Android中*_handle_t/ANativeWindowBuffer/ANativeWindow/GraphicBuffer/Surface的关系

在阅读SurfaceFlinger HardwareComposer以及gralloc相关代码的过程中,我们经常会遇到native_handle private_handle_t ANativeWindowBuffer ANativeWindow GraphicBuffer Surface等等一系列和memory相关的struct和class,他们相互之间到底是什么区别,又有什么联系呢?本文从struct/class的结构角度分析下上述类型之间的关联.

    概括来说,native_handle private_handle_t ANativeWindowBuffer GraphicBuffer这四个struct/class所描述的是一块memory,而ANativeWindow 和Surface所描述的是一系列上述memeory的组合和对buffer的操作方法.有的struct/class在比较低的level使用,和平台相关,而另外一些在比较高的level使用,和平台无关,还有一些介于低/高level之间,用以消除平台相关性,让Android可以方便的运行在不同的平台上.

    我们依次来看下上述struct/class的定义:

1. native_handle 

system/core/include/cutils/native_handle.h

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;

    native_handle/native_handle_t只是定义了一个描述buffer的结构体原型,这个原型是和平台无关的,方便buffer在各个进程之间传递,注意成员data是一个大小为0的数组,这意味着data指向紧挨着numInts后面的一个地址.我们可以把native_handle_t看成是一个纯虚的基类.

    一般来说,我们描述一块buffer,需要知道它在kernel中对应的fd,虚拟地址/物理地址,offset,size等等信息,后面我们在private_handle_t中就可以看到这些字段.
    android的gralloc模块负责从fb设备或者gpu中分配meomory,所以我们在gralloc中就可以找到native_handle的具体实现,gralloc中对buffer的描述就和具体的平台相关了,我们以aosp中最基本的gralloc为例,来看下gralloc中对native_handle是如何使用的.

2.private_handle_t

hardware/libhardware/modules/gralloc/gralloc_priv.h

#ifdef __cplusplus
//在c++编译环境下private_handle_t继承于native_handle
struct private_handle_t : public native_handle {
#else
//在c编译环境下,private_handle_t的第一个成员是native_handle类型,其实和c++的继承是一个意思,
//总之就是一个指向private_handle_t的指针同样也可以表示一个指向native_handle的指针.
struct private_handle_t {
    struct native_handle nativeHandle;
#endif
    // file-descriptors
    int     fd;
    // ints
    int     magic;
    int     flags;
    int     size;
    int     offset;
    // 因为native_handle的data成员是一个大小为0的数组,所以data[0]其实就是指向了fd,data[1]指向magic,以此类推.
    // 上面提到我们可以把native_handle看成是一个纯虚的基类,那么在private_handle_t这个派生类中,numFds=1 numInts=4.
    ...
}

    gralloc分配的buffer都可以用一个private_handle_t来描述,同时也可以用一个native_handle来描述.在不同的平台的实现上,private_handle_t可能会有不同的定义,所以private_handle_t在各个模块之间传递的时候很不方便,而如果用native_handle的身份来传递,就可以消除平台的差异性.在HardwareComposer中,由SurfaceFlinger传给hwc的handle即是native_handle类型,而hwc作为平台相关的模块,他需要知道native_handle中各个字段的具体含义,所以hwc往往会将native_handle指针转化为private_handle_t指针来使用.

3. buffer_handle_t

    标题中并没有提到这个类型,因为这个类型实在是太简单了,我们看code

system/core/include/system/window.h

typedef const native_handle_t* buffer_handle_t;

    在window.h中又把指向native_handle_t的指针define为buffer_handle_t,

sturct native_handle

native_handle_t

struct private_handle_t
这三个类型可以看作是同一个东西,而buffer_handle_t则是指向他们的指针.

    那么android是如何使用这些struct的,gralloc分配的buffer如何和android联系起来呢?
我们继续来看window.h

4. ANativeWindowBuffer和ANativeWindow

    在具体分析ANativeWindowBuffer和ANativeWindow之前,我们先来看下和这两个类型都相关的另外一个结构体android_native_base_t

system/core/include/system/window.h

typedef struct android_native_base_t
{
    /* a magic value defined by the actual EGL native type */
    int magic;

    /* the sizeof() of the actual EGL native type */
    int version;

    void* reserved[4];

    /* reference-counting interface */
    void (*incRef)(struct android_native_base_t* base);
    void (*decRef)(struct android_native_base_t* base);
} android_native_base_t;

incRef和decRef是为了把派生类和android所有class的老祖宗RefBase联系起来所预留的函数指针,在后面我们在会看到指针具体会指向哪些函数.

typedef struct ANativeWindowBuffer
{
#ifdef __cplusplus
    ANativeWindowBuffer() {
        // ANDROID_NATIVE_BUFFER_MAGIC的值是"_bfr"
        common.magic = ANDROID_NATIVE_BUFFER_MAGIC;
        common.version = sizeof(ANativeWindowBuffer);
        memset(common.reserved, 0, sizeof(common.reserved));
    }

    // Implement the methods that sp<ANativeWindowBuffer> expects so that it
    // can be used to automatically refcount ANativeWindowBuffer's.
    // 调用common,也就是android_native_base_t的incRef和decRef函数,具体函数是什么还不知道
    void incStrong(const void* /*id*/) const {
        common.incRef(const_cast<android_native_base_t*>(&common));
    }
    void decStrong(const void* /*id*/) const {
        common.decRef(const_cast<android_native_base_t*>(&common));
    }
#endif

    // common的incRef和decRef还没有明确是什么
   struct android_native_base_t common;

    int width;
    int height;
    int stride;
    int format;
    int usage;

    void* reserved[2];
    // buffer_handle_t是指向sturct native_handle, native_handle_t, struct private_handle_t的指针.
    buffer_handle_t handle;

    void* reserved_proc[8];
} ANativeWindowBuffer_t;
// Old typedef for backwards compatibility.
typedef ANativeWindowBuffer_t android_native_buffer_t;

ANativeWindow的定义如下

struct ANativeWindow
{
#ifdef __cplusplus
    ANativeWindow()
        : flags(0), minSwapInterval(0), maxSwapInterval(0), xdpi(0), ydpi(0)
    {
        // ANDROID_NATIVE_WINDOW_MAGIC的值是"_wnd"
        common.magic = ANDROID_NATIVE_WINDOW_MAGIC;
        common.version = sizeof(ANativeWindow);
        memset(common.reserved, 0, sizeof(common.reserved));
    }

    /* Implement the methods that sp<ANativeWindow> expects so that it
       can be used to automatically refcount ANativeWindow's. */
    // 调用common,也就是android_native_base_t的incRef和decRef函数,具体函数是什么还不知道
    void incStrong(const void* /*id*/) const {
        common.incRef(const_cast<android_native_base_t*>(&common));
    }
    void decStrong(const void* /*id*/) const {
        common.decRef(const_cast<android_native_base_t*>(&common));
    }
#endif
    // common的incRef和decRef还没有明确是什么
    struct android_native_base_t common;
    ...
    int     (*dequeueBuffer)(struct ANativeWindow* window,
                struct ANativeWindowBuffer** buffer, int* fenceFd);
    int     (*queueBuffer)(struct ANativeWindow* window,
                struct ANativeWindowBuffer* buffer, int fenceFd);
    int     (*cancelBuffer)(struct ANativeWindow* window,
                struct ANativeWindowBuffer* buffer, int fenceFd);
};
 /* Backwards compatibility: use ANativeWindow (struct ANativeWindow in C).
  * android_native_window_t is deprecated.
  */
typedef struct ANativeWindow ANativeWindow;
typedef struct ANativeWindow android_native_window_t __deprecated;

我们目前需要注意的是ANativeWindow的函数指针成员所指向的函数都需要一个struct ANativeWindowBuffer* buffer的参数.

ANativeWindowBuffer和ANativeWindow还是没有给android_native_base_t的incRef和decRef指针赋值,ANativeWindowBuffer和ANativeWindow两个还是可以理解为抽象类!

5. GraphicBuffer和Surface

frameworks/native/include/ui/GraphicBuffer.h 

class GraphicBuffer
    : public ANativeObjectBase< ANativeWindowBuffer, GraphicBuffer, RefBase >,
      public Flattenable<GraphicBuffer>
{
...
}

GraphicBuffer继承于模版类ANativeObjectBase,这个模版类有三个模版.

frameworks/native/include/ui/ANativeObjectBase.h

// NATIVE_TYPE=ANativeWindowBuffer TYPE=GraphicBuffer REF=RefBase
template <typename NATIVE_TYPE, typename TYPE, typename REF>
// ANativeObjectBase多重继承于ANativeWindowBuffer和RefBase
class ANativeObjectBase : public NATIVE_TYPE, public REF
{
public:
    // Disambiguate between the incStrong in REF and NATIVE_TYPE
    // incStrong和decStrong直接调用其中一个基类RefBase的对应函数
    void incStrong(const void* id) const {
        REF::incStrong(id);
    }
    void decStrong(const void* id) const {
        REF::decStrong(id);
    }

protected:
    // 给ANativeObjectBase取了个别名BASE
    typedef ANativeObjectBase<NATIVE_TYPE, TYPE, REF> BASE;
    ANativeObjectBase() : NATIVE_TYPE(), REF() {
        // 构造函数中给ANativeWindowBuffer.common的两个函数指针赋值了!这两个指针就是我们之前在分析ANativeWindowBuffer的时候悬而未决的地方.
        // incRef和decRef指针分别指向内部函数incRef和decRef
        NATIVE_TYPE::common.incRef = incRef;
        NATIVE_TYPE::common.decRef = decRef;
    }
    static inline TYPE* getSelf(NATIVE_TYPE* self) {
        return static_cast<TYPE*>(self);
    }
    static inline TYPE const* getSelf(NATIVE_TYPE const* self) {
        return static_cast<TYPE const *>(self);
    }
    static inline TYPE* getSelf(android_native_base_t* base) {
        return getSelf(reinterpret_cast<NATIVE_TYPE*>(base));
    }
    static inline TYPE const * getSelf(android_native_base_t const* base) {
        return getSelf(reinterpret_cast<NATIVE_TYPE const*>(base));
    }
    // 内部函数incRef和decRef调用上面的incStong和decStrong,也就是说ANativeWindowBuffer.common的两个函数指针最终会调用到RefBase的incStrong和decStrong.
    static void incRef(android_native_base_t* base) {
        ANativeObjectBase* self = getSelf(base);
        self->incStrong(self);
    }
    static void decRef(android_native_base_t* base) {
        ANativeObjectBase* self = getSelf(base);
        self->decStrong(self);
    }
};

    搞了半天,原来GraphicBuffer就是ANativeWindowBuffer一种具体实现,把ANativeWindowBuffer的common成员的两个函数指针incRef decRef指向了GraphicBuffer的另一个基类RefBase的incStrong和decStrong,而ANativeWindowBuffer无非就是把buffer_handle_t包了一层.我们看下另外一个从ANativeObjectBase派生的类,他就是大名鼎鼎的,Surface!

frameworks/native/include/gui/Surface.h

class Surface
    : public ANativeObjectBase<ANativeWindow, Surface, RefBase>
{
    enum { NUM_BUFFER_SLOTS = BufferQueue::NUM_BUFFER_SLOTS };
    ...
    struct BufferSlot {
        sp<GraphicBuffer> buffer;
        Region dirtyRegion;
    };
    // mSlots stores the buffers that have been allocated for each buffer slot.
    // It is initialized to null pointers, and gets filled in with the result of
    // IGraphicBufferProducer::requestBuffer when the client dequeues a buffer from a
    // slot that has not yet been used. The buffer allocated to a slot will also
    // be replaced if the requested buffer usage or geometry differs from that
    // of the buffer allocated to a slot.
    BufferSlot mSlots[NUM_BUFFER_SLOTS];
    ...
}

    Surface和GraphicBuffer都继承自模版类ANativeObjectBase,他使用的三个模版是ANativeWindow, Surface, RefBase,关于incRef和decRef两个函数指针的指向问题和上面GraphicBuffer是完全相同的, 这里就不赘述了.我们需要注意的是Surface有一个BufferSlot类型的成员数组mSlots,BufferSlot是GraphicBuffer的包装,所以我们可以理解为每个Surface中都有一个大小为NUM_BUFFER_SLOTS的GraphicBuffer数组.

    因为Surface继承自ANativeWindow,所以Surface需要实现ANativeWindow中定义的一些接口,这些实现在Surface的构造函数中:

frameworks/native/libs/gui/Surface.cpp

Surface::Surface(
        const sp<IGraphicBufferProducer>& bufferProducer,
        bool controlledByApp)
    : mGraphicBufferProducer(bufferProducer)
{
    // Initialize the ANativeWindow function pointers.
    ANativeWindow::setSwapInterval  = hook_setSwapInterval;
    ANativeWindow::dequeueBuffer    = hook_dequeueBuffer;
    ANativeWindow::cancelBuffer     = hook_cancelBuffer;
    ANativeWindow::queueBuffer      = hook_queueBuffer;
    ANativeWindow::query            = hook_query;
    ANativeWindow::perform          = hook_perform;

    ANativeWindow::dequeueBuffer_DEPRECATED = hook_dequeueBuffer_DEPRECATED;
    ANativeWindow::cancelBuffer_DEPRECATED  = hook_cancelBuffer_DEPRECATED;
    ANativeWindow::lockBuffer_DEPRECATED    = hook_lockBuffer_DEPRECATED;
    ANativeWindow::queueBuffer_DEPRECATED   = hook_queueBuffer_DEPRECATED;

    const_cast<int&>(ANativeWindow::minSwapInterval) = 0;
    const_cast<int&>(ANativeWindow::maxSwapInterval) = 1;
}

ANativeWindow定义的这些接口有什么用呢?谁会来call这些函数呢?举个例子来看.我们在EGL的api中可以找到eglCreateWindowSurface这个函数的定义:

frameworks/native/opengl/libs/EGL/eglApi.cpp

EGLSurface eglCreateWindowSurface(  EGLDisplay dpy, EGLConfig config,
                                    NativeWindowType window,
                                    const EGLint *attrib_list)
{
    ...
}

注意其中一个参数NativeWindowType window,这个NativeWindowType又是什么呢?

frameworks/native/opengl/include/EGL/eglplatform.h

typedef struct ANativeWindow*           EGLNativeWindowType;
typedef EGLNativeWindowType  NativeWindowType;

原来NativeWindowType在Android环境下,就是ANativeWindow*,也就是Surface*!

总结一下,

native_handle/native_handle_t是private_handle_t的抽象表示方法,消除平台相关性,方便private_handle_t所表示的memory信息在android各个层次之间传递.而buffer_handle_t是指向他们的指针.

ANativeWindowBuffer将buffer_handle_t进行了包装,ANativeWindow和ANativeWindowBuffer都继承于android_native_base_t,定义了common.incRef和common.decRef两个函数指针,但是并没有为函数指针赋值,所以ANativeWindow和ANativeWindowBuffer仍然是抽象类.

GraphicBuffer和Surface通过继承模版类ANativeObjectBase并指定其中一个模版是RefBase,为incRef和decRef两个指针分别赋值为RefBase的incStrong和decStrong,这样

GraphicBuffer继承了ANativeWindowBuffer,Surface继承了ANativeWindow,并且两者都具有的和RefBase同样的incStong decStrong成员函数.

Surface的成员BufferSlot mSlots[NUM_BUFFER_SLOTS];可以看作是sp<GraphicBuffer>类型的数组,也就是说每个Surface中都包含有NUM_BUFFER_SLOTS个sp<GraphicBuffer>.

关于ANativeWindow的使用方法,我们可以在SurfaceFlinger中找到一个很好的列子,就是SF的captureScreen接口.