清楚详解Android 进程间图传递图形buffer原理，

来源：开发者投稿于 2023-02-25 被查看 17704 次评论：147

清楚详解Android 进程间图传递图形buffer原理，

进程间图怎么传递图形buffer

写这篇文章的目的：讲解进程间图怎么传递图形buffer的

最近研究图形缓存怎么在进程之间传递的，谷歌了所有的博客，发现没人讲的清楚

图形缓存是Android绘制的核心内容，8.0版本后却没有讲清楚明白的。

source.android.com/docs/core/a… 这里handle中有些线索，但没细说。

Android 不同进程间，并不传递图形缓存，而是使用“共享内存”机制操作图形缓存。

但是“共享内存”用到的fd怎么传递的，没人讲清楚

fd 时进程级别的 int 数值，正常情况不同进程的 fd 并不能传递。而 GraphicBuffer 这个对象怎么做到传递共享内存fd 的？Java层的 Parcel 类有个 writeFileDescriptor 函数，用于传递 fd ，那么native层，hal层又是怎么传递的呢？

本文并不面面俱到，只讲核心内容。需要的一些比较硬的知识储备：

了解binder
- 用户层：binder数据传输中，数据对象Parcel类，生成序列化对象过程，以及解包过程。
- 内核层：binder 内核中，对不同数据对象的解析
内存知识：了解共享内存，了解mmap，了解内存分配和映射的本质
- 对内存了解的不够深的话，一些地方还是比较难去理解的，并不是单纯看代码看的懂的，这或许是大部分博客讲不明白的原因
了解Linux驱动
了解Surface 到 SurfaceFlinger 交互过程

大纲

一、Surface::dequeueBuffer 代码流程简述
二、进程间图传递图形buffer详解
- 【1】SurfaceFlinger进程和 IAllocator 服务之间传递图形显示的Buffer
- 【2】App进程同 SurfaceFlinger 进程之间传递 GraphicBuffer 对象服务端 requestBuffer 流程
- 【3】linux内核部分，binder驱动对 BINDER_TYPE_FDA 、BINDER_TYPE_FD 类型的处理
总结：

一、Surface.dequeueBuffer 代码流程简述

图形内存的分配核心在于 Surface.dequeueBuffer流程。

Surface.dequeueBuffer会调用 BufferQueueProducer.dequeueBuffer 去 SurfaceFlinger 端获取BufferSlot数组中可用Slot的下标值
- 这个 BufferSlot 如果没有 GraphicBuffer，就会去new一个，并在构造函数中申请图形缓存，并把图形缓存映射到当前进程
- 同时把 BufferQueueProducer::dequeueBuffer 返回值的标记位设置为 BUFFER_NEEDS_REALLOCATION
BufferQueueProducer.dequeueBuffer 的返回值如果带有 BUFFER_NEEDS_REALLOCATION标记，会调用 BufferQueueProducer.requestBuffer 获取 GraphicBuffer，同时把图形缓存映射到当前进程

调用过程

Surface.dequeueBuffer【App·进程端】
- BpGraphicBufferProducer.dequeueBuffer【接口层】
  - BufferQueueProducer.dequeueBuffer 【SurfaceFlinger 进程端】
    - dequeueBuffer 函数参数outSlot指针带回一个BufferSlot数组的下标，返回值返回标记位，但并未返回 GraphicBuffer
    - dequeueBuffer 函数中，在获取的 BufferSlot 没有GraphicBuffer时，会new一个GraphicBuffer，同时返回值的标记为 BUFFER_NEEDS_REALLOCATION
    - new GraphicBuffer( width, height, format, BQ_LAYER_COUNT, usage, {mConsumerName.string(), mConsumerName.size()});
      - GraphicBuffer 构造函数中会调用initWithSize，内部调用分配图形缓存的代码
      - initWithSize(inWidth, inHeight, inFormat, inLayerCount, inUsage, std::move(requestorName));
        GraphicBufferAllocator.allocate
        allocateHelper(width, height, format, layerCount, usage, handle, stride, requestorName, true)
        Gralloc4Allocator.allocate
        hwbinder 服务调用：
        IAllocator::getService()->allocate(descriptor, bufferCount,[&](const auto& tmpError, const auto& tmpStride,const auto& tmpBuffers){...}
        之后的代码需要看厂家的具体实现，最后无非是调用到内核驱动层分配内存，比如调用ion驱动层分配ion内存
        SurfaceFlinger 和 IAllocator 服务怎么传递共享内存的，转“进程间图传递图形buffer详解【1】”章节
        回调函数中调用 IMapper.importBuffer(tmpBuffers[i], &outBufferHandles[i]); // 内部使用 mmap 把内存映射到当前进程
- 在 dequeueBuffer 返回值的标记为 BUFFER_NEEDS_REALLOCATION 时，
  - App端需要调用 requestBuffer，获取 GraphicBuffer 对象，
  - 同时，把 SurfaceFlinger 分配的图形缓存，映射到App进程
- BpGraphicBufferProducer->requestBuffer(buf, &gbuf);【接口层】
  - BufferQueueProducer.requestBuffer-----请求返回 GraphicBuffer 对象
  - SurfaceFlinger 进程端 requestBuffer 代码非常简单，仅仅是把 dequeueBuffer 过程中分配的对象赋值给参数 gbuf ，传递给 App
  - 那么，图形缓存的 fd 是怎么传到App端的，App又是怎么映射的图形缓存呢？
  - 核心在 BpGraphicBufferProducer.requestBuffer 函数中 GraphicBuffer 对象的构建过程：
    - status_t result =remote()->transact(REQUEST_BUFFER, data, &reply);
      - 接下来GraphicBuffer 传输过程，见进程间图传递图形buffer详解【2】
    - *buf = new GraphicBuffer();
    - result = reply.read(**buf);
      - read 过程会调用 GraphicBuffer.unflatten
        GraphicBuffer.unflatten 函数内部调用了 GraphicBufferMapper.importBuffer
        内部也是调用IMapper.importBuffer，最终使用 mmap 把内存映射到当前进程
        调用 mmap 过程，可以参考 hardware/google/gchips/GrallocHAL/src/hidl_common/Mapper.cpp 代码
        cs.android.com/android/pla…

二、进程间图传递图形buffer详解

【1】SurfaceFlinger进程和 IAllocator 服务之间传递图形显示的Buffer

IAllocator 服务全称为 android.hardware.graphics.allocator@4.0::IAllocator/default

高通平台上的进程名为：vendor.qti.hardware.display.allocator-service

SurfaceFlinger IAllocator 接口的 allocate 函数

// frameworks/native/libs/ui/Gralloc4.cpp
status_t Gralloc4Allocator::allocate(std::string requestorName, uint32_t width, uint32_t height,
                                     android::PixelFormat format, uint32_t layerCount,
                                     uint64_t usage, uint32_t bufferCount, uint32_t* outStride,
                                     buffer_handle_t* outBufferHandles, bool importBuffers) const {
//...
	//===================关键代码============
    auto ret = mAllocator->allocate(descriptor, bufferCount,
                                    [&](const auto& tmpError, const auto& tmpStride,
                                        const auto& tmpBuffers) {// const auto& tmpBuffers 是个 hidl_handle 类型
                                        error = static_cast<status_t>(tmpError);
                                        if (tmpError != Error::NONE) {
                                            return;
                                        }
                                        if (importBuffers) {
                                            for (uint32_t i = 0; i < bufferCount; i++) {
                                                error = mMapper.importBuffer(tmpBuffers[i],
                                                                             &outBufferHandles[i]);
                                                if (error != NO_ERROR) {
                                                    for (uint32_t j = 0; j < i; j++) {
                                                        mMapper.freeBuffer(outBufferHandles[j]);
                                                        outBufferHandles[j] = nullptr;
                                                    }
                                                    return;
                                                }
                                            }
                                        } else {
											//....
                                        }
                                        *outStride = tmpStride;
                                    });
//...
    return (ret.isOk()) ? error : static_cast<status_t>(kTransactionError);
}

allocator服务端的hidl接口实现

不看具体的allocate函数实现，重点看数据传输过程

//out/soong/.intermediates/hardware/interfaces/graphics/allocator/4.0/android.hardware.graphics.allocator@4.0_genc++/gen/android/hardware/graphics/allocator/4.0/AllocatorAll.cpp  
// 这部分代码是 hidl 接口编译完成后，out目录自动生成的代码，源码目录下没有
// Methods from ::android::hardware::graphics::allocator::V4_0::IAllocator follow.
::android::status_t BnHwAllocator::_hidl_allocate(
        ::android::hidl::base::V1_0::BnHwBase* _hidl_this,
        const ::android::hardware::Parcel &_hidl_data,
        ::android::hardware::Parcel *_hidl_reply,
        TransactCallback _hidl_cb) {
 //...
//========================调用服务端真正的实现=====================
    ::android::hardware::Return<void> _hidl_ret = static_cast<IAllocator*>(_hidl_this->getImpl().get())->allocate(*descriptor, count, [&](const auto &_hidl_out_error, const auto &_hidl_out_stride, const auto &_hidl_out_buffers) {
        if (_hidl_callbackCalled) {
            LOG_ALWAYS_FATAL("allocate: _hidl_cb called a second time, but must be called once.");
        }
        _hidl_callbackCalled = true;
//===============函数调用完成后，开始写返回数据========================
        ::android::hardware::writeToParcel(::android::hardware::Status::ok(), _hidl_reply);
        _hidl_err = _hidl_reply->writeInt32((int32_t)_hidl_out_error);
        if (_hidl_err != ::android::OK) { goto _hidl_error; }
        // 返回数据 tmpStride 的值
        _hidl_err = _hidl_reply->writeUint32(_hidl_out_stride);
        if (_hidl_err != ::android::OK) { goto _hidl_error; }
        size_t _hidl__hidl_out_buffers_parent;
        _hidl_err = _hidl_reply->writeBuffer(&_hidl_out_buffers, sizeof(_hidl_out_buffers), &_hidl__hidl_out_buffers_parent);
        if (_hidl_err != ::android::OK) { goto _hidl_error; }
        size_t _hidl__hidl_out_buffers_child;
        _hidl_err = ::android::hardware::writeEmbeddedToParcel(
                _hidl_out_buffers,
                _hidl_reply,
                _hidl__hidl_out_buffers_parent,
                0 /* parentOffset */, &_hidl__hidl_out_buffers_child);
        if (_hidl_err != ::android::OK) { goto _hidl_error; }
//关键代码====传输上边的回调函数的参数 const auto& tmpBuffers 的每个元素， 数据类型是 hidl_handle 类型
        for (size_t _hidl_index_0 = 0; _hidl_index_0 < _hidl_out_buffers.size(); ++_hidl_index_0) {
            // 关键函数 android::hardware::writeEmbeddedToParcel
            _hidl_err = ::android::hardware::writeEmbeddedToParcel(
                    _hidl_out_buffers[_hidl_index_0],
                    _hidl_reply,
                    _hidl__hidl_out_buffers_child,
                    _hidl_index_0 * sizeof(::android::hardware::hidl_handle));
            if (_hidl_err != ::android::OK) { goto _hidl_error; }
        }
//...
        if (_hidl_err != ::android::OK) { return; }
        _hidl_cb(*_hidl_reply);
    });
    _hidl_ret.assertOk();
    if (!_hidl_callbackCalled) {
        LOG_ALWAYS_FATAL("allocate: _hidl_cb not called, but must be called once.");
    }
    return _hidl_err;
}

android::hardware::writeEmbeddedToParcel

// system/libhidl/transport/HidlBinderSupport.cpp
status_t writeEmbeddedToParcel(const hidl_handle &handle,
        Parcel *parcel, size_t parentHandle, size_t parentOffset) {
    //此处调用了 hwbinder/Parcel.cpp 的writeEmbeddedNativeHandle 函数
    status_t _hidl_err = parcel->writeEmbeddedNativeHandle(
            handle.getNativeHandle(),
            parentHandle,
            parentOffset + hidl_handle::kOffsetOfNativeHandle);
    return _hidl_err;
}
// system/libhwbinder/Parcel.cpp
status_t Parcel::writeEmbeddedNativeHandle(const native_handle_t *handle,
                                           size_t parent_buffer_handle,
                                           size_t parent_offset)
{
    return writeNativeHandleNoDup(handle, true /* embedded */, parent_buffer_handle, parent_offset);
}
status_t Parcel::writeNativeHandleNoDup(const native_handle_t *handle,
                                        bool embedded,
                                        size_t parent_buffer_handle,
                                        size_t parent_offset)
{
//...
    struct binder_fd_array_object fd_array {
        .hdr = { .type = BINDER_TYPE_FDA }, // 关键代码： BINDER_TYPE_FDA 类型，binder内核驱动代码对这个类型有专门的处理
        .num_fds = static_cast<binder_size_t>(handle->numFds),
        .parent = buffer_handle,
        .parent_offset = offsetof(native_handle_t, data),
    };
    return writeObject(fd_array);
}

之后的代码，见 binder驱动对 BINDER_TYPE_FDA 、BINDER_TYPE_FD 类型的处理

【2】App进程同 SurfaceFlinger 进程之间传递 GraphicBuffer 对象

GraphicBuffer 对象

服务端 requestBuffer 流程

// frameworks/native/libs/gui/IGraphicBufferProducer.cpp
status_t BnGraphicBufferProducer::onTransact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    switch(code) {
        case REQUEST_BUFFER: {
            CHECK_INTERFACE(IGraphicBufferProducer, data, reply);
            int bufferIdx = data.readInt32();
            sp<GraphicBuffer> buffer;
            int result = requestBuffer(bufferIdx, &buffer);
            reply->writeInt32(buffer != nullptr);
            if (buffer != nullptr) {
                reply->write(*buffer);// GraphicBuffer 对象回写========!!!!!!!!!!!!!!=====
            }
            reply->writeInt32(result);
            return NO_ERROR;
        }
//...
    }
//...
}

Parcel::write 写对象流程，

Parcel::write Parcel.h

// frameworks/native/libs/binder/include/binder/Parcel.h
template<typename T>
status_t Parcel::write(const Flattenable<T>& val) {// 对象需要继承 Flattenable
    const FlattenableHelper<T> helper(val);
    return write(helper);
}

Parcel::write Parcel.cpp

// frameworks/native/libs/binder/Parcel.cpp
status_t Parcel::write(const FlattenableHelperInterface& val)
{
    status_t err;
    // size if needed
    const size_t len = val.getFlattenedSize();
    // val.getFdCount(); 这个值为 GraphicBuffer.mTransportNumFds
    // 从这个接口获取
    // GrallocMapper::getTransportSize(buffer_handle_t bufferHandle, uint32_t* outNumFds, uint32_t* outNumInts)
    const size_t fd_count = val.getFdCount();// 这个值为 GraphicBuffer.mTransportNumFds
//...........
	// 调用对象的 flatten 写到缓存中
    err = val.flatten(buf, len, fds, fd_count);
    // fd_count 不为0，需要写 fd
    for (size_t i=0 ; i<fd_count && err==NO_ERROR ; i++) {
        err = this->writeDupFileDescriptor( fds[i] );
    }
    if (fd_count) {
        delete [] fds;
    }
    return err;
}

Parcel::writeDupFileDescriptor 写fd流程

// frameworks/native/libs/binder/Parcel.cpp
status_t Parcel::writeDupFileDescriptor(int fd)
{
    int dupFd;
    if (status_t err = dupFileDescriptor(fd, &dupFd); err != OK) {
        return err;
    }
    //=============!!!!!!!!!!!!!===========
    status_t err = writeFileDescriptor(dupFd, true /*takeOwnership*/);
    if (err != OK) {
        close(dupFd);
    }
    return err;
}
status_t Parcel::writeFileDescriptor(int fd, bool takeOwnership) {
//........
#ifdef BINDER_WITH_KERNEL_IPC // frameworks/native/libs/binder/Android.bp 中定义了此宏  "-DBINDER_WITH_KERNEL_IPC",
    flat_binder_object obj;
    obj.hdr.type = BINDER_TYPE_FD;// 类型为 fd ，内核会自动创建fd
    obj.flags = 0;
    obj.binder = 0; /* Don't pass uninitialized stack data to a remote process */
    obj.handle = fd;
    obj.cookie = takeOwnership ? 1 : 0;
    return writeObject(obj, true);
#else  // BINDER_WITH_KERNEL_IPC
    LOG_ALWAYS_FATAL("Binder kernel driver disabled at build time");
    (void)fd;
    (void)takeOwnership;
    return INVALID_OPERATION;
#endif // BINDER_WITH_KERNEL_IPC
}

之后的代码，见 binder驱动对 BINDER_TYPE_FDA 、BINDER_TYPE_FD 类型的处理

【3】linux内核部分，binder驱动对 BINDER_TYPE_FDA 、BINDER_TYPE_FD 类型的处理

binder_transaction

// 这里使用的 3.8 的内核版本，逻辑较为清晰
// 更新的内核版本需要看 binder_apply_fd_fixups 函数部分
// https://android.googlesource.com/kernel/msm/+/refs/heads/android-msm-coral-4.14-android10/drivers/android/binder.c
static void binder_transaction(struct binder_proc *proc,
			       struct binder_thread *thread,
			       struct binder_transaction_data *tr, int reply,
			       binder_size_t extra_buffers_size)
{
    //...
		case BINDER_TYPE_FD: {
			struct binder_fd_object *fp = to_binder_fd_object(hdr);
            //数据类型为 BINDER_TYPE_FD 时，调用了 binder_translate_fd
			int target_fd = binder_translate_fd(fp->fd, t, thread, in_reply_to);
			if (target_fd < 0) {
				return_error = BR_FAILED_REPLY;
				return_error_param = target_fd;
				return_error_line = __LINE__;
				goto err_translate_failed;
			}
			fp->pad_binder = 0;
			fp->fd = target_fd;
			binder_alloc_copy_to_buffer(&target_proc->alloc,
						    t->buffer, object_offset,
						    fp, sizeof(*fp));
		} break;
		case BINDER_TYPE_FDA: {
			struct binder_object ptr_object;
			binder_size_t parent_offset;
			struct binder_fd_array_object *fda =
				to_binder_fd_array_object(hdr);
			size_t num_valid = (buffer_offset - off_start_offset) /
						sizeof(binder_size_t);
			struct binder_buffer_object *parent =
				binder_validate_ptr(target_proc, t->buffer,
						    &ptr_object, fda->parent,
						    off_start_offset,
						    &parent_offset,
						    num_valid);
			if (!parent) {
				binder_user_error("%d:%d got transaction with invalid parent offset or type\n",
						  proc->pid, thread->pid);
				return_error = BR_FAILED_REPLY;
				return_error_param = -EINVAL;
				return_error_line = __LINE__;
				goto err_bad_parent;
			}
			if (!binder_validate_fixup(target_proc, t->buffer,
						   off_start_offset,
						   parent_offset,
						   fda->parent_offset,
						   last_fixup_obj_off,
						   last_fixup_min_off)) {
				binder_user_error("%d:%d got transaction with out-of-order buffer fixup\n",
						  proc->pid, thread->pid);
				return_error = BR_FAILED_REPLY;
				return_error_param = -EINVAL;
				return_error_line = __LINE__;
				goto err_bad_parent;
			}
            //数据类型为 BINDER_TYPE_FDA 时，调用了 binder_translate_fd
			ret = binder_translate_fd_array(fda, parent, t, thread, in_reply_to);
			if (ret < 0) {
				return_error = BR_FAILED_REPLY;
				return_error_param = ret;
				return_error_line = __LINE__;
				goto err_translate_failed;
			}
			last_fixup_obj_off = parent_offset;
			last_fixup_min_off =
				fda->parent_offset + sizeof(u32) * fda->num_fds;
		} break;
    //...
}

binder_translate_fd_array 函数中对每一个fd都调用了 binder_translate_fd 函数

binder_translate_fd

static int binder_translate_fd(int fd,
			       struct binder_transaction *t,
			       struct binder_thread *thread,
			       struct binder_transaction *in_reply_to)
{
	struct binder_proc *proc = thread->proc;
	struct binder_proc *target_proc = t->to_proc;
	int target_fd;
	struct file *file;
	int ret;
	bool target_allows_fd;
	if (in_reply_to)
		target_allows_fd = !!(in_reply_to->flags & TF_ACCEPT_FDS);
	else
		target_allows_fd = t->buffer->target_node->accept_fds;
	if (!target_allows_fd) {
		binder_user_error("%d:%d got %s with fd, %d, but target does not allow fds\n",
				  proc->pid, thread->pid,
				  in_reply_to ? "reply" : "transaction",
				  fd);
		ret = -EPERM;
		goto err_fd_not_accepted;
	}
	file = fget(fd);//从fd获取 file 对象
	if (!file) {
		binder_user_error("%d:%d got transaction with invalid fd, %d\n",
				  proc->pid, thread->pid, fd);
		ret = -EBADF;
		goto err_fget;
	}
    //se权限处理
	ret = security_binder_transfer_file(proc->tsk, target_proc->tsk, file);
	if (ret < 0) {
		ret = -EPERM;
		goto err_security;
	}
    //在目标进程中找到一个可用的fd
	target_fd = task_get_unused_fd_flags(target_proc, O_CLOEXEC);
	if (target_fd < 0) {
		ret = -ENOMEM;
		goto err_get_unused_fd;
	}
    // 调用task_fd_install将 file对象 关联到目标进程中的fd
	task_fd_install(target_proc, target_fd, file);
	trace_binder_transaction_fd(t, fd, target_fd);
	binder_debug(BINDER_DEBUG_TRANSACTION, "        fd %d -> %d\n",
		     fd, target_fd);
	return target_fd;
err_get_unused_fd:
err_security:
	fput(file);
err_fget:
err_fd_not_accepted:
	return ret;
}

附、图形缓存的几个重要数据类型

1、App端 Surface 同 SurfaceFlinger 用于传递共享内存 fd 的对象 GraphicBuffer

GraphicBuffer

// frameworks/native/libs/ui/include/ui/GraphicBuffer.h
class GraphicBuffer
    : public ANativeObjectBase<ANativeWindowBuffer, GraphicBuffer, RefBase>,
      public Flattenable<GraphicBuffer>
      { //...
    	status_t flatten(void*& buffer, size_t& size, int*& fds, size_t& count) const;
    	status_t unflatten(void const*& buffer, size_t& size, int const*& fds, size_t& count);
        //...
      }
// frameworks/native/libs/ui/include/ui/ANativeObjectBase.h
/*
 * This helper class turns a ANativeXXX object type into a C++
 * reference-counted object; with proper type conversions.
 */
template <typename NATIVE_TYPE, typename TYPE, typename REF,
        typename NATIVE_BASE = android_native_base_t>
class ANativeObjectBase : public NATIVE_TYPE, public REF
{
    //...
}
//转换后：
class ANativeObjectBase : public ANativeWindowBuffer, public RefBase
{
    //...
}

GraphicBuffer 继承 ANativeWindowBuffer Flattenable
Flattenable 两个关键函数 flatten unflatten，用于binder序列化时使用。

ANativeWindowBuffer

// /frameworks/native/libs/nativebase/include/nativebase/nativebase.h
// 图形Buffer的Size = stride * height * 每像素字节数
typedef struct ANativeWindowBuffer
{   
    ...
    int width;      // 图形Buffer的宽度
    int height;     // 图形Buffer的高度
    int stride;     // 图形Buffer的步长，为了处理对齐问题，与width可能不同
    int format;     // 图形Buffer的像素格式
    const native_handle_t* handle; // 指向一块图形Buffer
    uint64_t usage; // 图形Buffer的使用规则（gralloc会分配不同属性的图形Buffer）
    ...
} ANativeWindowBuffer_t;

native_handle_t

// system/core/libcutils/include/cutils/native_handle.h
typedef struct native_handle
{
    int version;        /* sizeof(native_handle_t) */
    // //data[0]中的文件描述符个数
    int numFds;         /* number of file-descriptors at &data[0] */
    // //&data[numFds]中int的个数
    int numInts;        /* number of ints at &data[numFds] */
    int data[0];        /* numFds + numInts ints */
} native_handle_t;

buffer_handle_t 同 native_handle_t

// system/core/libcutils/include/cutils/native_handle.h
typedef const native_handle_t* buffer_handle_t;

2、hidl接口进程间传递 fd 使用的数据类型 (HWbinder 传递 fd 的对象)

hidl_handle 用于 SurfaceFlinger 同 IAllocator HIDL接口的服务之间传递共享内存fd
- 高通平台上，这个 HIDL 服务端对应的进程是 vendor.qti.hardware.display.allocator-service

hidl_handle

struct hidl_handle {
    hidl_handle();
    ~hidl_handle();
    hidl_handle(const native_handle_t *handle);
    // copy constructor.
    hidl_handle(const hidl_handle &other);
    // move constructor.
    hidl_handle(hidl_handle &&other) noexcept;
    // assignment operators
    hidl_handle &operator=(const hidl_handle &other);
    hidl_handle &operator=(const native_handle_t *native_handle);
    hidl_handle &operator=(hidl_handle &&other) noexcept;
    void setTo(native_handle_t* handle, bool shouldOwn = false);
    const native_handle_t* operator->() const;
    // implicit conversion to const native_handle_t*
    operator const native_handle_t *() const;
    // explicit conversion
    const native_handle_t *getNativeHandle() const;
    // offsetof(hidl_handle, mHandle) exposed since mHandle is private.
    static const size_t kOffsetOfNativeHandle;
private:
    void freeHandle();
	// 核心数据 native_handle_t mHandle;
    details::hidl_pointer<const native_handle_t> mHandle;
    bool mOwnsHandle;
    uint8_t mPad[7];
};

总结：

SurfaceFlinger进程和 IAllocator服务进程之间通过 hidl_handle 类型的数据传递图形buffer共享内存的fd
- 数据传输中对 hidl_handle 类型数据特化处理，并把binder数据类型设置为 BINDER_TYPE_FDA
- binder内核对 BINDER_TYPE_FDA 类型数据特化处理
- 同时在 IAllocator.allocate 的回调函数中调用 IMapper.importBuffer 把内存映射到当前进程
App进程同 SurfaceFlinger进程之间使用 GraphicBuffer 对象传递图形buffer共享内存的fd
- 数据传输中对 GraphicBuffer 中的 native_handle_t 数据特化处理，并把binder数据类型设置为 BINDER_TYPE_FD
- binder内核对 BINDER_TYPE_FD 类型数据特化处理
- 同时在从binder读取数据创建GraphicBuffer对象时，调用 GraphicBuffer.unflatten，内部调用 IMapper.importBuffer 把内存映射到当前进程

后记

Android12 之后使用 BLASTBufferQueue ，虽然有些变化，但是理解了 GraphicBuffer 和 hidl_handle 传递 fd 的过程，这些都游刃有余

Android 的 aidl接口层、hidl接口层、binder bp 接口层都隐藏了很多关键代码，导致看代码时，感觉总是云里雾里

像hidl接口，生成的大量代码，在out/soong目录下，仅仅看源码树目录下的代码根本找不到好吧。

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