Android4.0.3 显示系统深入理解

 

1. 简介      

        网上已经有很多兄弟对Android的显示系统做了深入解剖，很是佩服。可最近小弟在研究Android4.0时发现出入比较大，也许是Android4.0的修改比较多吧！因为小弟没有看Android4.0以前的代码。

       面对这么复杂一个Android显示系统，如何入手呢? 根据以前的经验，不管它有多么复杂，其功能不就是以下三步曲吗？

　　1）显示系统的创建及初始化

       2）画图

       3）销毁

       哪我的分析就从显示系统的创建及初始化开始吧！由于小弟对Java没有什么研究兴趣，所有重点就分析Native部分。当然Native的入口就在android_view_Surface.cpp中，此文件主要包含以下两部分给Java层调用：

       1）gSurfaceSessionMethods： 操作SurfaceSession的方法

       2）gSurfaceMethods：操作Surface的方法

2. android_view_Surface.cpp

 

2.1 SurfaceSession操作方法

 

static JNINativeMethod gSurfaceSessionMethods[] = {  
    {"init",     "()V",  (void*)SurfaceSession_init }, //创建SurfaceComposerClient 
    {"destroy",  "()V",  (void*)SurfaceSession_destroy }, //直接销毁SurfaceComposerClient 
    {"kill",     "()V",  (void*)SurfaceSession_kill },//先clear,再销毁SurfaceComposerClient
};  

 2.1.1 SurfaceSession_init

        其功能如下：

        1）创建SurfaceComposerClient对象

        2）调用SurfaceComposerClient::onFirstRef方法

        现在已经进入到SurfaceComposerClient的地盘，根据其名字含义，它应该是一个进行Surface合成的客户端，通过它发命令给SurfaceFlinger来进行需要的操作。其初始化流程如下图所示：

2.1.2 SurfaceComposerClient.cpp中的宝贝

        为了方便后面的理解，先看看SurfaceComposerClient中有些什么宝贝来完成这个任务。在其中定义了如下几个类：

2.1.2.1 ComposerService(获取SurfaceFlinger服务)

        一看到名字为Service，应该是用于从SurfaceFlinger中获取Service以建立连接关系<它是一个单实例，一个进程有且只有一个实例对象>，然后供后面进行相关的操作。其构造函数代码如下：      

 

class ComposerService : public Singleton
{
    //实质为BpSurfaceComposer,通过它与SurfaceFlinger进行通信，
    //BnSurfaceComposer是SurfaceFlinger基类中的一个
    sp mComposerService;

    //实质为BpMemoryHeap,它在SurfaceFlinger中对应为管理一个4096字节的
    //一个MemoryHeapBase对象，在SurfaceFlinger::readyToRun中创建
    sp mServerCblkMemory;
    
    //为MemoryHeapBase管理的内存在用户空间的基地址，通过mmap而来，
    //具体见MemoryHeapBase::mapfd
    surface_flinger_cblk_t volatile* mServerCblk;
    ComposerService();
    friend class Singleton;
public:
    static sp getComposerService();
    static surface_flinger_cblk_t const volatile * getControlBlock();
};

ComposerService::ComposerService()
: Singleton() {
    const String16 name("SurfaceFlinger");
    //获取SurfaceFlinger服务，即BpSurfaceComposer对象
    while (getService(name, &mComposerService) != NO_ERROR) {
        usleep(250000);
    }
    //获取共享内存块
    mServerCblkMemory = mComposerService->getCblk();
    //获取共享内存块基地址
    mServerCblk = static_cast(
            mServerCblkMemory->getBase());
}

      由此可见，ComposerService主要是获取SurfaceFlinger服务、获取在SurfaceFlinger::readyToRun中创建的共享内存块及其基地址。在Client中，谁要想与SurfaceFlinger通信，需要通过接口getComposerService来获取此BpSurfaceComposer。

     此ComposerService是在调用ComposerService::getInstance时进行有且只有一个的实例化，因为前面讲过，它是一个单实例。

 

2.1.2.2 Composer

      它也是一个单实例，管理并发送每个layer的ComposerState。其定义如下：

 

struct ComposerState {
    sp client;
    layer_state_t state;
    status_t    write(Parcel& output) const;
    status_t    read(const Parcel& input);
};

class Composer : public Singleton
{
    friend class Singleton;

    mutable Mutex               mLock;
    //SurfaceComposerClient+SurfaceID与一个ComposerState一一对应
    SortedVector mStates;     
    int                         mOrientation;//整个屏幕的方向
    Composer() : Singleton(),
        mOrientation(ISurfaceComposer::eOrientationUnchanged) { }
    //通过BpSurfaceComposer把mStates发送给SurfaceFlinger处理
    void closeGlobalTransactionImpl();

    //根据client和id从mStates中获取对应原ComposerState,从而获取对应的layer_state_t
    layer_state_t* getLayerStateLocked(
            const sp& client, SurfaceID id);

public:
    //设置与client和id对应的layer_state_t中的位置信息,并保存在mStates中
    status_t setPosition(const sp& client, SurfaceID id,
            float x, float y);
    //设置与client和id对应的layer_state_t中的Size信息,并保存在mStates中
    status_t setSize(const sp& client, SurfaceID id,
            uint32_t w, uint32_t h);
    //设置与client和id对应的layer_state_t中的z-order信息,并保存在mStates中
    status_t setLayer(const sp& client, SurfaceID id,
            int32_t z);
    //设置与client和id对应的layer_state_t中的flags信息,并保存在mStates中
    status_t setFlags(const sp& client, SurfaceID id,
            uint32_t flags, uint32_t mask);
    //设置与client和id对应的layer_state_t中的透明区域信息,并保存在mStates中
    status_t setTransparentRegionHint(
            const sp& client, SurfaceID id,
            const Region& transparentRegion);
    //设置与client和id对应的layer_state_t中的alpha信息,并保存在mStates中
    status_t setAlpha(const sp& client, SurfaceID id,
            float alpha);
    //设置与client和id对应的layer_state_t中的矩阵信息,并保存在mStates中
    status_t setMatrix(const sp& client, SurfaceID id,
            float dsdx, float dtdx, float dsdy, float dtdy);
    //设置与client和id对应的layer_state_t中的位置信息,并保存在mStates中
    status_t setFreezeTint(
            const sp& client, SurfaceID id,
            uint32_t tint);
    //设置整个屏幕的方向
    status_t setOrientation(int orientation);
    //通过BpSurfaceComposer把mStates发送给SurfaceFlinger处理
    static void closeGlobalTransaction() {
        Composer::getInstance().closeGlobalTransactionImpl();
    }
}

      把上面的comments看完就明白了，Composer管理每个SurfaceComposerClient中的每一个Surface的状态，并记录在ComposerState的layer_state_t中，然后调用者可以调用其closeGlobalTransaction方法把这些mStates发送给SurfaceFlinger处理（处理函数为：SurfaceFlinger::setTransactionState）。

      谁来调用它的方法设置层的属性及发送mStates呢? -----答案是由SurfaceComposerClient来调用。

  2.1.2.3  SurfaceComposerClient

       前面介绍的两个类一个用于获取SurfaceFlinger服务；一个用于记录每个Layer的状态，且可按要求把这些CoposerState发送给SurfaceFlinger。这个类是不是来使用前面两个类提供的服务呢? --答案是肯定的。其定义及详细注释如下：

 

#define NUM_DISPLAY_MAX 4  //最多支持四个显示屏
struct display_cblk_t //每个显示屏的配置参数
{
    uint16_t    w;
    uint16_t    h;
    uint8_t     format;
    uint8_t     orientation;
    uint8_t     reserved[2];
    float       fps;
    float       density;
    float       xdpi;
    float       ydpi;
    uint32_t    pad[2];
};
//在SurfaceFlinger::readyToRun中创建的共享控制块
struct surface_flinger_cblk_t   // 4KB max,管理系统中所有的显示屏
{
    uint8_t         connected; //每一个bit表示一个显示屏
    uint8_t         reserved[3];
    uint32_t        pad[7];
    display_cblk_t  displays[NUM_DISPLAY_MAX];
};

class SurfaceComposerClient : public RefBase
{
    friend class Composer;
public:         
    //获取Composer实例，并保存在mComposer中
                SurfaceComposerClient();
    virtual     ~SurfaceComposerClient();

    //通过BpSurfaceComposerClient创建Surface，
    //同时通过ISurfaceComposerClient::surface_data_t返回SurfaceID.然后创建一个SurfaceControl
    //并把返回的BpSurface和当前的SurfaceComposerClient保存在SurfaceControl中,
    //然后返回此SurfaceControl
    sp createSurface(
            const String8& name,// name of the surface
            DisplayID display,  // Display to create this surface on
            uint32_t w,         // width in pixel
            uint32_t h,         // height in pixel
            PixelFormat format, // pixel-format desired
            uint32_t flags = 0  // usage flags
    );


    // Composer parameters <合成参数>
    //所有的合成参数必须在一个transaction中被修改，多个surface可在一个transaction中被更新，
    //所有的变化在关闭transaction时被一次性提交(通过调用closeGlobalTransaction来提交所有变化)。

    //什么都没有做
    static void openGlobalTransaction();
        
    //通过调用Composer::closeGlobalTransaction(),
    // 把Composer中记录的ComposerState(即mStates)发送给SurfaceFlinger
    static void closeGlobalTransaction();
    
    //什么都没做
    static status_t freezeDisplay(DisplayID dpy, uint32_t flags = 0);
        
    //什么都没做
    static status_t unfreezeDisplay(DisplayID dpy, uint32_t flags = 0);

    //把新的显示方向保存在Composer实例中
    static int setOrientation(DisplayID dpy, int orientation, uint32_t flags);

    //从surface_flinger_cblk_t.connected中获取显示屏个数
    static ssize_t getNumberOfDisplays();

    //获取显示屏的信息
    static status_t getDisplayInfo(DisplayID dpy, DisplayInfo* info);
    static ssize_t getDisplayWidth(DisplayID dpy);
    static ssize_t getDisplayHeight(DisplayID dpy);
    static ssize_t getDisplayOrientation(DisplayID dpy);
    
    //通过注册，当Binder异常退出时，可以获得通知
    status_t linkToComposerDeath(const sp& recipient,
            void* cookie = NULL, uint32_t flags = 0);

    //Start####: 以下函数都是把相应的修改状态记录在Composer的mStates中
    //调用Composer::setFlags来设置对应(client+id)的layer状态〈即ComposerState中的layer_state_t〉
    status_t    hide(SurfaceID id);
    status_t    show(SurfaceID id, int32_t layer = -1);
    status_t    freeze(SurfaceID id);
    status_t    unfreeze(SurfaceID id);
    status_t    setFlags(SurfaceID id, uint32_t flags, uint32_t mask);
    //调用Composer::setTransparentRegionHint
    status_t    setTransparentRegionHint(SurfaceID id, const Region& transparent);
    //调用Composer::setLayer
    status_t    setLayer(SurfaceID id, int32_t layer);
    //调用Composer::setAlpha
    status_t    setAlpha(SurfaceID id, float alpha=1.0f);
    //调用Composer::setFreezeTint
    status_t    setFreezeTint(SurfaceID id, uint32_t tint);
    //调用Composer::setMatrix
    status_t    setMatrix(SurfaceID id, float dsdx, float dtdx, float dsdy, float dtdy);
    //调用Composer::setPosition
    status_t    setPosition(SurfaceID id, float x, float y);
    //调用Composer::setSize
    status_t    setSize(SurfaceID id, uint32_t w, uint32_t h);
    //End####:
    status_t    destroySurface(SurfaceID sid);//通过BpSurfaceComposerClient销毁Surface

private:
    //通过BpSurfaceComposer从SurfaceFlinger获取BpSurfaceComposerClient,
    //并把它保存在mClient中
    virtual void onFirstRef();
    Composer& getComposer();

    mutable     Mutex                       mLock;
                status_t                    mStatus;
                //实质为BpSurfaceComposerClient,与SurfaceFlinger.cpp中的Client相对应 
                  sp  mClient; 
                //Composer实例
                  Composer&                   mComposer;
}

    其功能列表如下：

     1）获取BpSurfaceComposerClient（即mClient），在onFirstRef中实现

     2）通过BpSurfaceComposerClient（即mClient）创建和销毁Surface

     3）通过Composer来记录Surface和显示屏状态变化，及在Composer中通过BpSurfaceComposer把状态变化发给SurfaceFlinger处理

     至此，SurfaceComposerClient功能已经分析清楚。可是从这三个类中，我们已经看到三个Bp（BpSurfaceComposer，BpSurfaceComposerClient和BpSurface）及三个对应的接口。下面总结一下，每个接口的功能，在客户端由谁使用，在服务器端谁来实现。

 2.1.2.4 Surface相关接口总结

     

 

 2.2 Surface操作

其相关接口如下：

static JNINativeMethod gSurfaceMethods[] = {
    {"nativeClassInit",     "()V",  (void*)nativeClassInit },
    {"init",                "(Landroid/view/SurfaceSession;ILjava/lang/String;IIIII)V",  (void*)Surface_init },
    {"init",                "(Landroid/os/Parcel;)V",  (void*)Surface_initParcel },
    {"initFromSurfaceTexture", "(Landroid/graphics/SurfaceTexture;)V", (void*)Surface_initFromSurfaceTexture },
    {"getIdentity",         "()I",  (void*)Surface_getIdentity },
    {"destroy",             "()V",  (void*)Surface_destroy },
    {"release",             "()V",  (void*)Surface_release },
    {"copyFrom",            "(Landroid/view/Surface;)V",  (void*)Surface_copyFrom },
    {"isValid",             "()Z",  (void*)Surface_isValid },
    {"lockCanvasNative",    "(Landroid/graphics/Rect;)Landroid/graphics/Canvas;",  (void*)Surface_lockCanvas },
    {"unlockCanvasAndPost", "(Landroid/graphics/Canvas;)V", (void*)Surface_unlockCanvasAndPost },
    {"unlockCanvas",        "(Landroid/graphics/Canvas;)V", (void*)Surface_unlockCanvas },
    {"openTransaction",     "()V",  (void*)Surface_openTransaction },
    {"closeTransaction",    "()V",  (void*)Surface_closeTransaction },
    {"setOrientation",      "(III)V", (void*)Surface_setOrientation },
    {"freezeDisplay",       "(I)V", (void*)Surface_freezeDisplay },
    {"unfreezeDisplay",     "(I)V", (void*)Surface_unfreezeDisplay },
    {"screenshot",          "(II)Landroid/graphics/Bitmap;", (void*)Surface_screenshotAll },
    {"screenshot",          "(IIII)Landroid/graphics/Bitmap;", (void*)Surface_screenshot },
    {"setLayer",            "(I)V", (void*)Surface_setLayer },
    {"setPosition",         "(FF)V",(void*)Surface_setPosition },
    {"setSize",             "(II)V",(void*)Surface_setSize },
    {"hide",                "()V",  (void*)Surface_hide },
    {"show",                "()V",  (void*)Surface_show },
    {"freeze",              "()V",  (void*)Surface_freeze },
    {"unfreeze",            "()V",  (void*)Surface_unfreeze },
    {"setFlags",            "(II)V",(void*)Surface_setFlags },
    {"setTransparentRegionHint","(Landroid/graphics/Region;)V", (void*)Surface_setTransparentRegion },
    {"setAlpha",            "(F)V", (void*)Surface_setAlpha },
    {"setMatrix",           "(FFFF)V",  (void*)Surface_setMatrix },
    {"setFreezeTint",       "(I)V",  (void*)Surface_setFreezeTint },
    {"readFromParcel",      "(Landroid/os/Parcel;)V", (void*)Surface_readFromParcel },
    {"writeToParcel",       "(Landroid/os/Parcel;I)V", (void*)Surface_writeToParcel },
};

2.2.1 Surface_init调用流程

     在SurfaceFlinger端创建BSurface,在客户端返回SurfaceControl,同时在SurfaceControl中拥有了BpSurface用于与BSurface交互。

 

2.2.1.1 调用流程分析

     BpSurfaceComposerClient->createSurface返回BpSurface。且通过参数返回ISurfaceComposerClient::surface_data_t，其定义如下：

     其中token在SurfaceComposerClient的函数参数中，对应于SurfaceID。即在客户端，它就是SurfaceID。

     token: 加入到Client::mLayers中的序号，在Client中单调递增,初始值为:1，一个Layer创建一个BSurface

     identity: LayerBaseClient中的mIdentity,在所有的Layer中单调递增,初始值为:1

    struct surface_data_t {
        int32_t             token;  //加入到Client::mLayers中的序号，在Client中单调递增,初始值为:1
        int32_t             identity; //LayerBaseClient中的mIdentity,在所有的Layer中单调递增,初始值为:1
        status_t readFromParcel(const Parcel& parcel);
        status_t writeToParcel(Parcel* parcel) const;
    };

2.2.1.2 创建真正的Surface

      在Layer::createSurface中创建真正的BSurface,在SurfaceFlinger::createSurface中调用layer->getSurface时创建的。此BSurface定义如下：

sp Layer::createSurface()
{
    class BSurface : public BnSurface, public LayerCleaner {
        wp mOwner;
        virtual sp getSurfaceTexture() const { //实现了ISurface的接口
            sp res;
            sp that( mOwner.promote() );
            if (that != NULL) {
                res = that->mSurfaceTexture;
            }
            return res;
        }
    public:
        BSurface(const sp& flinger,
                const sp& layer)
            : LayerCleaner(flinger, layer), mOwner(layer) { }
    };
    sp sur(new BSurface(mFlinger, this));
    return sur;
}

         在此BSurface中实现了ISurface的接口getSurfaceTexture，在此接口中返回Layer::mSurfaceTexture(类型为:SurfaceTextureLayer,它才是真正操作内存的东东),此成员在Layer::onFirstRef中创建，SurfaceTextureLayer是SurfaceTexture的派生类，代码如下：

void Layer::onFirstRef()
{
    LayerBaseClient::onFirstRef();

    struct FrameQueuedListener : public SurfaceTexture::FrameAvailableListener {
        FrameQueuedListener(Layer* layer) : mLayer(layer) { }
    private:
        wp mLayer;
        virtual void onFrameAvailable() {
            sp that(mLayer.promote());
            if (that != 0) {
                that->onFrameQueued();
            }
        }
    };
    mSurfaceTexture = new SurfaceTextureLayer(mTextureName, this); //创建Layer中的mSurfaceTexture
    mSurfaceTexture->setFrameAvailableListener(new FrameQueuedListener(this));
    mSurfaceTexture->setSynchronousMode(true);
    mSurfaceTexture->setBufferCountServer(2);
}

2.2.1.3 不得不说的SurfaceControl

       本来Surface_init调用SurfaceComposerClient::createSurface创建一个Surface,可却返回了一个SurfaceControl,下面看看SurfaceCotrol到底做了些什么，以及如何做的？

        相关数据结构如下图所示：

 

 SurfaceControl定义如下：

 

class SurfaceControl : public RefBase
{
public:
    // release surface data from java
    void        clear();
    
    //调用SurfaceComposerClient中对应方法，把对应信息保存在
    //Composer的ComposerState中    
    status_t    setLayer(int32_t layer);
    status_t    setPosition(int32_t x, int32_t y);
    status_t    setSize(uint32_t w, uint32_t h);
    status_t    hide();
    status_t    show(int32_t layer = -1);
    status_t    freeze();
    status_t    unfreeze();
    status_t    setFlags(uint32_t flags, uint32_t mask);
    status_t    setTransparentRegionHint(const Region& transparent);
    status_t    setAlpha(float alpha=1.0f);
    status_t    setMatrix(float dsdx, float dtdx, float dsdy, float dtdy);
    status_t    setFreezeTint(uint32_t tint);
    
    //把SurfaceControl中的mSurface和mIdentity写入parcel
    static status_t writeSurfaceToParcel(
            const sp& control, Parcel* parcel);
    
    //以SurfaceControl为参数创建一个Surface返回，此Surface派生关系如下：
    //class Surface : public SurfaceTextureClient
    //class SurfaceTextureClient: public ANativeWindow, RefBase
    //struct ANativeWindow
    sp getSurface() const;

private:
    SurfaceControl(
            const sp& client,
            const sp& surface,
            const ISurfaceComposerClient::surface_data_t& data);

    ~SurfaceControl();

    void destroy();
    
    sp   mClient;
    sp                mSurface;
    SurfaceID                   mToken;  //对应SurfaceID,在Client中单调递增
    uint32_t                    mIdentity;  //Layer在系统中唯一的序列号,在系统中单调递增
    mutable Mutex               mLock;
    
    mutable sp         mSurfaceData;
}

       从其定义中可以看出，在getSurface中将有新花样，其它操作函数都是直接以mToken作为SurfaceID,直接调用SurfaceComposerClient中对应方法。 经过这样一分析，SurfaceControl也没什么神秘的了。但它的getSurface到有点神秘。

 

 2.2.2 getSurface流程

      getSurface在客户端返回Surface(派生于SurfaceTextureClient)，并在Surface的mSurfaceTexture域中保存了BpSurfaceTexture。

      前面Surface初始化之后，就可以getSurface了。getSurface流程如下图所示：

           有了Surface，且在Surface中又有了BpSurfaceTexture，下一步就操作GraphicBuffer了。

 

 3. 画图流程

对于画图流程，可以从ViewRootImpl（ViewRootImpl.java）的draw函数看起，在画图之间，它要调用java层的surface.lockCanvas，画完图之后调用surface.unlockCanvasAndPost来提交显示。

surface.lockCanvas->

lockCanvasNative(Java)->

(C++)Surface_lockCanvas

surface.unlockCanvasAndPost(Java)->

(C++)Surface_unlockCanvasAndPost

本章主要分析这两个函数到底做了些什么>

 3.1 Surface_lockCanvas

       Android图形系统中一个重要的概念是surface。View及其子类（如TextView, Button）要画在surface上。每个surface创建一个Canvas对象（但属性时常改变），用来管理view在surface上的绘图操作，如画点画线。每个canvas对象对应一个bitmap，存储画在surface上的内容。

3.1.1 相关数据结构定义

 3.1.1.1 ANativeWindow_Buffer

typedef struct ANativeWindow_Buffer {
    // The number of pixels that are show horizontally.
    int32_t width;

    // The number of pixels that are shown vertically.
    int32_t height;

    // The number of *pixels* that a line in the buffer takes in
    // memory.  This may be >= width.
    int32_t stride;

    // The format of the buffer.  One of WINDOW_FORMAT_*
    int32_t format;

    // The actual bits.
    void* bits;  //显示内存基地址，通过服务器端fd通过flat_binder_object传给客户端， 然后客户端通过mmap获取。
    
    // Do not touch.
    uint32_t reserved[6];
} ANativeWindow_Buffer;

3.1.1.2 SurfaceInfo

    struct SurfaceInfo {
        uint32_t    w;
        uint32_t    h;
        uint32_t    s;
        uint32_t    usage;
        PixelFormat format;
        void*       bits;//显示内存基地址，通过服务器端fd通过flat_binder_object传给客户端， 然后客户端通过mmap获取。
        uint32_t    reserved[2];
    };

3.1.1.3 二者对应关系

SurfaceInfo* other;
ANativeWindow_Buffer outBuffer;
other->w = uint32_t(outBuffer.width);
other->h = uint32_t(outBuffer.height);
other->s = uint32_t(outBuffer.stride);
other->usage = GRALLOC_USAGE_SW_READ_OFTEN | GRALLOC_USAGE_SW_WRITE_OFTEN;
other->format = uint32_t(outBuffer.format);
other->bits = outBuffer.bits;

 3.1.1.4 GraphicBuffer

     在分析下面的流程时， 不得不对GraphicBuffer进行深入了解，特别是其Flattenable interface，这是实现画图buffer的关键。其相关定义如下：

typedef struct native_handle
{
    int version;        /* sizeof(native_handle_t) */
    int numFds;         /* number of file-descriptors at &data[0] */
    int numInts;        /* number of ints at &data[numFds] */
    int data[0];        /* numFds + numInts ints */
} native_handle_t;

typedef const native_handle_t* buffer_handle_t;

class GraphicBuffer
    : public EGLNativeBase<
        ANativeWindowBuffer,
        GraphicBuffer, 
        LightRefBase >, public Flattenable
{
    ...
    // Flattenable interface
    size_t getFlattenedSize() const;
    size_t getFdCount() const;
    status_t flatten(void* buffer, size_t size,
            int fds[], size_t count) const;
    status_t unflatten(void const* buffer, size_t size,
            int fds[], size_t count);
    ...
    buffer_handle_t handle; //定义于基类ANativeWindowBuffer中
};

3.1.1.5 Flattenable interface

下面看看每个Flattenable interface是如何实现的：

3.1.1.5.1 getFlattenedSize

size_t GraphicBuffer::getFlattenedSize() const {
    return (8 + (handle ? handle->numInts : 0))*sizeof(int);
}

3.1.1.5.2 getFdCount

size_t GraphicBuffer::getFdCount() const {
    return handle ? handle->numFds : 0;
}

3.1.1.5.3 flatten

status_t GraphicBuffer::flatten(void* buffer, size_t size,
        int fds[], size_t count) const
{
    size_t sizeNeeded = GraphicBuffer::getFlattenedSize();
    if (size < sizeNeeded) return NO_MEMORY;

    size_t fdCountNeeded = GraphicBuffer::getFdCount();
    if (count < fdCountNeeded) return NO_MEMORY;

    int* buf = static_cast(buffer);
    buf[0] = 'GBFR';
    buf[1] = width;
    buf[2] = height;
    buf[3] = stride;
    buf[4] = format;
    buf[5] = usage;
    buf[6] = 0;
    buf[7] = 0;

    if (handle) {
        buf[6] = handle->numFds;
        buf[7] = handle->numInts;
        native_handle_t const* const h = handle;
        memcpy(fds,     h->data,             h->numFds*sizeof(int));
        memcpy(&buf[8], h->data + h->numFds, h->numInts*sizeof(int));
    }

    return NO_ERROR;
}

     把handle中的numFds拷贝到fds中，把handle中的numInts拷贝到buffer中。

 3.1.1.5.4 unflatten 

status_t GraphicBuffer::unflatten(void const* buffer, size_t size,
        int fds[], size_t count)
{
    if (size < 8*sizeof(int)) return NO_MEMORY;

    int const* buf = static_cast(buffer);
    if (buf[0] != 'GBFR') return BAD_TYPE;

    const size_t numFds  = buf[6];
    const size_t numInts = buf[7];

    const size_t sizeNeeded = (8 + numInts) * sizeof(int);
    if (size < sizeNeeded) return NO_MEMORY;

    size_t fdCountNeeded = 0;
    if (count < fdCountNeeded) return NO_MEMORY;

    if (handle) {
        // free previous handle if any
        free_handle();
    }

    if (numFds || numInts) {
        width  = buf[1];
        height = buf[2];
        stride = buf[3];
        format = buf[4];
        usage  = buf[5];
        native_handle* h = native_handle_create(numFds, numInts);
        memcpy(h->data,          fds,     numFds*sizeof(int));
        memcpy(h->data + numFds, &buf[8], numInts*sizeof(int));
        handle = h;
    } else {
        width = height = stride = format = usage = 0;
        handle = NULL;
    }

    mOwner = ownHandle;

    if (handle != 0) {
        mBufferMapper.registerBuffer(handle);
    }

    return NO_ERROR;
}

       把width,height,stride,format和usage保存到成员变量中，并创建一个native_handle，然后把numFds和numInts拷贝到handle的data中。同时把此handle注册到mBufferMapper中，mBufferMapper的注册函数实现代码如下：

status_t GraphicBufferMapper::registerBuffer(buffer_handle_t handle)
{
    status_t err;
    //gralloc_module_t const *mAllocMod;是一个硬件抽象层实现。通过hw_get_module(GRALLOC_HARDWARE_MODULE_ID, &module)方式获取
    err = mAllocMod->registerBuffer(mAllocMod, handle);

    LOGW_IF(err, "registerBuffer(%p) failed %d (%s)",
            handle, err, strerror(-err));
    return err;
}

 

GraphicBufferMapper::GraphicBufferMapper()
    : mAllocMod(0)
{
    hw_module_t const* module;
    int err = hw_get_module(GRALLOC_HARDWARE_MODULE_ID, &module);
    LOGE_IF(err, "FATAL: can't find the %s module", GRALLOC_HARDWARE_MODULE_ID);
    if (err == 0) {
        mAllocMod = (gralloc_module_t const *)module;
    }
}

 3.1.1.5.4  GRALLOC_HARDWARE_MODULE_ID实例

      对于GRALLOC_HARDWARE_MODULE_ID，以hardware/msm7k/libgralloc/gralloc.cpp为例进行分析。其registerBuffer实现函数：gralloc_register_buffer（hardware/msm7k/libgralloc/mapper.cpp)，其相关代码如下：

int gralloc_register_buffer(gralloc_module_t const* module,
        buffer_handle_t handle)
{
    if (private_handle_t::validate(handle) < 0)
        return -EINVAL;

    // if this handle was created in this process, then we keep it as is.
    int err = 0;
    private_handle_t* hnd = (private_handle_t*)handle;
    if (hnd->pid != getpid()) {
        hnd->base = NULL;
        if (!(hnd->flags & private_handle_t::PRIV_FLAGS_USES_GPU)) {
            void *vaddr;
            err = gralloc_map(module, handle, &vaddr);
        }
    }
    return err;
}

static int gralloc_map(gralloc_module_t const* module,
        buffer_handle_t handle,
        void** vaddr)
{
    private_handle_t* hnd = (private_handle_t*)handle;
    if (!(hnd->flags & private_handle_t::PRIV_FLAGS_FRAMEBUFFER)) {
        size_t size = hnd->size;
#if PMEM_HACK
        size += hnd->offset;
#endif
        void* mappedAddress = mmap(0, size,
                PROT_READ|PROT_WRITE, MAP_SHARED, hnd->fd, 0);
        if (mappedAddress == MAP_FAILED) {
            LOGE("Could not mmap handle %p, fd=%d (%s)",
                    handle, hnd->fd, strerror(errno));
            hnd->base = 0;
            return -errno;
        }
        hnd->base = intptr_t(mappedAddress) + hnd->offset;
        //LOGD("gralloc_map() succeeded fd=%d, off=%d, size=%d, vaddr=%p", 
        //        hnd->fd, hnd->offset, hnd->size, mappedAddress);
    }
    *vaddr = (void*)hnd->base;
    return 0;
}

    从gralloc_map可以看出，这个registerBuffer主要做了一件事：

    1）根据handle中传过来的fd和size进行mmap映射（把kernel中的内存映射到用户空间），映射之后的地址再加上hnd->offset便获得hnd->base供后面使用。

      从这里可以初步看出，这个图形buffer数据并不是真正的从client传递到server，而是在lock是从server把fd传递给client，由客户端进行mmap，然后进行使用。关于这个是怎么实现的，后面将详细分析其实现过程。

    对于如何从native_handle转换为private_handle_t，且在private_handle_t中可以获取fd和offset? 看一下其数据结构和flatten的实现方式就可以得知：
native_handle：

typedef struct native_handle
{
    int version;        /* sizeof(native_handle_t) */
    int numFds;         /* number of file-descriptors at &data[0] */
    int numInts;        /* number of ints at &data[numFds] */
    int data[0];        /* numFds + numInts ints */
} native_handle_t;

     这个data[0]是关键，虽然分配了哪么多buffer,但实质上native_handle只占了3个int.其它的数据由包含它的数据结构来解析。

private_handle_t：

struct private_handle_t {
    native_handle_t nativeHandle;
#endif
    
    enum {
        PRIV_FLAGS_FRAMEBUFFER = 0x00000001,
        PRIV_FLAGS_USES_PMEM   = 0x00000002,
        PRIV_FLAGS_USES_GPU    = 0x00000004,
    };

    // file-descriptors
    int     fd;
    // ints
    int     magic;
    int     flags;
    int     size;
    int     offset;
    int     gpu_fd; // stored as an int, b/c we don't want it marshalled

    // FIXME: the attributes below should be out-of-line
    int     base;
    int     map_offset;
    int     pid;

#ifdef __cplusplus
    static const int sNumInts = 8;  //numInts在这儿明确指定
    static const int sNumFds = 1;   //numFds在这儿明确指定
    static const int sMagic = 'gmsm';

    private_handle_t(int fd, int size, int flags) :
        fd(fd), magic(sMagic), flags(flags), size(size), offset(0),
        base(0), pid(getpid())
    {
        version = sizeof(native_handle);
        numInts = sNumInts;
        numFds = sNumFds;
    }
    ~private_handle_t() {
        magic = 0;
    }

    static int validate(const native_handle* h) {
        const private_handle_t* hnd = (const private_handle_t*)h;
        if (!h || h->version != sizeof(native_handle) ||
                h->numInts != sNumInts || h->numFds != sNumFds ||
                hnd->magic != sMagic) 
        {
            LOGE("invalid gralloc handle (at %p)", h);
            return -EINVAL;
        }
        return 0;
    }
#endif
}

3.1.2 Surface_lockCanvas执行流程

查看高清大图

3.1.3 Surface_lockCanvas总结 

功能：Surface_lockCanvas获取显示buffer在本进程用户空间的地址，并据此创建一个SkBitmap给Java使用。
关键技术：BINDER_TYPE_FD类型的Binder、mmap、gralloc硬件抽象层

 

3.1.4 SurfaceTexture::dequeueBuffer如何创建GraphicBuffer

       相关代码如下：

    const sp& buffer(mSlots[buf].mGraphicBuffer);
    if ((buffer == NULL) ||
        (uint32_t(buffer->width)  != w) ||
        (uint32_t(buffer->height) != h) ||
        (uint32_t(buffer->format) != format) ||
        ((uint32_t(buffer->usage) & usage) != usage))
    {
        usage |= GraphicBuffer::USAGE_HW_TEXTURE;
        status_t error;
        sp graphicBuffer( //创建GraphicBuffer
                mGraphicBufferAlloc->createGraphicBuffer(
                        w, h, format, usage, &error));
        if (graphicBuffer == 0) {
            ST_LOGE("dequeueBuffer: SurfaceComposer::createGraphicBuffer "
                    "failed");
            return error;
        }
        if (updateFormat) {
            mPixelFormat = format;
        }
        mSlots[buf].mGraphicBuffer = graphicBuffer;
        mSlots[buf].mRequestBufferCalled = false;
        if (mSlots[buf].mEglImage != EGL_NO_IMAGE_KHR) {
            eglDestroyImageKHR(mSlots[buf].mEglDisplay, mSlots[buf].mEglImage);
            mSlots[buf].mEglImage = EGL_NO_IMAGE_KHR;
            mSlots[buf].mEglDisplay = EGL_NO_DISPLAY;
        }
        returnFlags |= ISurfaceTexture::BUFFER_NEEDS_REALLOCATION;
    }

     mGraphicBufferAlloc也是通过调用BpSurfaceComposer->createGraphicBufferAlloc而获取，它对应的服务器为SufaceFlinger中的GraphicBufferAlloc。

     mGraphicBufferAlloc实质为一个BpGraphicBufferAlloc,它真正创建GraphicBuffer的代码位于GraphicBufferAlloc::createGraphicBuffer中。代码关键调用流程如下：

     new GraphicBuffer(w, h, format, usage)->

       initSize(w, h, reqFormat, reqUsage)->

         GraphicBufferAllocator::get()->

            allocator.alloc(w, h, format, reqUsage, &handle, &stride)->

            返回handle,此handle为ANativeWindowBuffer成员，类型为native_handle。

              GraphicBufferAllocator::alloc->

                mAllocDev->alloc->

                 mAllocDev类型为alloc_device_t，它通过gralloc_open向

                 GRALLOC_HARDWARE_MODULE_ID获取，根据上面的实例msm7k，

                 它最终执行gralloc_device_open而获取gralloc_context_t.device.common，

                  alloc的实现函数为gralloc_alloc.

                   gralloc_alloc->

                    gralloc_alloc_buffer->

                     1）获取GPU内存(调用SimpleBestFitAllocator::allocate进行分配)

                     2）fd = open("/dev/null", O_RDONLY)获取fd

                     3）根据fd、size和flags创建private_handle_t，其相关代码如下：

 

        private_handle_t* hnd = new private_handle_t(fd, size, flags);
        if (base == NULL) {...
            }
        } else {
            private_module_t* m = reinterpret_cast(
                    dev->common.module);
            hnd->offset = offset;
            hnd->base = int(base)+offset;
            hnd->gpu_fd = gpu_fd;
            hnd->map_offset = m->fb_map_offset;
            *pHandle = hnd;
        }

3.2 Surface_unlockCanvasAndPost