【Android】Android(安卓)SurfaceFlinger之NativeWindow

1、EGLNativeWindowType

Android的GUI构建于OpenGL ES，对于2D图形来说还可以使用Skia库（https://skia.org/）。在OpenGL ES与底层的framebuffer之间，还有一层Native介质，如NativeWindow、NativeDisplay和NativePixmap，这里讨论NativeWindow。OpenGL ES是跨平台的，为了适配不同的运行环境，需要EGL将其Native化，如下所示的eglCreateWindowSurface。

EGLAPI EGLSurface EGLAPIENTRY eglCreateWindowSurface(EGLDisplay dpy, EGLConfig config,                  EGLNativeWindowType win,                  const EGLint *attrib_li

eglCreateWindowSurface中的EGLNativeWindowType就是个平台相关的类型，如下所示：

#if defined(_WIN32) || defined(__VC32__) && !defined(__CYGWIN__) && !defined(__SCITECH_SNAP__) /* Win32 and WinCE */#ifndef WIN32_LEAN_AND_MEAN#define WIN32_LEAN_AND_MEAN 1#endif#include typedef HDC     EGLNativeDisplayType;typedef HBITMAP EGLNativePixmapType;typedef HWND    EGLNativeWindowType;#elif defined(__WINSCW__) || defined(__SYMBIAN32__)  /* Symbian */typedef int   EGLNativeDisplayType;typedef void *EGLNativeWindowType;typedef void *EGLNativePixmapType;#elif defined(__ANDROID__) || defined(ANDROID)#include struct egl_native_pixmap_t;typedef struct ANativeWindow*           EGLNativeWindowType;typedef struct egl_native_pixmap_t*     EGLNativePixmapType;typedef void*                           EGLNativeDisplayType;#elif defined(__unix__)/* X11 (tentative)  */#include #include typedef Display *EGLNativeDisplayType;typedef Pixmap   EGLNativePixmapType;typedef Window   EGLNativeWindowType;#else#error "Platform not recognized"#endif/* EGL 1.2 types, renamed for consistency in EGL 1.3 */typedef EGLNativeDisplayType NativeDisplayType;typedef EGLNativePixmapType  NativePixmapType;typedef EGLNativeWindowType  NativeWindowType;

可以看出，在Android系统上，EGLNativeWindowType为指向ANativeWindow结构的指针，如下所示：

struct ANativeWindow{#ifdef __cplusplus    ANativeWindow()        : flags(0), minSwapInterval(0), maxSwapInterval(0), xdpi(0), ydpi(0)    {        common.magic = ANDROID_NATIVE_WINDOW_MAGIC;        common.version = sizeof(ANativeWindow);        memset(common.reserved, 0, sizeof(common.reserved));    }    void incStrong(const void* id) const {        common.incRef(const_cast(&common));    }    void decStrong(const void* id) const {        common.decRef(const_cast(&common));    }#endif    struct android_native_base_t common;    uint32_t flags;    int   minSwapInterval;    int   maxSwapInterval;    float xdpi;    float ydpi;    intptr_t    oem[4];    int     (*setSwapInterval)(struct ANativeWindow* window,                int interval);    int     (*dequeueBuffer)(struct ANativeWindow* window,                struct ANativeWindowBuffer** buffer);    int     (*lockBuffer)(struct ANativeWindow* window,                struct ANativeWindowBuffer* buffer);    int     (*queueBuffer)(struct ANativeWindow* window,                struct ANativeWindowBuffer* buffer);    int     (*query)(const struct ANativeWindow* window,                int what, int* value);    int     (*perform)(struct ANativeWindow* window,                int operation, ... );    int     (*cancelBuffer)(struct ANativeWindow* window,                struct ANativeWindowBuffer* buffer);    void* reserved_proc[2];};

2、FramebufferNativeWindow

在老式的Android版本中有个专门的FramebufferNativeWindow用来实现ANativeWindow的功能，如下所示：

class FramebufferNativeWindow     : public EGLNativeBase<        ANativeWindow,         FramebufferNativeWindow,         LightRefBase<FramebufferNativeWindow> >

不过在最新的Android系统（Android 7）中，FramebufferNativeWindow已被移除，看到Android源码中有如下一条提交记录：

libui: Remove FramebufferNativeWindowWe no longer support the framebuffer device, so FramebufferNativeWindow is no longer relevant.

FramebufferNativeWindow是属于libui库的，虽被移除，但其实现逻辑值得借鉴。在FramebufferNativeWindow的构造函数中，首先打开gralloc模块，即dlopen相关的gralloc库，然后打开fb0和gpu0设备，再分配需要的buffer（用于framebuffer device），这个buffer即NativeBuffer的数据结构如下：

class NativeBuffer     : public EGLNativeBase<        ANativeWindowBuffer,         NativeBuffer,         LightRefBase >typedef struct ANativeWindowBuffer{#ifdef __cplusplus    ANativeWindowBuffer() {        common.magic = ANDROID_NATIVE_BUFFER_MAGIC;        common.version = sizeof(ANativeWindowBuffer);        memset(common.reserved, 0, sizeof(common.reserved));    }    void incStrong(const void* id) const {        common.incRef(const_cast(&common));    }    void decStrong(const void* id) const {        common.decRef(const_cast(&common));    }#endif    struct android_native_base_t common;    int width;    int height;    int stride;    int format;    int usage;    void* reserved[2];    buffer_handle_t handle;    void* reserved_proc[8];} ANativeWindowBuffer_t;

最后是一些属性和接口赋值操作，其中最重要的一点是多缓冲技术，至少2个buffer，可以理解为一个screen-buffer（用户直接看到的东西）和一个off-screen-buffer（即将呈现给用户的东西），这就需要所谓的swap buffer，因为在屏幕上绘制东西需要时间，而屏幕刷新有一定的帧率（fps），为了保证画面流畅，就要在off-screen-buffer准备好数据后通过swap buffer把数据发送到screen-buffer以显示。举一个例子，对于单个buffer来说，如果绘制一个画面需要10秒，而屏幕每隔1秒刷新一次，用户就会感觉很卡，前9秒看到的都是不同的且不完整的画面。在NativeWindow中有两个重要的接口dequeueBuffer和queueBuffer，EGL通过dequeueBuffer来申请一个buffer，当EGL对一块buffer渲染完成后，queueBuffer用来unlock和post buffer，其中用到了条件变量用于同步操作。

3、Surface

另一个本地窗口是Surface，针对应用程序，同样继承自ANativeWindow：

class Surface    : public ANativeObjectBase<ANativeWindow, Surface, RefBase>

Surface属于libgui库的东西，承担着应用进程中的UI显示需求，其分配的内存空间不属于帧缓冲区，系统中所有应用程序绘制的图像数据由SurfaceFlinger收集。Surface有两个重要的成员变量，mGraphicBufferProducer和mSlots，前者用来获取buffer，为buffer的生产者，后者则记录这些缓冲区，NUM_BUFFER_SLOTS为64。

sp<IGraphicBufferProducer> mGraphicBufferProducer;BufferSlot mSlots[NUM_BUFFER_SLOTS];struct BufferSlot {        sp<GraphicBuffer> buffer;        Region dirtyRegion;    };

在Surface的构造函数中同样是给其成员变量赋值，做一些初始化工作，比如说如下的dequeueBuffer：

ANativeWindow::dequeueBuffer    = hook_dequeueBuffer;int Surface::hook_dequeueBuffer(ANativeWindow* window,        ANativeWindowBuffer** buffer, int* fenceFd) {    Surface* c = getSelf(window);    return c->dequeueBuffer(buffer, fenceFd);}int Surface::dequeueBuffer(android_native_buffer_t** buffer, int* fenceFd) {    ATRACE_CALL();    ALOGV("Surface::dequeueBuffer");    uint32_t reqWidth;    uint32_t reqHeight;    PixelFormat reqFormat;    uint32_t reqUsage;    {        Mutex::Autolock lock(mMutex);        reqWidth = mReqWidth ? mReqWidth : mUserWidth;        reqHeight = mReqHeight ? mReqHeight : mUserHeight;        reqFormat = mReqFormat;        reqUsage = mReqUsage;        if (mSharedBufferMode && mAutoRefresh && mSharedBufferSlot !=                BufferItem::INVALID_BUFFER_SLOT) {            sp& gbuf(mSlots[mSharedBufferSlot].buffer);            if (gbuf != NULL) {                *buffer = gbuf.get();                *fenceFd = -1;                return OK;            }        }    } // Drop the lock so that we can still touch the Surface while blocking in IGBP::dequeueBuffer    int buf = -1;    sp fence;    nsecs_t now = systemTime();    status_t result = mGraphicBufferProducer->dequeueBuffer(&buf, &fence,            reqWidth, reqHeight, reqFormat, reqUsage);    mLastDequeueDuration = systemTime() - now;    if (result < 0) {        ALOGV("dequeueBuffer: IGraphicBufferProducer::dequeueBuffer"                "(%d, %d, %d, %d) failed: %d", reqWidth, reqHeight, reqFormat,                reqUsage, result);        return result;    }    Mutex::Autolock lock(mMutex);    sp& gbuf(mSlots[buf].buffer);    // this should never happen    ALOGE_IF(fence == NULL, "Surface::dequeueBuffer: received null Fence! buf=%d", buf);    if (result & IGraphicBufferProducer::RELEASE_ALL_BUFFERS) {        freeAllBuffers();    }    if ((result & IGraphicBufferProducer::BUFFER_NEEDS_REALLOCATION) || gbuf == 0) {        result = mGraphicBufferProducer->requestBuffer(buf, &gbuf);        if (result != NO_ERROR) {            ALOGE("dequeueBuffer: IGraphicBufferProducer::requestBuffer failed: %d", result);            mGraphicBufferProducer->cancelBuffer(buf, fence);            return result;        }    }    if (fence->isValid()) {        *fenceFd = fence->dup();        if (*fenceFd == -1) {            ALOGE("dequeueBuffer: error duping fence: %d", errno);            // dup() should never fail; something is badly wrong. Soldier on            // and hope for the best; the worst that should happen is some            // visible corruption that lasts until the next frame.        }    } else {        *fenceFd = -1;    }    *buffer = gbuf.get();    if (mSharedBufferMode && mAutoRefresh) {        mSharedBufferSlot = buf;        mSharedBufferHasBeenQueued = false;    } else if (mSharedBufferSlot == buf) {        mSharedBufferSlot = BufferItem::INVALID_BUFFER_SLOT;        mSharedBufferHasBeenQueued = false;    }    return OK;}

dequeueBuffer最终通过Surface的成员函数dequeueBuffer实现，关键操作由其成员变量mGraphicBufferProducer这个buffer生产者来完成，既然有生产者就有消费者，消费者为SurfaceFlinger（libsurfaceflinger中的东西）。Surface由SurfaceControl创建、管理，如下所示：

sp<Surface> SurfaceControl::getSurface() const{    Mutex::Autolock _l(mLock);    if (mSurfaceData == 0) {        // This surface is always consumed by SurfaceFlinger, so the        // producerControlledByApp value doesn't matter; using false.        mSurfaceData = new Surface(mGraphicBufferProducer, false);    }    return mSurfaceData;}

SurfaceControl由Surface合成的客户端SurfaceComposerClient创建，如下所示：

sp<SurfaceControl> SurfaceComposerClient::createSurface(        const String8& name,        uint32_t w,        uint32_t h,        PixelFormat format,        uint32_t flags){    sp<SurfaceControl> sur;    if (mStatus == NO_ERROR) {        sp<IBinder> handle;        sp<IGraphicBufferProducer> gbp;        status_t err = mClient->createSurface(name, w, h, format, flags,                &handle, &gbp);        ALOGE_IF(err, "SurfaceComposerClient::createSurface error %s", strerror(-err));        if (err == NO_ERROR) {            sur = new SurfaceControl(this, handle, gbp);        }    }    return sur;}

在SurfaceComposerClient的createSurface中，通过mClient（类型为sp）来createSurface，其中有两个关键变量handle（IBinder）和gbp（IGraphicBufferProducer），mClient在onFirstRef函数中获取与SurfaceFlinger（服务端）的连接，如下代码：

void SurfaceComposerClient::onFirstRef() {    sp<ISurfaceComposer> sm(ComposerService::getComposerService());    if (sm != 0) {        sp<ISurfaceComposerClient> conn = sm->createConnection();        if (conn != 0) {            mClient = conn;            mStatus = NO_ERROR;        }    }}

这样，一个普通的Surface就与SurfaceFlinger建立了联系。由此可见，ISurfaceComposerClient是由ISurfaceComposer::createConnection生成的，在这一过程中，总共涉及了三个匿名的Binder服务，分别为ISurfaceComposer、ISurfaceComposerClient和IGraphicBufferProducer，它们是紧紧相扣的，只能按上面的顺序访问。这些匿名Binder由实名Binder即在ServiceManager中注册过的SurfaceFlinger提供，从下面的函数中可以看出：

/*static*/ 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;}void ComposerService::connectLocked() {    const String16 name("SurfaceFlinger"); // 轮询查找SurfaceFlinger服务    while (getService(name, &mComposerService) != NO_ERROR) {        usleep(250000);    }    assert(mComposerService != NULL);    // Create the death listener.    class DeathObserver : public IBinder::DeathRecipient {        ComposerService& mComposerService;        virtual void binderDied(const wp& who) {            ALOGW("ComposerService remote (surfaceflinger) died [%p]",                  who.unsafe_get());            mComposerService.composerServiceDied();        }     public:        explicit DeathObserver(ComposerService& mgr) : mComposerService(mgr) { }    };    mDeathObserver = new DeathObserver(*const_cast(this));    IInterface::asBinder(mComposerService)->linkToDeath(mDeathObserver);}

前面介绍了Android的GUI系统中两个重要的本地窗口，FramebuferNativeWindow和Surface，前者是专门为SurfaceFlinger服务的，由Gralloc提供支持，后者为应用程序服务，但也由SurfaceFlinger统一管理。

1、EGLNativeWindowType

2、FramebufferNativeWindow

3、Surface

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