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.