该系列文章总纲链接：专题分纲目录 Android Framework 音频子系统
本章关键点总结 & 说明：
本章节主要关注➕ 以上思维导图下边 AudioPolicyService 部分 即可。主要是对AudioPolicyService启动的分析，涉及从AudioPolicyService启动流程 到 AudioPolicyManager流程和AudioPolicyManager详细分析流程（加载配置文件、loadHwModule
、openOutput，这里涉及的AudioFlinger相关操作在下一节中详细分析其流程）。
1 从AudioPolicyService启动流程 到 AudioPolicyManager
AudioPolicyService的启动是在Main_mediaserver.cpp中实现，代码如下：
int main(int argc __unused, char** argv)
{
signal(SIGPIPE, SIG_IGN);
char value[PROPERTY_VALUE_MAX];
//...
sp<ProcessState> proc(ProcessState::self());
sp<IServiceManager> sm = defaultServiceManager();
//...
AudioFlinger::instantiate();//AudioFlinger
MediaPlayerService::instantiate();
CameraService::instantiate();
AudioPolicyService::instantiate();//AudioPolicyService
SoundTriggerHwService::instantiate();
registerExtensions();
ProcessState::self()->startThreadPool();
IPCThreadState::self()->joinThreadPool();
//...
}这里AudioFlinger服务先启动，之后AudioPolicyService启动就可以直接使用AudioFlinger服务了。本章节专注分析AudioPolicyService::instantiate()的实现，代码如下：
static void instantiate() { publish(); }继续分析publish，代码如下：
static status_t publish(bool allowIsolated = false) {
sp<IServiceManager> sm(defaultServiceManager());
return sm->addService(//注册服务到servicemanager中
String16(SERVICE::getServiceName()),
new SERVICE(), allowIsolated);
}这里将服务注册到service_manager 中。同时这里会 创建 一个AudioPolicyService，即执行AudioPolicyService的构造器，代码如下：
AudioPolicyService::AudioPolicyService()
: BnAudioPolicyService(), mpAudioPolicyDev(NULL), mpAudioPolicy(NULL),
mAudioPolicyManager(NULL), mAudioPolicyClient(NULL), mPhoneState(AUDIO_MODE_INVALID)
{
}主要是一些变量的初始化，之后主要看onFirstRef的实现，代码如下：
void AudioPolicyService::onFirstRef()
{
char value[PROPERTY_VALUE_MAX];
const struct hw_module_t *module;
int forced_val;
int rc;
{
Mutex::Autolock _l(mLock);
//创建3个线程：
//用于播放tone音
mTonePlaybackThread = new AudioCommandThread(String8("ApmTone"), this);
//用于执行audio命令
mAudioCommandThread = new AudioCommandThread(String8("ApmAudio"), this);
//用于执行输出命令
mOutputCommandThread = new AudioCommandThread(String8("ApmOutput"), this);
#ifdef USE_LEGACY_AUDIO_POLICY
//老版本的实现，这里忽略
#else
//AudioFlinger客户端实现，调用AudioFlinger的一些服务
mAudioPolicyClient = new AudioPolicyClient(this);
mAudioPolicyManager = createAudioPolicyManager(mAudioPolicyClient);
#endif
}
// load audio processing modules
sp<AudioPolicyEffects>audioPolicyEffects = new AudioPolicyEffects();
{
Mutex::Autolock _l(mLock);
mAudioPolicyEffects = audioPolicyEffects;
}
}这里主要关注 createAudioPolicyManager的实现，代码如下：
extern "C" AudioPolicyInterface* createAudioPolicyManager(
AudioPolicyClientInterface *clientInterface)
{
return new AudioPolicyManager(clientInterface);
}接下里继续分析AudioPolicyManager的实现。
2 AudioPolicyManager详细分析
AudioPolicyManager的代码实现如下：
AudioPolicyManager::AudioPolicyManager(AudioPolicyClientInterface *clientInterface)
://test...
mPrimaryOutput((audio_io_handle_t)0),
mPhoneState(AUDIO_MODE_NORMAL),
mLimitRingtoneVolume(false), mLastVoiceVolume(-1.0f),
//...
{
//...
mDefaultOutputDevice = new DeviceDescriptor(String8(""), AUDIO_DEVICE_OUT_SPEAKER);
//关键点1，加载配置文件 audio_policy.conf
/*
系统会首先加载vendor/etc目录下的configure文件，再加载system/etc目录下的configure文件。
若这两者加载都发生错误的话，系统会加载default配置文件，并命名为primary module，从这可以看
出，音频系统中一定必须存在的module就是primary了。
*/
if (loadAudioPolicyConfig(AUDIO_POLICY_VENDOR_CONFIG_FILE) != NO_ERROR) {
if (loadAudioPolicyConfig(AUDIO_POLICY_CONFIG_FILE) != NO_ERROR) {
ALOGE("could not load audio policy configuration file, setting defaults");
defaultAudioPolicyConfig();
}
}
// mAvailableOutputDevices and mAvailableInputDevices now contain all attached devices
//初始化各种音频流对应的音量调节点
initializeVolumeCurves();
// open all output streams needed to access attached devices
audio_devices_t outputDeviceTypes = mAvailableOutputDevices.types();
//input等价操作
for (size_t i = 0; i < mHwModules.size(); i++) {
//关键点2，使用AudioFlinger加载audio policy硬件抽象库
mHwModules[i]->mHandle = mpClientInterface->loadHwModule(mHwModules[i]->mName);
if (mHwModules[i]->mHandle == 0) {
ALOGW("could not open HW module %s", mHwModules[i]->mName);
continue;
}
for (size_t j = 0; j < mHwModules[i]->mOutputProfiles.size(); j++)
{
const sp<IOProfile> outProfile = mHwModules[i]->mOutputProfiles[j];
if (outProfile->mSupportedDevices.isEmpty()) {
ALOGW("Output profile contains no device on module %s", mHwModules[i]->mName);
continue;
}
if ((outProfile->mFlags & AUDIO_OUTPUT_FLAG_DIRECT) != 0) {
continue;
}
audio_devices_t profileType = outProfile->mSupportedDevices.types();
if ((profileType & mDefaultOutputDevice->mDeviceType) != AUDIO_DEVICE_NONE) {
profileType = mDefaultOutputDevice->mDeviceType;
} else {
for (size_t k = 0; k < outProfile->mSupportedDevices.size(); k++) {
profileType = outProfile->mSupportedDevices[k]->mDeviceType;
if ((profileType & outputDeviceTypes) != 0) {
break;
}
}
}
if ((profileType & outputDeviceTypes) == 0) {
continue;
}
//通过outProfile 构建AudioOutputDescriptor类型的变量outputDesc
sp<AudioOutputDescriptor> outputDesc = new AudioOutputDescriptor(outProfile);
outputDesc->mDevice = profileType;
audio_config_t config = AUDIO_CONFIG_INITIALIZER;
config.sample_rate = outputDesc->mSamplingRate;
config.channel_mask = outputDesc->mChannelMask;
config.format = outputDesc->mFormat;
audio_io_handle_t output = AUDIO_IO_HANDLE_NONE;
//关键点3：openOutput
status_t status = mpClientInterface->openOutput(outProfile->mModule->mHandle,
&output,
&config,
&outputDesc->mDevice,
String8(""),
&outputDesc->mLatency,
outputDesc->mFlags);
if (status != NO_ERROR) {
//...
} else {
outputDesc->mSamplingRate = config.sample_rate;
outputDesc->mChannelMask = config.channel_mask;
outputDesc->mFormat = config.format;
//...
/*保存输出设备描述符对象outputDesc到mOutputs，表示已经打开的Output
*这样以后就可以根据一个整数output 找到对应的thread和outputDesc
*/
addOutput(output, outputDesc);
//设置输出设备
setOutputDevice(output,
outputDesc->mDevice,
true);
}
}
// open input streams needed to access attached devices to validate
// mAvailableInputDevices list
//input等价操作
}
// make sure all attached devices have been allocated a unique ID
for (size_t i = 0; i < mAvailableOutputDevices.size();) {
if (mAvailableOutputDevices[i]->mId == 0) {
ALOGW("Input device %08x unreachable", mAvailableOutputDevices[i]->mDeviceType);
mAvailableOutputDevices.remove(mAvailableOutputDevices[i]);
continue;
}
i++;
}
//更新输出设备
updateDevicesAndOutputs();
//test...
}这里主要分析 3个关键点：加载配置文件 和 loadHwModule和打开output。
2.1 加载配置文件
loadAudioPolicyConfig的代码实现如下：
status_t AudioPolicyManager::loadAudioPolicyConfig(const char *path)
{
cnode *root;
char *data;
data = (char *)load_file(path, NULL);
if (data == NULL) {
return -ENODEV;
}
root = config_node("", "");
config_load(root, data);
loadHwModules(root);
// legacy audio_policy.conf files have one global_configuration section
loadGlobalConfig(root, getModuleFromName(AUDIO_HARDWARE_MODULE_ID_PRIMARY));
config_free(root);
free(root);
free(data);
return NO_ERROR;
}
这里本质上是解析一个文件audio_policy.conf，这个文件的构造如下：
# Global configuration section: lists input and output devices always present on the device
# as well as the output device selected by default.
# Devices are designated by a string that corresponds to the enum in audio.h
global_configuration {
attached_output_devices AUDIO_DEVICE_OUT_SPEAKER
default_output_device AUDIO_DEVICE_OUT_SPEAKER
attached_input_devices AUDIO_DEVICE_IN_BUILTIN_MIC|AUDIO_DEVICE_IN_VOICE_CALL|AUDIO_DEVICE_IN_REMOTE_SUBMIX
}
# audio hardware module section: contains descriptors for all audio hw modules present on the
# device. Each hw module node is named after the corresponding hw module library base name.
# For instance, "primary" corresponds to audio.primary.<device>.so.
# The "primary" module is mandatory and must include at least one output with
# AUDIO_OUTPUT_FLAG_PRIMARY flag.
# Each module descriptor contains one or more output profile descriptors and zero or more
# input profile descriptors. Each profile lists all the parameters supported by a given output
# or input stream category.
# The "channel_masks", "formats", "devices" and "flags" are specified using strings corresponding
# to enums in audio.h and audio_policy.h. They are concatenated by use of "|" without space or "\n".
audio_hw_modules {
primary { #一个module对应厂家提供一个so文件
outputs { #一个module可以有多个output
primary { #一个module里表明它的参数
sampling_rates 48000
channel_masks AUDIO_CHANNEL_OUT_STEREO
formats AUDIO_FORMAT_PCM_16_BIT
devices AUDIO_DEVICE_OUT_SPEAKER|AUDIO_DEVICE_OUT_WIRED_HEADSET|AUDIO_DEVICE_OUT_WIRED_HEADPHONE|AUDIO_DEVICE_OUT_ALL_SCO
flags AUDIO_OUTPUT_FLAG_PRIMARY #默认设备
}
}
inputs { #一个module可以有多个input
primary {
sampling_rates 8000|11025|12000|16000|22050|24000|32000|44100|48000
channel_masks AUDIO_CHANNEL_IN_MONO|AUDIO_CHANNEL_IN_STEREO
formats AUDIO_FORMAT_PCM_16_BIT
devices AUDIO_DEVICE_IN_BUILTIN_MIC|AUDIO_DEVICE_IN_BLUETOOTH_SCO_HEADSET|AUDIO_DEVICE_IN_WIRED_HEADSET|AUDIO_DEVICE_IN_VOICE_CALL
}
}
}
a2dp {
outputs {
a2dp {
sampling_rates 44100
channel_masks AUDIO_CHANNEL_OUT_STEREO
formats AUDIO_FORMAT_PCM_16_BIT
devices AUDIO_DEVICE_OUT_ALL_A2DP
}
}
}
//...
}总结下，这部分代码主要是加载解析/vendor/etc/audio_policy.conf或/system/etc/audio_policy.conf,主要分为3个步骤：
- 对于配置文件里的每一个module项, new HwModule(name), 放入mHwModules数组
- 对于module里的每一个output, new IOProfile, 放入module的mOutputProfiles
- 对于module里的每一个input, new IOProfile, 放入module的mInputProfiles
这个文件解析用一张图来表示，这样可以更清楚的看到层级关系与数据结构的关系，如下所示：
2.2 loadHwModule的实现
mpClientInterface->loadHwModule(mHwModules[i]->mName)，最后调用的是AudioFlinger的loadHwModule方法，它的代码实现如下：
audio_module_handle_t AudioPolicyService::AudioPolicyClient::loadHwModule(const char *name)
{
sp<IAudioFlinger> af = AudioSystem::get_audio_flinger();
if (af == 0) {
ALOGW("%s: could not get AudioFlinger", __func__);
return 0;
}
return af->loadHwModule(name);
}
这里最后调用了AudioFlinger的loadHwModule方法。下一章节我们针对此进行详细分析
2.3 openOutput的实现
mpClientInterface->openOutput，最后调用的是AudioFlinger的openOutput方法，它的代码实现如下：
status_t AudioPolicyService::AudioPolicyClient::openOutput(audio_module_handle_t module,
//...
audio_output_flags_t flags)
{
sp<IAudioFlinger> af = AudioSystem::get_audio_flinger();
if (af == 0) {
ALOGW("%s: could not get AudioFlinger", __func__);
return PERMISSION_DENIED;
}
return af->openOutput(module, output, config, devices, address, latencyMs, flags);
}这里最后调用了AudioFlinger的output (这里最后会创建一个MixerThread线程与对应的output相关联，之后应用程序就可以把数据传递给线程了，进而传递给硬件设备了)。下一章节我们针对此进行详细分析。
