Android4.4深入浅出之SurfaceFlinger与Client通信框架(一)


               SurfaceFlinger框架是基于Binder进程间通信机制搭建的,SF作为一个服务进程,用户程序想要跟它通信必然要经过Binder机制。首先说一下,用户要跟SF通信,那么SF必须出现在ServiceManager中,因为SF也是一个服务,所有的服务都由ServiceManager来进行统一管理。在系统启动的过程中,SF就在ServiceManager中注册好了,注册好之后,SF在后台中监视一些surface的变化从而做出处理。

             而启动之后,用户程序想操作一些跟surface有关的动作,就必须和SF进行交互。而这种交互是基于Binder进程间通信机制的。下面是一张图简单说明了SF的基本框架:

Android4.4深入浅出之SurfaceFlinger与Client通信框架(一)_第1张图片
大体结构是这样,有些地方有些小错误用图形不能很好的表达。用文字来说明一下:
当应用程序需要请求SurfaceFlinger服务时,首先需要构造SurfaceComposerClient对象,通过SurfaceComposerClient对象就可以访问SurfaceFlinger服务了。基本的服务流程都是这么走的,我们看一下SurfaceComposerClient的构造函数:
 
SurfaceComposerClient::SurfaceComposerClient()
    : mStatus(NO_INIT), mComposer(Composer::getInstance())
{
}
可想而知没做啥事啊,SurfaceComposerClient 继承于RefBase类,在创建SurfaceComposerClient对象,系统第一次引用SurfaceComposerClient对象时,onFirstRef函数自动调用,我们看一下它的onFirstRef
 
void SurfaceComposerClient::onFirstRef() {
    sp sm(ComposerService::getComposerService());
    if (sm != 0) {
        sp conn = sm->createConnection();
        if (conn != 0) {
            mClient = conn;
            mStatus = NO_ERROR;
        }
    }
}

上面代码,第二行的作用是获得SurfaceFlinger这个服务,我们跟踪一下:
 
sp ComposerService::getComposerService() {
    ComposerService& instance = ComposerService::getInstance();
    Mutex::Autolock _l(instance.mLock);
    if (instance.mComposerService == NULL) {
        ComposerService::getInstance().connectLocked();
        assert(instance.mComposerService != NULL);
        ALOGD("ComposerService reconnected");
    }
    return instance.mComposerService;
}
第二行得到了ComposerService的对象,是为了下一步访问它的成员变量mComposerService(ISurfaceComposer类的),跟踪到第五行的函数 connectLocked():
 
void ComposerService::connectLocked() {
    const String16 name("SurfaceFlinger");
    while (getService(name, &mComposerService) != NO_ERROR) {
        usleep(250000);
    }
    assert(mComposerService != NULL);                                                                                                         //省略了一些东西。。。
 
           }

这里看到第三行getService(),第一个参数是“SurfaceFlinger”,第二个参数是mComposerService,记住它的类型的强引用的ISurfaceComposer,对就是这句得到了一个SurfaceFlinger服务的代理对象,看一下这个的函数的实体:
template
status_t getService(const String16& name, sp* outService)
{
    const sp sm = defaultServiceManager();
    if (sm != NULL) {
        *outService = interface_cast(sm->getService(name));
        if ((*outService) != NULL) return NO_ERROR;
    }
    return NAME_NOT_FOUND;
}
到第四行这里跟之前启动过程中SurfaceFlinger的创建一模一样了,最后outService会被强转化成BpSurfaceComposer类,BpSurfaceComposer就是SurfaceFlinger在客户端这边的Binder代理,与之对应的是BnSurfaceComposer相当于SurfaceFlinger这边与BpSurfaceComposer进行交互的一方。获得了代理之后,我们再回到之前的onFirstRef函数:
void SurfaceComposerClient::onFirstRef() {
    sp sm(ComposerService::getComposerService());
    if (sm != 0) {
        sp conn = sm->createConnection();
        if (conn != 0) {
            mClient = conn;
            mStatus = NO_ERROR;
        }
    }
}

这时候走到if,sm有值了,所以往下走,看名字可以得出这是客户端在请求连接到SurfaceFlinger服务。我们跟踪一下createConnection:
 
virtual sp createConnection()
    {
        uint32_t n;
        Parcel data, reply;
        data.writeInterfaceToken(ISurfaceComposer::getInterfaceDescriptor());
        remote()->transact(BnSurfaceComposer::CREATE_CONNECTION, data, &reply);
        return interface_cast(reply.readStrongBinder());
    }

函数的大概意思是把一些连接信息写到数据包data里,然后通过transact这个函数传出去,remote()之前有研究过他是IBinder类的,这里重点看transact这个函数,他是IBinder类的成员函数故它的实现必定在BpBinder类里,我们跳到这个函数的实体:
 
status_t BpBinder::transact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    // Once a binder has died, it will never come back to life.
    if (mAlive) {
        status_t status = IPCThreadState::self()->transact(
            mHandle, code, data, reply, flags);
        if (status == DEAD_OBJECT) mAlive = 0;
        return status;
    }

    return DEAD_OBJECT;
}

发现他又调用了transact这个函数,不同的是 是在IPCThreadState类中的 我们继续跟踪:
status_t IPCThreadState::transact(int32_t handle,
                                  uint32_t code, const Parcel& data,
                                  Parcel* reply, uint32_t flags)
{
    status_t err = data.errorCheck();

    flags |= TF_ACCEPT_FDS;

    IF_LOG_TRANSACTIONS() {
        TextOutput::Bundle _b(alog);
        alog << "BC_TRANSACTION thr " << (void*)pthread_self() << " / hand "
            << handle << " / code " << TypeCode(code) << ": "
            << indent << data << dedent << endl;
    }
    
    if (err == NO_ERROR) {
        LOG_ONEWAY(">>>> SEND from pid %d uid %d %s", getpid(), getuid(),
            (flags & TF_ONE_WAY) == 0 ? "READ REPLY" : "ONE WAY");
        err = writeTransactionData(BC_TRANSACTION, flags, handle, code, data, NULL);
    }
    
    if (err != NO_ERROR) {
        if (reply) reply->setError(err);
        return (mLastError = err);
    }
    
    if ((flags & TF_ONE_WAY) == 0) {
        #if 0
        if (code == 4) { // relayout
            ALOGI(">>>>>> CALLING transaction 4");
        } else {
            ALOGI(">>>>>> CALLING transaction %d", code);
        }
        #endif
        if (reply) {
            err = waitForResponse(reply);
        } else {
            Parcel fakeReply;
            err = waitForResponse(&fakeReply);
        }
        #if 0
        if (code == 4) { // relayout
            ALOGI("<<<<<< RETURNING transaction 4");
        } else {
            ALOGI("<<<<<< RETURNING transaction %d", code);
        }
        #endif
        
        IF_LOG_TRANSACTIONS() {
            TextOutput::Bundle _b(alog);
            alog << "BR_REPLY thr " << (void*)pthread_self() << " / hand "
                << handle << ": ";
            if (reply) alog << indent << *reply << dedent << endl;
            else alog << "(none requested)" << endl;
        }
    } else {
        err = waitForResponse(NULL, NULL);
    }
    
    return err;
}

这里已经扯到Binder机制的基本原理了,简单说就是客户端的SF代理通过BpBinder跟/dev/binder设备进行交互,向binder里写东西,函数err = writeTransactionData(BC_TRANSACTION, flags, handle, code, data, NULL);就是实现了这个功能,写完后他又等待服务端(SurfaceFlinger)那端的回应,err = waitForResponse(reply);当然服务端那边同样有人跟他进行交互。到这里,如果服务端收到消息并且返回了一个消息给客户端,这说明客户端请求连接成功。

     请求成功后conn就不为0了,回到onFirstRef,成功后把coon赋给mClient。

既然客户端有请求了,那么服务端肯定有东西会去留意这个消息,回到更以前,做出向服务端请求连接这件事其实是SurfaceFlinger在客户端这边的代理BpSurfaceComposer完成的,那么之前说过,与之对应的就是BnSurfaceComposer。我们看一下他类的定义:

 

class BnSurfaceComposer: public BnInterface {
public:
    enum {
        // Note: BOOT_FINISHED must remain this value, it is called from
        // Java by ActivityManagerService.
        BOOT_FINISHED = IBinder::FIRST_CALL_TRANSACTION,
        CREATE_CONNECTION,
        CREATE_GRAPHIC_BUFFER_ALLOC,
        CREATE_DISPLAY_EVENT_CONNECTION,
        CREATE_DISPLAY,
        DESTROY_DISPLAY,
        GET_BUILT_IN_DISPLAY,
        SET_TRANSACTION_STATE,
        AUTHENTICATE_SURFACE,
        BLANK,
        UNBLANK,
        GET_DISPLAY_INFO,
        CONNECT_DISPLAY,
        CAPTURE_SCREEN,
    };

    virtual status_t onTransact(uint32_t code, const Parcel& data,
            Parcel* reply, uint32_t flags = 0);
 }

发现他没有构造函数只有一个成员函数 onTransact。跟踪一下:

status_t BnSurfaceComposer::onTransact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    switch(code) {
        case CREATE_CONNECTION: {
            CHECK_INTERFACE(ISurfaceComposer, data, reply);
            sp b = createConnection()->asBinder();
            reply->writeStrongBinder(b);
            return NO_ERROR;
        }
        case CREATE_GRAPHIC_BUFFER_ALLOC: {
            CHECK_INTERFACE(ISurfaceComposer, data, reply);
            sp b = createGraphicBufferAlloc()->asBinder();
            reply->writeStrongBinder(b);
            return NO_ERROR;
        }
        case SET_TRANSACTION_STATE: {
            CHECK_INTERFACE(ISurfaceComposer, data, reply);
            size_t count = data.readInt32();
            ComposerState s;
            Vector state;
            state.setCapacity(count);
            for (size_t i=0 ; i displays;
            displays.setCapacity(count);
            for (size_t i=0 ; i display = data.readStrongBinder();
            sp producer =
                    interface_cast(data.readStrongBinder());
            uint32_t reqWidth = data.readInt32();
            uint32_t reqHeight = data.readInt32();
            uint32_t minLayerZ = data.readInt32();
            uint32_t maxLayerZ = data.readInt32();
            status_t res = captureScreen(display, producer,
                    reqWidth, reqHeight, minLayerZ, maxLayerZ);
            reply->writeInt32(res);
            return NO_ERROR;
        }
        case AUTHENTICATE_SURFACE: {
            CHECK_INTERFACE(ISurfaceComposer, data, reply);
            sp bufferProducer =
                    interface_cast(data.readStrongBinder());
            int32_t result = authenticateSurfaceTexture(bufferProducer) ? 1 : 0;
            reply->writeInt32(result);
            return NO_ERROR;
        }
        case CREATE_DISPLAY_EVENT_CONNECTION: {
            CHECK_INTERFACE(ISurfaceComposer, data, reply);
            sp connection(createDisplayEventConnection());
            reply->writeStrongBinder(connection->asBinder());
            return NO_ERROR;
        }
        case CREATE_DISPLAY: {
            CHECK_INTERFACE(ISurfaceComposer, data, reply);
            String8 displayName = data.readString8();
            bool secure = bool(data.readInt32());
            sp display(createDisplay(displayName, secure));
            reply->writeStrongBinder(display);
            return NO_ERROR;
        }
        case DESTROY_DISPLAY: {
            CHECK_INTERFACE(ISurfaceComposer, data, reply);
            sp display = data.readStrongBinder();
            destroyDisplay(display);
            return NO_ERROR;
        }
        case GET_BUILT_IN_DISPLAY: {
            CHECK_INTERFACE(ISurfaceComposer, data, reply);
            int32_t id = data.readInt32();
            sp display(getBuiltInDisplay(id));
            reply->writeStrongBinder(display);
            return NO_ERROR;
        }
        case BLANK: {
            CHECK_INTERFACE(ISurfaceComposer, data, reply);
            sp display = data.readStrongBinder();
            blank(display);
            return NO_ERROR;
        }
        case UNBLANK: {
            CHECK_INTERFACE(ISurfaceComposer, data, reply);
            sp display = data.readStrongBinder();
            unblank(display);
            return NO_ERROR;
        }
        case GET_DISPLAY_INFO: {
            CHECK_INTERFACE(ISurfaceComposer, data, reply);
            DisplayInfo info;
            sp display = data.readStrongBinder();
            status_t result = getDisplayInfo(display, &info);
            memcpy(reply->writeInplace(sizeof(DisplayInfo)), &info, sizeof(DisplayInfo));
            reply->writeInt32(result);
            return NO_ERROR;
        }
        default: {
            return BBinder::onTransact(code, data, reply, flags);
        }
    }
    // should be unreachable
    return NO_ERROR;
}

代码虽然有点长,但是整体框架就是一个swith语句,看到第一个分支,马上就能看出来这是一个对请求连接信号的处理,这里大概能知道这个onTransact的功能就是判断收到的消息即参数code是哪种类型从而做出相应的处理。在server测,一开始创建SF服务的时候,在线城池创建的时候就已经开启线程去监听binder设备了我们来跟踪下:

 从最开始SF服务启动的源文件开始:

int main(int argc, char** argv) {
    // When SF is launched in its own process, limit the number of
    // binder threads to 4.
    ProcessState::self()->setThreadPoolMaxThreadCount(4);

    // start the thread pool
    sp ps(ProcessState::self());
    ps->startThreadPool();

是一个片段,最后一句就是开始线程池。跟踪其代码:

void ProcessState::startThreadPool()
{
    AutoMutex _l(mLock);
    if (!mThreadPoolStarted) {
        mThreadPoolStarted = true;
        spawnPooledThread(true);
    }
}

核心也在最后一句,

void ProcessState::spawnPooledThread(bool isMain)
{
    if (mThreadPoolStarted) {
        String8 name = makeBinderThreadName();
        ALOGV("Spawning new pooled thread, name=%s\n", name.string());
        sp t = new PoolThread(isMain);
        t->run(name.string());
    }
}


到这里,可以看到实质的线程创建和开始运行。接下来我用一张图来表示他的函数调用的走向:

Android4.4深入浅出之SurfaceFlinger与Client通信框架(一)_第2张图片

这里一旦检测到有连接请求消息就会跳到executeCommand的switch分支BR_TRANSACTION,去执行BBinder的transact,transact函数会调用onTransact函数,这个onTransact由其子类实现,在这里由BnSurfaceComposer完成,即调用的是BnSurfaceComposer::onTransact(),函数的具体内容在上面已贴出,就是处理不同的请求。

     到此为止,客户端已经与SF进行连接,也就是交手过了,那么自然连接了,一定要做些事情比如客户端请求渲染一个surface,或者等等其他的。接下去的工作就是SurfaceFlinger的事,对客户端不同的请求而进行不同的处理,这才是SF核心工作所在。当然所有的通信机制都是基于binder机制,而求也有负责这个方面的客户端这边的binder代理BpSurfaceComposerClient和服务端这边的本地对象 BnSurfaceComposerClient,而这里有点不一样的是BnSurfaceComposerClient派生出Client所以事情都交与Client做了。


 




 

 
 
      


 
 
 
 

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