清楚详解Android 进程间图传递图形buffer原理
作者:拉普
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
- Gralloc4Allocator.allocate
- allocateHelper(width, height, format, layerCount, usage, handle, stride, requestorName, true)
- GraphicBufferAllocator.allocate
- BufferQueueProducer.dequeueBuffer 【SurfaceFlinger 进程端】
- 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…
- GraphicBuffer.unflatten 函数内部调用了 GraphicBufferMapper.importBuffer
- read 过程会调用 GraphicBuffer.unflatten
- status_t result =remote()->transact(REQUEST_BUFFER, data, &reply);
- BpGraphicBufferProducer.dequeueBuffer【接口层】
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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