Android音频框架之一 详解audioPolicy流程及HAL驱动加载与配置

一、 AndroidRuntime.so 引发思考

android 系统 framework 代码起点, frameworks/base/core/jni/AndroidRuntime.cpp 文件，
此文件是android系统主线程代码，代码内容涉及系统很多模块，此程序主要是注册模块的JNI接口方法。其中涉及到模块 native、sensorHal、media、audioflinger、displayflinger、camera、serialport、binder等，
从各模块名称上可看出是 android 系统核心组件内容，由此可见 AndroidRuntime 是系统框架的入口。
启动注册 audioRecord、audioSystem、audioTrack 模块,此模块注册是在 mediaService 模块启动之前。

defaultServiceManager?定义在 frameworks\native\libs\binder\IServiceManager.cpp

sp<IServiceManager> defaultServiceManager()
{
    std::call_once(gSmOnce, []() {
        sp<AidlServiceManager> sm = nullptr;
        while (sm == nullptr) {
            sm = interface_cast<AidlServiceManager>(ProcessState::self()->getContextObject(nullptr));
            if (sm == nullptr) {
                ALOGE("Waiting 1s on context object on %s.", ProcessState::self()->getDriverName().c_str());
                sleep(1);
            }
        }

        gDefaultServiceManager = sp<ServiceManagerShim>::make(sm);
    });

    return gDefaultServiceManager;
}

sp<IServiceManager> sm(defaultServiceManager());

安卓源码中大量使用 sm 对象来管理系统的各种服务，添加、释放服务。用户 app 通过反射的方式获取系统的各种服务，
由此可以对安卓系统服务概念理解会更加深刻。

Binder学习之获取ServiceManager对象_sp<iservicemanager> sm = defaultservicemanager();-CSDN博客

二、 Android 音频框架

Android音频系统有两大服务：一是AudioFlinger，二是AudioPolicyService。 AudioFlinger负责向下 访问AudioHardwareInterface，
实现音频PCM数据的混音/输入/输出，实现音量调节；
AudioPolicyService负责音 频输入输出设备的连接状态，音频策略调度即音频设备（如本地CODEC、Bluetooth A2DP、Headset）的切换
策略（注意它只是负责策略，真正的切换操作是在AudioFlinger中的openOutput，毕竟 AudioFlinger负责操作底层音频硬件）。

2.0> mediaserver 和 audioserver 本地服务

在 android 系统初始化 frameworks\av\media\mediaserver\mediaserver.rc?中，启动 mediaserver 服务, 线程名称: /system/bin/mediaserver ,启动多媒体服务。

on property:init.svc.media=*
    setprop init.svc.mediadrm ${init.svc.media}

service media /system/bin/mediaserver
    class main
    user media
    group audio camera inet net_bt net_bt_admin net_bw_acct drmrpc mediadrm
    ioprio rt 4
    task_profiles ProcessCapacityHigh HighPerformance

frameworks/av/media/mediaserver/main_mediaserver.cpp 是入口函数，?

int main(int argc __unused, char **argv __unused)
{
    signal(SIGPIPE, SIG_IGN);

    sp<ProcessState> proc(ProcessState::self());
    sp<IServiceManager> sm(defaultServiceManager());
    ALOGI("ServiceManager: %p", sm.get());
    MediaPlayerService::instantiate();
    ResourceManagerService::instantiate();
    registerExtensions();
    ::android::hardware::configureRpcThreadpool(16, false);
    ProcessState::self()->startThreadPool();
    IPCThreadState::self()->joinThreadPool();
    ::android::hardware::joinRpcThreadpool();
}

?

system/core/rootdir/init.zygote32.rc 中启动 audioserver 服务，内容如下:

service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
    class main
    socket zygote stream 660 root system
    onrestart write /sys/android_power/request_state wake
    onrestart write /sys/power/state on
    onrestart restart audioserver          ## 启动 audioserver 服务
    onrestart restart cameraserver         ## 启动 cameraserver 服务
    onrestart restart media
    onrestart restart netd
    writepid /dev/cpuset/foreground/tasks

frameworks/av/media/audioserver/main_audioserver.cpp 此程序是入口函数?

int main(int argc __unused, char **argv)
{
    // TODO: update with refined parameters
    limitProcessMemory(
        "audio.maxmem", /* "ro.audio.maxmem", property that defines limit */
        (size_t)512 * (1 << 20), /* SIZE_MAX, upper limit in bytes */
        20 /* upper limit as percentage of physical RAM */);

    signal(SIGPIPE, SIG_IGN);

#if 1
    // FIXME See bug 165702394 and bug 168511485
    const bool doLog = false;
#else
    bool doLog = (bool) property_get_bool("ro.test_harness", 0);
#endif

    pid_t childPid;
    // FIXME The advantage of making the process containing media.log service the parent process of
    // the process that contains the other audio services, is that it allows us to collect more
    // detailed information such as signal numbers, stop and continue, resource usage, etc.
    // But it is also more complex.  Consider replacing this by independent processes, and using
    // binder on death notification instead.
    if (doLog && (childPid = fork()) != 0) {
        // media.log service
        //prctl(PR_SET_NAME, (unsigned long) "media.log", 0, 0, 0);
        // unfortunately ps ignores PR_SET_NAME for the main thread, so use this ugly hack
        strcpy(argv[0], "media.log");
        sp<ProcessState> proc(ProcessState::self());
        MediaLogService::instantiate();
        ProcessState::self()->startThreadPool();
        IPCThreadState::self()->joinThreadPool();
        for (;;) {
            siginfo_t info;
            int ret = waitid(P_PID, childPid, &info, WEXITED | WSTOPPED | WCONTINUED);
            if (ret == EINTR) {
                continue;
            }
            if (ret < 0) {
                break;
            }
            char buffer[32];
            const char *code;
            switch (info.si_code) {
            case CLD_EXITED:
                code = "CLD_EXITED";
                break;
            case CLD_KILLED:
                code = "CLD_KILLED";
                break;
            case CLD_DUMPED:
                code = "CLD_DUMPED";
                break;
            case CLD_STOPPED:
                code = "CLD_STOPPED";
                break;
            case CLD_TRAPPED:
                code = "CLD_TRAPPED";
                break;
            case CLD_CONTINUED:
                code = "CLD_CONTINUED";
                break;
            default:
                snprintf(buffer, sizeof(buffer), "unknown (%d)", info.si_code);
                code = buffer;
                break;
            }
            struct rusage usage;
            getrusage(RUSAGE_CHILDREN, &usage);
            ALOG(LOG_ERROR, "media.log", "pid %d status %d code %s user %ld.%03lds sys %ld.%03lds",
                    info.si_pid, info.si_status, code,
                    usage.ru_utime.tv_sec, usage.ru_utime.tv_usec / 1000,
                    usage.ru_stime.tv_sec, usage.ru_stime.tv_usec / 1000);
            sp<IServiceManager> sm = defaultServiceManager();
            sp<IBinder> binder = sm->getService(String16("media.log"));
            if (binder != 0) {
                Vector<String16> args;
                binder->dump(-1, args);
            }
            switch (info.si_code) {
            case CLD_EXITED:
            case CLD_KILLED:
            case CLD_DUMPED: {
                ALOG(LOG_INFO, "media.log", "exiting");
                _exit(0);
                // not reached
                }
            default:
                break;
            }
        }
    } else {
        // all other services
        if (doLog) {
            prctl(PR_SET_PDEATHSIG, SIGKILL);   // if parent media.log dies before me, kill me also
            setpgid(0, 0);                      // but if I die first, don't kill my parent
        }
        android::hardware::configureRpcThreadpool(4, false /*callerWillJoin*/);
        sp<ProcessState> proc(ProcessState::self());
        sp<IServiceManager> sm = defaultServiceManager();
        ALOGI("ServiceManager: %p", sm.get());
        AudioFlinger::instantiate();
        AudioPolicyService::instantiate();

        // AAudioService should only be used in OC-MR1 and later.
        // And only enable the AAudioService if the system MMAP policy explicitly allows it.
        // This prevents a client from misusing AAudioService when it is not supported.
        aaudio_policy_t mmapPolicy = property_get_int32(AAUDIO_PROP_MMAP_POLICY,
                                                        AAUDIO_POLICY_NEVER);
        if (mmapPolicy == AAUDIO_POLICY_AUTO || mmapPolicy == AAUDIO_POLICY_ALWAYS) {
            AAudioService::instantiate();
        }

        ProcessState::self()->startThreadPool();
        IPCThreadState::self()->joinThreadPool();
    }
}

此线程启动服务有:AudioFlinger、AudioPolicyService、RadioService、SoundTriggerHwService 服务；其中 RadioService 是电话服务，不是本次讨论内容略过。

2.1> audioflinger 本地混音管理框架

AudioFlinger(下面简称AF)是整个音频系统的核心与难点。作为 Android 系统中的音频中枢，它同时也是一个系统服务，启到承上(为上层提供访问接口)启下(通过HAL来管理音频设备)的作用。 AudioFlinger 向下访问 AudioHardware，实现输出音频数据，控制音频参数。
首先看 AudioFlinger 的继承关系及父类内容,在 audioserver 函数中调用 AudioFlinger::instantiate() 函数.

void AudioFlinger::instantiate() {
    sp<IServiceManager> sm(defaultServiceManager());
    sm->addService(String16(IAudioFlinger::DEFAULT_SERVICE_NAME),
                   new AudioFlingerServerAdapter(new AudioFlinger()), false,
                   IServiceManager::DUMP_FLAG_PRIORITY_DEFAULT);
}

在 系统服务中添加 audioService 服务内容，供用户通过服务的方式使用音频设备

AudioFlinger 构造函数内容如下:

AudioFlinger::AudioFlinger()
    : mMediaLogNotifier(new AudioFlinger::MediaLogNotifier()),
      mPrimaryHardwareDev(NULL),
      mAudioHwDevs(NULL),
      mHardwareStatus(AUDIO_HW_IDLE),
      mMasterVolume(1.0f),
      mMasterMute(false),
      // mNextUniqueId(AUDIO_UNIQUE_ID_USE_MAX),
      mMode(AUDIO_MODE_INVALID),
      mBtNrecIsOff(false),
      mIsLowRamDevice(true),
      mIsDeviceTypeKnown(false),
      mTotalMemory(0),
      mClientSharedHeapSize(kMinimumClientSharedHeapSizeBytes),
      mGlobalEffectEnableTime(0),
      mPatchPanel(this),
      mDeviceEffectManager(this),
      mSystemReady(false)
{
    // Move the audio session unique ID generator start base as time passes to limit risk of
    // generating the same ID again after an audioserver restart.
    // This is important because clients will reuse previously allocated audio session IDs
    // when reconnecting after an audioserver restart and newly allocated IDs may conflict with
    // active clients.
    // Moving the base by 1 for each elapsed second is a good compromise between avoiding overlap
    // between allocation ranges and not reaching wrap around too soon.
    timespec ts{};
    clock_gettime(CLOCK_MONOTONIC, &ts);
    // zero ID has a special meaning, so start allocation at least at AUDIO_UNIQUE_ID_USE_MAX
    uint32_t movingBase = (uint32_t)std::max((long)1, ts.tv_sec);
    // unsigned instead of audio_unique_id_use_t, because ++ operator is unavailable for enum
    for (unsigned use = AUDIO_UNIQUE_ID_USE_UNSPECIFIED; use < AUDIO_UNIQUE_ID_USE_MAX; use++) {
        mNextUniqueIds[use] =
                ((use == AUDIO_UNIQUE_ID_USE_SESSION || use == AUDIO_UNIQUE_ID_USE_CLIENT) ?
                        movingBase : 1) * AUDIO_UNIQUE_ID_USE_MAX;
    }

#if 1
    // FIXME See bug 165702394 and bug 168511485
    const bool doLog = false;
#else
    const bool doLog = property_get_bool("ro.test_harness", false);
#endif
    if (doLog) {
        mLogMemoryDealer = new MemoryDealer(kLogMemorySize, "LogWriters",
                MemoryHeapBase::READ_ONLY);
        (void) pthread_once(&sMediaLogOnce, sMediaLogInit);
    }

    // reset battery stats.
    // if the audio service has crashed, battery stats could be left
    // in bad state, reset the state upon service start.
    BatteryNotifier::getInstance().noteResetAudio();

    mDevicesFactoryHal = DevicesFactoryHalInterface::create();
    mEffectsFactoryHal = EffectsFactoryHalInterface::create();

    mMediaLogNotifier->run("MediaLogNotifier");
    std::vector<pid_t> halPids;
    mDevicesFactoryHal->getHalPids(&halPids);
    TimeCheck::setAudioHalPids(halPids);

    // Notify that we have started (also called when audioserver service restarts)
    mediametrics::LogItem(mMetricsId)
        .set(AMEDIAMETRICS_PROP_EVENT, AMEDIAMETRICS_PROP_EVENT_VALUE_CTOR)
        .record();
}

?C++ 中智能指针创建对象时，第一次创建会调用onFirstRef()函数，在本例中将调用 AudioFlinger::onFirstRef() 函数，
内容如下:frameworks/av/services/audioflinger/audioFlinger.cpp

void AudioFlinger::onFirstRef()
{
    Mutex::Autolock _l(mLock);

    /* TODO: move all this work into an Init() function */
    char val_str[PROPERTY_VALUE_MAX] = { 0 };
    if (property_get("ro.audio.flinger_standbytime_ms", val_str, NULL) >= 0) {
        uint32_t int_val;
        if (1 == sscanf(val_str, "%u", &int_val)) {
            mStandbyTimeInNsecs = milliseconds(int_val);
            ALOGI("Using %u mSec as standby time.", int_val);
        } else {
            mStandbyTimeInNsecs = kDefaultStandbyTimeInNsecs;
            ALOGI("Using default %u mSec as standby time.",
                    (uint32_t)(mStandbyTimeInNsecs / 1000000));
        }
    }

    mMode = AUDIO_MODE_NORMAL;

    gAudioFlinger = this;  // we are already refcounted, store into atomic pointer.

    mDevicesFactoryHalCallback = new DevicesFactoryHalCallbackImpl;
    mDevicesFactoryHal->setCallbackOnce(mDevicesFactoryHalCallback);
}

?至此 audioFlinger 对象构建完成

?