Android 5.1 Camera 架构学习之Camera初始化

Android Camera 采用C/S架构，client 与server两个独立的线程之间（CameraService）使用Binder通信。

一 CameraService的注册。

1.手机开机后，会走init.rc流程，init.rc会启动MediaServer Service。

service media /system/bin/mediaserver
    class main
    user root ####
#   google default ####
#   user media    ####
    group audio camera inet net_bt net_bt_admin net_bw_acct drmrpc mediadrm media sdcard_r system net_bt_stack ####
#   google default ####
#   group audio camera inet net_bt net_bt_admin net_bw_acct drmrpc mediadrm ####

    ioprio rt 4

2.MediaServer的main函数位于frameworks/base/media/mediaserver/main_mediaserver.cpp中。

在Main_MediaServer.cpp的main函数中，CameraService完成了注册

int main(int argc __unused, char** argv)
{
    signal(SIGPIPE, SIG_IGN);
    char value[PROPERTY_VALUE_MAX];
    bool doLog = (property_get("ro.test_harness", value, "0") > 0) && (atoi(value) == 1);
    pid_t childPid;
..............................
        sp<ProcessState> proc(ProcessState::self());
        sp<IServiceManager> sm = defaultServiceManager();
        ALOGI("ServiceManager: %p", sm.get());
        AudioFlinger::instantiate();
        MediaPlayerService::instantiate();
#ifdef MTK_AOSP_ENHANCEMENT
        MemoryDumper::instantiate();
#endif
        CameraService::instantiate();
......................................

3. instantiate的实现在CameraService的父类中，

namespace android {

template<typename SERVICE>
class BinderService
{
public:
    static status_t publish(bool allowIsolated = false) {
        sp<IServiceManager> sm(defaultServiceManager());
        return sm->addService(
                String16(SERVICE::getServiceName()),
                new SERVICE(), allowIsolated);
    }

    static void publishAndJoinThreadPool(bool allowIsolated = false) {
        publish(allowIsolated);
        joinThreadPool();
    }

    static void instantiate() { publish(); }

    static status_t shutdown() { return NO_ERROR; }

private:
    static void joinThreadPool() {
        sp<ProcessState> ps(ProcessState::self());
        ps->startThreadPool();
        ps->giveThreadPoolName();
        IPCThreadState::self()->joinThreadPool();
    }
};


}; // namespace android

可以发现在publish(）函数中，CameraService完成服务的注册 。SERVICE是个模板，这里是注册CameraService，所以可用CameraService代替

  return sm->addService(String16(CameraService::getServiceName()), new CameraService());  

这样，Camera就在ServiceManager完成服务注册，提供给client随时使用。

二  client如何连上server端，并打开camera模块

1.Client如何连接到server端。

我们从Camera.open()开始往framework进行分析，调用frameworksasecorejavaandroidhardwareCamera.java类的open方法 。

public static Camera open() {
............................................
    return new Camera(cameraId);
.............................................
}

这里调用了Camera的构造函数,在构造函数中调用了cameraInitVersion

    private int cameraInitVersion(int cameraId, int halVersion) {
        ..................................................
        return native_setup(new WeakReference<Camera>(this), cameraId, halVersion, packageName);
    }

此后进入JNI层android_hardware_camera.cpp

// connect to camera service
static jint android_hardware_Camera_native_setup(JNIEnv *env, jobject thiz,
    jobject weak_this, jint cameraId, jint halVersion, jstring clientPackageName)
{
   .......................
        camera = Camera::connect(cameraId, clientName,
                Camera::USE_CALLING_UID);
    ......................

    sp<JNICameraContext> context = new JNICameraContext(env, weak_this, clazz, camera);
     ...........................
    camera->setListener(context);
    ..........................

}

JNI函数里面，我们找到Camera C/S架构的客户端了（即红色加粗的那个Camera），它调用connect函数向服务端发送连接请求。JNICameraContext这个类是一个监听类，用于处理底层Camera回调函数传来的数据和消息

sp<Camera> Camera::connect(int cameraId, const String16& clientPackageName,
        int clientUid)
{
    return CameraBaseT::connect(cameraId, clientPackageName, clientUid);
}

template <typename TCam, typename TCamTraits>
sp<TCam> CameraBase<TCam, TCamTraits>::connect(int cameraId,
                                               const String16& clientPackageName,
                                               int clientUid)
{
    ALOGV("%s: connect", __FUNCTION__);
    sp<TCam> c = new TCam(cameraId);
    sp<TCamCallbacks> cl = c;
    status_t status = NO_ERROR;
    const sp<ICameraService>& cs = getCameraService();

    if (cs != 0) {
        TCamConnectService fnConnectService = TCamTraits::fnConnectService;
        status = (cs.get()->*fnConnectService)(cl, cameraId, clientPackageName, clientUid,
                                             /*out*/ c->mCamera);
    }
    if (status == OK && c->mCamera != 0) {
        c->mCamera->asBinder()->linkToDeath(c);
        c->mStatus = NO_ERROR;
    } else {
        ALOGW("An error occurred while connecting to camera: %d", cameraId);
        c.clear();
    }
    return c;
}

// establish binder interface to camera service
template <typename TCam, typename TCamTraits>
const sp<ICameraService>& CameraBase<TCam, TCamTraits>::getCameraService()
{
    Mutex::Autolock _l(gLock);
    if (gCameraService.get() == 0) {
        sp<IServiceManager> sm = defaultServiceManager();
        sp<IBinder> binder;
        do {
            binder = sm->getService(String16(kCameraServiceName));
            if (binder != 0) {
                break;
            }
            ALOGW("CameraService not published, waiting...");
            usleep(kCameraServicePollDelay);
        } while(true);
        if (gDeathNotifier == NULL) {
            gDeathNotifier = new DeathNotifier();
        }
        binder->linkToDeath(gDeathNotifier);
        gCameraService = interface_cast<ICameraService>(binder);
    }
    ALOGE_IF(gCameraService == 0, "no CameraService!?");
    return gCameraService;
}

此处终于获得CameraService实例了，该CameraService实例是通过binder获取的。

再来看fnConnectService是什么，

在Camera.cpp中，有

CameraTraits<Camera>::TCamConnectService CameraTraits<Camera>::fnConnectService =
        &ICameraService::connect;

这样也就是说fnConnectService就是使用CameraService调用connect，从这里开始，终于从进入了服务端的流程：

status_t CameraService::connect(
        const sp<ICameraClient>& cameraClient,
        int cameraId,
        const String16& clientPackageName,
        int clientUid,
        /*out*/
        sp<ICamera>& device) { 

.........................
    status_t status = validateConnect(cameraId, /*inout*/clientUid);
..................................

if (!canConnectUnsafe(cameraId, clientPackageName,
                              cameraClient->asBinder(),
                              /*out*/clientTmp)) {
            return -EBUSY;
        } 

        status = connectHelperLocked(/*out*/client,
                                     cameraClient,
                                     cameraId,
                                     clientPackageName,
                                     clientUid,
                                     callingPid);

  device = client;  

return OK;
}

status_t CameraService::connectHelperLocked(
        /*out*/
        sp<Client>& client,
        /*in*/
        const sp<ICameraClient>& cameraClient,
        int cameraId,
        const String16& clientPackageName,
        int clientUid,
        int callingPid,
        int halVersion,
        bool legacyMode) {

....................................
            client = new CameraClient(this, cameraClient,
                    clientPackageName, cameraId,
                    facing, callingPid, clientUid, getpid(), legacyMode);
....................................       
    status_t status = connectFinishUnsafe(client, client->getRemote());
............................................

}

在connectHelpLocked，CameraService返回一个其实是它内部类的client——CameraClient。（注意此client是CameraService内部的CameraClient，不是Camera客户端），此client还赋值给了device，而这个device，就是在客户端CameraBase.cpp中c->mCamera,

        status = (cs.get()->*fnConnectService)(cl, cameraId, clientPackageName, clientUid,
                                             /*out*/ c->mCamera);

而这个c->mCamera就是每一次客户端Camera.cpp中调用一些函数（如preview/takepicture）时的mCamera，这样每一次客户端调用preview/takepicture，就直接调用的是CameraClient中的相关函数。这样就真正建立了客户端和服务端的关系。如Camera.cpp中的startPreview

// start preview mode
status_t Camera::startPreview()
{
    ALOGV("startPreview");
    sp <ICamera> c = mCamera;
    if (c == 0) return NO_INIT;
    return c->startPreview();
}

其实直接就会调用到CameraService中CameraClient.cpp中

// start preview mode
status_t CameraClient::startPreview() {
    LOG1("startPreview (pid %d)", getCallingPid());
    return startCameraMode(CAMERA_PREVIEW_MODE);
}

2.service端（其实就是CameraClient）的初始化和运行过程。

刚才我们知道，camera客户端与服务端的调用关系，就是camera.cpp中的函数，对应到CameraClient中的函数。下面我们再看看CameraClient的初始化过程。

在connectFinishUnsafe中，

    status_t status = client->initialize(mModule);

我们再来看CameraClient类的initialize函数

status_t CameraClient::initialize(camera_module_t *module) {
    int callingPid = getCallingPid();
    status_t res;

    mHardware = new CameraHardwareInterface(camera_device_name);
    res = mHardware->initialize(&module->common);

}

CameraClient的初始化就是：先实例化Camera Hal接口 CameraHardwareInterface，CameraHardwareInterface调用initialize()进入HAL层打开Camera底层驱动

    status_t initialize(hw_module_t *module)
    {
        ALOGI("Opening camera %s", mName.string());
        camera_module_t *cameraModule = reinterpret_cast<camera_module_t *>(module);
        camera_info info;
        status_t res = cameraModule->get_camera_info(atoi(mName.string()), &info);
        if (res != OK) return res;

        int rc = OK;
        if (module->module_api_version >= CAMERA_MODULE_API_VERSION_2_3 &&
            info.device_version > CAMERA_DEVICE_API_VERSION_1_0) {
            // Open higher version camera device as HAL1.0 device.
            rc = cameraModule->open_legacy(module, mName.string(),
                                               CAMERA_DEVICE_API_VERSION_1_0,
                                               (hw_device_t **)&mDevice);
        } else {
            rc = CameraService::filterOpenErrorCode(module->methods->open(
                module, mName.string(), (hw_device_t **)&mDevice));
        }
        if (rc != OK) {
            ALOGE("Could not open camera %s: %d", mName.string(), rc);
            return rc;
        }
        initHalPreviewWindow();
        return rc;
    }

hardware->initialize(&mModule->common)中mModule模块是一个结构体camera_module_t，他是怎么初始化的呢？我们发现CameraService里面有个函数

void CameraService::onFirstRef()
{
    BnCameraService::onFirstRef();

    if (hw_get_module(CAMERA_HARDWARE_MODULE_ID,
                (const hw_module_t **)&mModule) < 0) {
        LOGE("Could not load camera HAL module");
        mNumberOfCameras = 0;
    }
 
}

了解HAL层的都知道hw_get_module函数就是用来获取模块的Hal stub，这里通过CAMERA_HARDWARE_MODULE_ID 获取Camera Hal层的代理stub，并赋值给mModule，后面就可通过操作mModule完成对Camera模块的控制。那么onFirstRef()函数又是何时调用的？

onFirstRef（）属于其父类RefBase，该函数在强引用sp新增引用计数时调用。就是当 有sp包装的类初始化的时候调用，那么camera是何时调用的呢？可以发现在

客户端发起连接时候

sp Camera::connect(int cameraId)
{
    LOGV("connect");
    sp c = new Camera();
    const sp& cs = getCameraService();

}

    这个时候初始化了一个CameraService实例，且用Sp包装，这个时候sp将新增计数，相应的CameraService实例里面onFirstRef（）函数完成调用。
CameraService::connect（）即实例化CameraClient并打开驱动，返回CameraClient的时候，就表明客户端和服务端连接建立。Camera完成初始化。

总结：Camera的初始化流程简单的说就是：

->先是系统注册CameraService的服务

->AP层调用Camera.open()

->Camera.java调用JNI native_setup()

->JNI层调用 android_hardware_Camera_native_setup

-> HAL 客户端(Camera.cpp)调用connect与服务端(CameraService.cpp)连接，并得到CameraService中的CameraClient的一个实例

->服务端CameraClient的初始化，实例化Camera Hal接口 CameraHardwareInterface

->CameraHardwareInterface 打开Camera驱动，初始化完毕

最终的结果就是客户端会得到一个服务端CameraService中的CameraClient的一个实例，客户端的每一个函数操作其实最终都是调用CameraClient的函数

参考文档：

Android 4.0 Camera架构分析之Camera初始化 http://blog.csdn.net/dnfchan/article/details/7594590

Android 5.1 Camera Framework源码