该条件成立，我们做了 mute 操作，此处要做恢复
if (output == mHardwareOutput && AudioSystem::popCount(prevDevice) == 2) {
setStrategyMute(STRATEGY_MEDIA, false, output, delayMs);
}
}
返回到函数 AudioPolicyManagerBase::startOutput
----------------------------AudioPolicyManagerBase::setOutputDevice------------------------------------

// handle special case for sonification while in call
if (isInCall()) {
handleIncallSonification(stream, true, false);
++++++++++++++++++++++++++++AudioPolicyManagerBase::handleIncallSonification++++++++++++++++++++++++++++++++++++
从函数 AudioPolicyManagerBase::startOutput 进入：

void AudioPolicyManagerBase::handleIncallSonification(int stream, bool starting, bool stateChange)
{
// if the stream pertains to sonification strategy and we are in call we must
// mute the stream if it is low visibility. If it is high visibility, we must play a tone
// in the device used for phone strategy and play the tone if the selected device does not
// interfere with the device used for phone strategy
// if stateChange is true, we are called from setPhoneState() and we must mute or unmute as
// many times as there are active tracks on the output
// 如果 stream 是 sonification strategy 的，并且其可见性低， 如果有电话打入则 mute 该 stream 。
// 如果其可见性高，我们必须在 phone strategy 使用的 device 中播放 tone ，并且如果选定的 device
// 与 phone strategy 不相干的话，也要播放 tone 。
// 如果 stateChange 是 true ，则我们肯定是从函数 setPhoneState 进来的，
// 我们必须 mute / unmute output 中所有的 active track 。

if (getStrategy((AudioSystem::stream_type)stream) == STRATEGY_SONIFICATION) {
AudioOutputDescriptor *outputDesc = mOutputs.valueFor(mHardwareOutput);
LOGV("handleIncallSonification() stream %d starting %d device %x stateChange %d",
stream, starting, outputDesc->mDevice, stateChange);
if (outputDesc->mRefCount[stream]) {
// 只有 output 中存在 active 的该类型的 stream 时才做处理
int muteCount = 1;
if (stateChange) {
muteCount = outputDesc->mRefCount[stream];
}
if (AudioSystem::isLowVisibility((AudioSystem::stream_type)stream)) {
++++++++++++++++++++++++++++AudioSystem::isLowVisibility++++++++++++++++++++++++++++++++++++
从函数 AudioPolicyManagerBase::handleIncallSonification 进入：

bool AudioSystem::isLowVisibility(stream_type stream)
{
// 以下这几种 stream 的可见性低，其他的都是高的
if (stream == AudioSystem::SYSTEM ||
stream == AudioSystem::NOTIFICATION ||
stream == AudioSystem::RING) {
return true;
} else {
return false;
}
}
返回到函数 AudioPolicyManagerBase::handleIncallSonification
----------------------------AudioSystem::isLowVisibility------------------------------------
LOGV("handleIncallSonification() low visibility, muteCount %d", muteCount);
for (int i = 0; i < muteCount; i++) {
setStreamMute(stream, starting, mHardwareOutput);
}
} else {
LOGV("handleIncallSonification() high visibility");
if (outputDesc->device() & getDeviceForStrategy(STRATEGY_PHONE)) {
LOGV("handleIncallSonification() high visibility muted, muteCount %d", muteCount);
for (int i = 0; i < muteCount; i++) {
setStreamMute(stream, starting, mHardwareOutput);
}
}
if (starting) {
mpClientInterface->startTone(ToneGenerator::TONE_SUP_CALL_WAITING, AudioSystem::VOICE_CALL);
} else {
mpClientInterface->stopTone();
}
}
}
}
}
返回到函数 AudioPolicyManagerBase::startOutput
----------------------------AudioPolicyManagerBase::handleIncallSonification------------------------------------
}

// apply volume rules for current stream and device if necessary
checkAndSetVolume(stream, mStreams[stream].mIndexCur, output, outputDesc->device());

return NO_ERROR;
}
返回到函数 AudioPolicyService::startOutput
-----------------------------AudioPolicyManagerBase::startOutput-----------------------------------
}
返回到函数 AudioSystem::startOutput
-----------------------------AudioPolicyService::startOutput-----------------------------------
}
返回到函数 AudioFlinger::PlaybackThread::Track::start
----------------------------AudioSystem::startOutput------------------------------------
thread->mLock.lock();
}
if (status == NO_ERROR) {
PlaybackThread *playbackThread = (PlaybackThread *)thread.get();
playbackThread->addTrack_l(this);
} else {
mState = state;
}
} else {
status = BAD_VALUE;
}
return status;
}
###########################################################

&&&&&&&&&&&&&&&&&&&&&&&总结&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
各种声音控制策略的实现主要在 Audio Policy Manager 中。
其中会根据 stream type 和当前的状态对各种 stream 的音量作必要的修改。
以达成以下的优先级效果：

Priority Strategy Type Stream Type
4 STRATEGY_PHONE VOICE_CALL
BLUETOOTH_SCO
3 STRATEGY_SONIFICATION RING
NOTIFICATION
ALARM
ENFORCED_AUDIBLE
2 STRATEGY_MEDIA SYSTEM
TTS
MUSIC
1 STRATEGY_DTMF DTMF

&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&

摘自：江风专栏