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Android音频框架之一 详解audioPolicy流程及HAL驱动加载与配置_安卓音频开发和音频驱动

作者：小丑西瓜9 | 2024-03-26 19:06:16

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安卓音频开发和音频驱动

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

  1. sp<IServiceManager> defaultServiceManager()
  2. {
  3.     std::call_once(gSmOnce, []() {
  4.         sp<AidlServiceManager> sm = nullptr;
  5.         while (sm == nullptr) {
  6.             sm = interface_cast<AidlServiceManager>(ProcessState::self()->getContextObject(nullptr));
  7.             if (sm == nullptr) {
  8.                 ALOGE("Waiting 1s on context object on %s.", ProcessState::self()->getDriverName().c_str());
  9.                 sleep(1);
  10.             }
  11.         }
  12.  
  13.         gDefaultServiceManager = sp<ServiceManagerShim>::make(sm);
  14.     });
  15.  
  16.     return gDefaultServiceManager;
  17. }
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。

  1. on property:init.svc.media=*
  2.     setprop init.svc.mediadrm ${init.svc.media}
  3.  
  4. service media /system/bin/mediaserver
  5.     class main
  6.     user media
  7.     group audio camera inet net_bt net_bt_admin net_bw_acct drmrpc mediadrm
  8.     ioprio rt 4
  9.     task_profiles ProcessCapacityHigh HighPerformance

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

  1. int main(int argc __unused, char **argv __unused)
  2. {
  3.     signal(SIGPIPE, SIG_IGN);
  4.  
  5.     sp<ProcessState> proc(ProcessState::self());
  6.     sp<IServiceManager> sm(defaultServiceManager());
  7.     ALOGI("ServiceManager: %p", sm.get());
  8.     MediaPlayerService::instantiate();
  9.     ResourceManagerService::instantiate();
  10.     registerExtensions();
  11.     ::android::hardware::configureRpcThreadpool(16, false);
  12.     ProcessState::self()->startThreadPool();
  13.     IPCThreadState::self()->joinThreadPool();
  14.     ::android::hardware::joinRpcThreadpool();
  15. }

 

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

  1. service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
  2.     class main
  3.     socket zygote stream 660 root system
  4.     onrestart write /sys/android_power/request_state wake
  5.     onrestart write /sys/power/state on
  6.     onrestart restart audioserver          ## 启动 audioserver 服务
  7.     onrestart restart cameraserver         ## 启动 cameraserver 服务
  8.     onrestart restart media
  9.     onrestart restart netd
  10.     writepid /dev/cpuset/foreground/tasks

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

  1. int main(int argc __unused, char **argv)
  2. {
  3.     // TODO: update with refined parameters
  4.     limitProcessMemory(
  5.         "audio.maxmem", /* "ro.audio.maxmem", property that defines limit */
  6.         (size_t)512 * (1 << 20), /* SIZE_MAX, upper limit in bytes */
  7.         20 /* upper limit as percentage of physical RAM */);
  8.  
  9.     signal(SIGPIPE, SIG_IGN);
  10.  
  11. #if 1
  12.     // FIXME See bug 165702394 and bug 168511485
  13.     const bool doLog = false;
  14. #else
  15.     bool doLog = (bool) property_get_bool("ro.test_harness", 0);
  16. #endif
  17.  
  18.     pid_t childPid;
  19.     // FIXME The advantage of making the process containing media.log service the parent process of
  20.     // the process that contains the other audio services, is that it allows us to collect more
  21.     // detailed information such as signal numbers, stop and continue, resource usage, etc.
  22.     // But it is also more complex.  Consider replacing this by independent processes, and using
  23.     // binder on death notification instead.
  24.     if (doLog && (childPid = fork()) != 0) {
  25.         // media.log service
  26.         //prctl(PR_SET_NAME, (unsigned long) "media.log", 0, 0, 0);
  27.         // unfortunately ps ignores PR_SET_NAME for the main thread, so use this ugly hack
  28.         strcpy(argv[0], "media.log");
  29.         sp<ProcessState> proc(ProcessState::self());
  30.         MediaLogService::instantiate();
  31.         ProcessState::self()->startThreadPool();
  32.         IPCThreadState::self()->joinThreadPool();
  33.         for (;;) {
  34.             siginfo_t info;
  35.             int ret = waitid(P_PID, childPid, &info, WEXITED | WSTOPPED | WCONTINUED);
  36.             if (ret == EINTR) {
  37.                 continue;
  38.             }
  39.             if (ret < 0) {
  40.                 break;
  41.             }
  42.             char buffer[32];
  43.             const char *code;
  44.             switch (info.si_code) {
  45.             case CLD_EXITED:
  46.                 code = "CLD_EXITED";
  47.                 break;
  48.             case CLD_KILLED:
  49.                 code = "CLD_KILLED";
  50.                 break;
  51.             case CLD_DUMPED:
  52.                 code = "CLD_DUMPED";
  53.                 break;
  54.             case CLD_STOPPED:
  55.                 code = "CLD_STOPPED";
  56.                 break;
  57.             case CLD_TRAPPED:
  58.                 code = "CLD_TRAPPED";
  59.                 break;
  60.             case CLD_CONTINUED:
  61.                 code = "CLD_CONTINUED";
  62.                 break;
  63.             default:
  64.                 snprintf(buffer, sizeof(buffer), "unknown (%d)", info.si_code);
  65.                 code = buffer;
  66.                 break;
  67.             }
  68.             struct rusage usage;
  69.             getrusage(RUSAGE_CHILDREN, &usage);
  70.             ALOG(LOG_ERROR, "media.log", "pid %d status %d code %s user %ld.%03lds sys %ld.%03lds",
  71.                     info.si_pid, info.si_status, code,
  72.                     usage.ru_utime.tv_sec, usage.ru_utime.tv_usec / 1000,
  73.                     usage.ru_stime.tv_sec, usage.ru_stime.tv_usec / 1000);
  74.             sp<IServiceManager> sm = defaultServiceManager();
  75.             sp<IBinder> binder = sm->getService(String16("media.log"));
  76.             if (binder != 0) {
  77.                 Vector<String16> args;
  78.                 binder->dump(-1, args);
  79.             }
  80.             switch (info.si_code) {
  81.             case CLD_EXITED:
  82.             case CLD_KILLED:
  83.             case CLD_DUMPED: {
  84.                 ALOG(LOG_INFO, "media.log", "exiting");
  85.                 _exit(0);
  86.                 // not reached
  87.                 }
  88.             default:
  89.                 break;
  90.             }
  91.         }
  92.     } else {
  93.         // all other services
  94.         if (doLog) {
  95.             prctl(PR_SET_PDEATHSIG, SIGKILL);   // if parent media.log dies before me, kill me also
  96.             setpgid(0, 0);                      // but if I die first, don't kill my parent
  97.         }
  98.         android::hardware::configureRpcThreadpool(4, false /*callerWillJoin*/);
  99.         sp<ProcessState> proc(ProcessState::self());
  100.         sp<IServiceManager> sm = defaultServiceManager();
  101.         ALOGI("ServiceManager: %p", sm.get());
  102.         AudioFlinger::instantiate();
  103.         AudioPolicyService::instantiate();
  105.         // AAudioService should only be used in OC-MR1 and later.
  106.         // And only enable the AAudioService if the system MMAP policy explicitly allows it.
  107.         // This prevents a client from misusing AAudioService when it is not supported.
  108.         aaudio_policy_t mmapPolicy = property_get_int32(AAUDIO_PROP_MMAP_POLICY,
  109.                                                         AAUDIO_POLICY_NEVER);
  110.         if (mmapPolicy == AAUDIO_POLICY_AUTO || mmapPolicy == AAUDIO_POLICY_ALWAYS) {
  111.             AAudioService::instantiate();
  112.         }
  114.         ProcessState::self()->startThreadPool();
  115.         IPCThreadState::self()->joinThreadPool();
  116.     }
  117. }

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.

  1. void AudioFlinger::instantiate() {
  2.     sp<IServiceManager> sm(defaultServiceManager());
  3.     sm->addService(String16(IAudioFlinger::DEFAULT_SERVICE_NAME),
  4.                    new AudioFlingerServerAdapter(new AudioFlinger()), false,
  5.                    IServiceManager::DUMP_FLAG_PRIORITY_DEFAULT);
  6. }

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:

  1. AudioFlinger::AudioFlinger()
  2.     : mMediaLogNotifier(new AudioFlinger::MediaLogNotifier()),
  3.       mPrimaryHardwareDev(NULL),
  4.       mAudioHwDevs(NULL),
  5.       mHardwareStatus(AUDIO_HW_IDLE),
  6.       mMasterVolume(1.0f),
  7.       mMasterMute(false),
  8.       // mNextUniqueId(AUDIO_UNIQUE_ID_USE_MAX),
  9.       mMode(AUDIO_MODE_INVALID),
  10.       mBtNrecIsOff(false),
  11.       mIsLowRamDevice(true),
  12.       mIsDeviceTypeKnown(false),
  13.       mTotalMemory(0),
  14.       mClientSharedHeapSize(kMinimumClientSharedHeapSizeBytes),
  15.       mGlobalEffectEnableTime(0),
  16.       mPatchPanel(this),
  17.       mDeviceEffectManager(this),
  18.       mSystemReady(false)
  19. {
  20.     // Move the audio session unique ID generator start base as time passes to limit risk of
  21.     // generating the same ID again after an audioserver restart.
  22.     // This is important because clients will reuse previously allocated audio session IDs
  23.     // when reconnecting after an audioserver restart and newly allocated IDs may conflict with
  24.     // active clients.
  25.     // Moving the base by 1 for each elapsed second is a good compromise between avoiding overlap
  26.     // between allocation ranges and not reaching wrap around too soon.
  27.     timespec ts{};
  28.     clock_gettime(CLOCK_MONOTONIC, &ts);
  29.     // zero ID has a special meaning, so start allocation at least at AUDIO_UNIQUE_ID_USE_MAX
  30.     uint32_t movingBase = (uint32_t)std::max((long)1, ts.tv_sec);
  31.     // unsigned instead of audio_unique_id_use_t, because ++ operator is unavailable for enum
  32.     for (unsigned use = AUDIO_UNIQUE_ID_USE_UNSPECIFIED; use < AUDIO_UNIQUE_ID_USE_MAX; use++) {
  33.         mNextUniqueIds[use] =
  34.                 ((use == AUDIO_UNIQUE_ID_USE_SESSION || use == AUDIO_UNIQUE_ID_USE_CLIENT) ?
  35.                         movingBase : 1) * AUDIO_UNIQUE_ID_USE_MAX;
  36.     }
  37.  
  38. #if 1
  39.     // FIXME See bug 165702394 and bug 168511485
  40.     const bool doLog = false;
  41. #else
  42.     const bool doLog = property_get_bool("ro.test_harness", false);
  43. #endif
  44.     if (doLog) {
  45.         mLogMemoryDealer = new MemoryDealer(kLogMemorySize, "LogWriters",
  46.                 MemoryHeapBase::READ_ONLY);
  47.         (void) pthread_once(&sMediaLogOnce, sMediaLogInit);
  48.     }
  49.  
  50.     // reset battery stats.
  51.     // if the audio service has crashed, battery stats could be left
  52.     // in bad state, reset the state upon service start.
  53.     BatteryNotifier::getInstance().noteResetAudio();
  54.  
  55.     mDevicesFactoryHal = DevicesFactoryHalInterface::create();
  56.     mEffectsFactoryHal = EffectsFactoryHalInterface::create();
  57.  
  58.     mMediaLogNotifier->run("MediaLogNotifier");
  59.     std::vector<pid_t> halPids;
  60.     mDevicesFactoryHal->getHalPids(&halPids);
  61.     TimeCheck::setAudioHalPids(halPids);
  62.  
  63.     // Notify that we have started (also called when audioserver service restarts)
  64.     mediametrics::LogItem(mMetricsId)
  65.         .set(AMEDIAMETRICS_PROP_EVENT, AMEDIAMETRICS_PROP_EVENT_VALUE_CTOR)
  66.         .record();
  67. }

 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

  1. void AudioFlinger::onFirstRef()
  2. {
  3.     Mutex::Autolock _l(mLock);
  4.  
  5.     /* TODO: move all this work into an Init() function */
  6.     char val_str[PROPERTY_VALUE_MAX] = { 0 };
  7.     if (property_get("ro.audio.flinger_standbytime_ms", val_str, NULL) >= 0) {
  8.         uint32_t int_val;
  9.         if (1 == sscanf(val_str, "%u", &int_val)) {
  10.             mStandbyTimeInNsecs = milliseconds(int_val);
  11.             ALOGI("Using %u mSec as standby time.", int_val);
  12.         } else {
  13.             mStandbyTimeInNsecs = kDefaultStandbyTimeInNsecs;
  14.             ALOGI("Using default %u mSec as standby time.",
  15.                     (uint32_t)(mStandbyTimeInNsecs / 1000000));
  16.         }
  17.     }
  18.  
  19.     mMode = AUDIO_MODE_NORMAL;
  20.  
  21.     gAudioFlinger = this;  // we are already refcounted, store into atomic pointer.
  22.  
  23.     mDevicesFactoryHalCallback = new DevicesFactoryHalCallbackImpl;
  24.     mDevicesFactoryHal->setCallbackOnce(mDevicesFactoryHalCallback);
  25. }

 至此 audioFlinger 对象构建完成

 

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