Android IPC机制—Binder的工作机制

Binder是一种进程间通信机制

Binder架构

• Binder通信机制采用C/S架构

  
  
  
• Binder框架中主要涉及到4个角色Client、Server、Service Manager及Binder驱动，其中Client、Server、Service Manager运行在用户空间，Binder驱动运行在内核空间
• Client代表客户端进程，Server代表客户端进程提供各种服务，如音视频等
• Service Manager用来管理各种系统服务
• Binder驱动提供进程间通信的能力
• 用户空间的Client、Server、ServiceManager通过open、mmap和ioctl等标准文件操作(详见Unix环境编程)来访问/dev/binder，进而实现进程间通信

基础数据结构

Binder驱动

/kernel/drivers/staging/android/binder.c
device_initcall(binder_init);

设备初始化时候会调用binder_init进行binder驱动初始化

/kernel/drivers/staging/android/binder.c
//绑定binder驱动操作函数
static const struct file_operations binder_fops = {
    .owner = THIS_MODULE,
    .poll = binder_poll,
    .unlocked_ioctl = binder_ioctl,
    .compat_ioctl = binder_ioctl,
    .mmap = binder_mmap,
    .open = binder_open,
    .flush = binder_flush,
    .release = binder_release,
};

//创建misc类型的驱动
static struct miscdevice binder_miscdev = {
    .minor = MISC_DYNAMIC_MINOR,
    .name = \"binder\",
    .fops = &binder_fops//绑定binder驱动操作函数
};

//binder驱动初始化
static int __init binder_init(void)
{
    int ret;

    binder_deferred_workqueue = create_singlethread_workqueue(\"binder\");
    if (!binder_deferred_workqueue)
        return -ENOMEM;

    //创建目录/binder
    binder_debugfs_dir_entry_root = debugfs_create_dir(\"binder\", NULL);
    if (binder_debugfs_dir_entry_root)
        //创建目录/binder/proc
        binder_debugfs_dir_entry_proc = debugfs_create_dir(\"proc\",
                         binder_debugfs_dir_entry_root);
    //注册binder驱动
    ret = misc_register(&binder_miscdev);
    //创建其他文件
    if (binder_debugfs_dir_entry_root) {
        //创建文件/binder/proc/state
        debugfs_create_file(\"state\",
                    S_IRUGO,
                    binder_debugfs_dir_entry_root,
                    NULL,
                    &binder_state_fops);
        //创建文件/binder/proc/stats
        debugfs_create_file(\"stats\",
                    S_IRUGO,
                    binder_debugfs_dir_entry_root,
                    NULL,
                    &binder_stats_fops);
        //创建文件/binder/proc/transactions
        debugfs_create_file(\"transactions\",
                    S_IRUGO,
                    binder_debugfs_dir_entry_root,
                    NULL,
                    &binder_transactions_fops);
        //创建文件/binder/proc/transaction_log
        debugfs_create_file(\"transaction_log\",
                    S_IRUGO,
                    binder_debugfs_dir_entry_root,
                    &binder_transaction_log,
                    &binder_transaction_log_fops);
        //创建文件/binder/proc/failed_transaction_log
        debugfs_create_file(\"failed_transaction_log\",
                    S_IRUGO,
                    binder_debugfs_dir_entry_root,
                    &binder_transaction_log_failed,
                    &binder_transaction_log_fops);
    }
    return ret;
}

初始化主要做了两件事情

• 初始化存储binder存储信息的目录
• 创建binder设备，并绑定操作函数如binder_open、binder_mmap、binder_ioctl等
• 设备启动时候，会调用binder_init，主要做两件事情
• 1.创建/binder/proc目录，之后在这个目录下创建state、stats、transactions、transaction_log、failed_transaction_log文件夹，分别存储进程通信的各种数据
• 2.注册驱动，并绑定文件操作函数binder_open、binder_mmap、binder_ioctl等，之后就可以通过RPC机制去访问binder驱动

Native层级的Binder结构

Binder通信机制

Binder进程与线程

ServiceManager启动

预备知识补充

ServiceManager启动流程主要分为三个流程
1.以系统服务形式启动service_manager,之后通过在binder_open函数打开驱动设备，驱动层相应的就会创建service_manager对应的binder_proc，并且这是个特殊的service
2.调用binder_become_context_manager通过ioctl调用内核中的binder_ioctl,经过一系列处理后，binder驱动会将这个特殊的binder_node存到静态指针binder_context_mgr_node

static struct binder_node *binder_context_mgr_node;
...
static int binder_ioctl_set_ctx_mgr(struct file *filp)
{
    ...
    //注意这里后续两个参数都是0
    binder_context_mgr_node = binder_new_node(proc, 0, 0);
    ...
}
...

3.调用binder_loop进入循环解析的过程

int main(int argc, char** argv){
    ...
    //进入循环，等待或处理Client进程的通信请求
    binder_loop(bs, svcmgr_handler);
    ...
}

这里指定循环处理函数为svcmgr_handler,后续再仔细分析这个函数，先看binder_loop实现

void binder_loop(struct binder_state *bs, binder_handler func)
{
    ...
    //通信数据
    struct binder_write_read bwr;
    
     bwr.write_size = 0;
    bwr.write_consumed = 0;
    bwr.write_buffer = 0;

    readbuf[0] = BC_ENTER_LOOPER;
    binder_write(bs, readbuf, sizeof(uint32_t));
    
    for (;;) {
        bwr.read_size = sizeof(readbuf);
        bwr.read_consumed = 0;
        bwr.read_buffer = (uintptr_t) readbuf;
        
        res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr);
        ...
        //解析
        res = binder_parse(bs, 0, (uintptr_t) readbuf, bwr.read_consumed, func);
        ...
    }
    
}

int binder_write(struct binder_state *bs, void *data, size_t len)
{
    struct binder_write_read bwr;
    int res;

    bwr.write_size = len;
    bwr.write_consumed = 0;
    bwr.write_buffer = (uintptr_t) data;
    bwr.read_size = 0;
    bwr.read_consumed = 0;
    bwr.read_buffer = 0;
    res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr);
    ...
    return res;
}

• 首先创建一个binder_write_read结构体，然后通过binder_write向Binder驱动写入命令协议BC_ENTER_LOOPER，注意这个理输出缓冲区是没有数据的,binder驱动经过一系列处理进入循环状态，之后通过一个死循环来不断的从Binder驱动读取数据，之后交由binder_parse去解析各种协议数据，后续再分析细节
• Binder驱动是如何处理交互细节的，我们来看下binder_ioctl_write_read的实现

static int binder_ioctl_write_read(struct file *filp,
                unsigned int cmd, unsigned long arg,
                struct binder_thread *thread)
{
    ...
    //从文件句柄取出进程信息
    struct binder_proc *proc = filp->private_data;
    //命令协议
    unsigned int size = _IOC_SIZE(cmd);
    ...
    struct binder_write_read bwr;
    //取出bwr进程通信协议载体
    if (copy_from_user(&bwr, ubuf, sizeof(bwr))) {
        ...
    }
    //如果有写入数据，就交由binder_thread_write去处理，之后
    //通过copy_to_user将数据返还给用户空间
    if (bwr.write_size > 0) {
        ret = binder_thread_write(proc, thread,
                      bwr.write_buffer,
                      bwr.write_size,
                      &bwr.write_consumed);
        ...
        if (ret < 0) {
            bwr.read_consumed = 0;
            if (copy_to_user(ubuf, &bwr, sizeof(bwr)))
                ...
            goto out;
        }
    }
    
    //如果有输出数据，则调用binder_thread_read解析，
    //之后判断进程的事务队里是否为空，如果不为空就等待执行
    if (bwr.read_size > 0) {
        ret = binder_thread_read(proc, thread, bwr.read_buffer,
                     bwr.read_size,
                     &bwr.read_consumed,
                     filp->f_flags & O_NONBLOCK);
        ...
        if (!list_empty(&proc->todo))
            wake_up_interruptible(&proc->wait);
        if (ret < 0) {
            //将写出数据返还给用户空间
            if (copy_to_user(ubuf, &bwr, sizeof(bwr)))
                ...
            goto out;
        }
    }
    ...
out:
    return ret;
}

• 至于binder_thread_write与binder_thread_read则是处理命令协(binder_driver_command_protocol)与返回协议(binder_driver_return_protocol )详见/drivers/staging/android/binder.h

ServiceManager注册服务

• service的注册流程如上图，这里以media_server的注册为例子去看代码

/frameworks/av/services/mediadrm/mediadrmserver.rc
service mediadrm /system/bin/mediadrmserver
    class main
    user media
    group mediadrm drmrpc
    ioprio rt 4
    writepid /dev/cpuset/foreground/tasks

• 可以看到以用户权限启动mediadrm服务，文件位于/system/bin/mediadrmserver
• 接下来看main_mediadrmserver.cpp

/frameworks/av/services/mediadrm/mediadrmserver.cpp
int main()
{
    ...
    sp<ProcessState> proc(ProcessState::self());
    sp<IServiceManager> sm = defaultServiceManager();
    ...
    MediaDrmService::instantiate();
    ProcessState::self()->startThreadPool();
    IPCThreadState::self()->joinThreadPool();
}

• main()中主要做了两件事情：
  1.初始化MediaDrmService
  2.线程池初始化
  先看第一个

/frameworks/av/services/mediadrm/MediaDrmService.cpp
...
void MediaDrmService::instantiate() {
    defaultServiceManager()->addService(
            String16(\"media.drm\"), new MediaDrmService());
}
...

• 这里面主要通过defaultServiceManager()获取一个BpServiceManager指针然后调用addService，注意这里传入的两个参数“media.drm”和一个MediaDrmService对象

ServiceManager代理对象的获取

• 先来看defaultServiceManager()调用

/frameworks/native/libs/binder/Static.cpp
...
sp<IServiceManager> gDefaultServiceManager;
...

/frameworks/native/libs/binder/ISerivceManager.cpp
...
sp<IServiceManager> defaultServiceManager()
{
    if (gDefaultServiceManager != NULL) 
        return gDefaultServiceManager;
    {
        ...
        while (gDefaultServiceManager == NULL) {
            gDefaultServiceManager = interface_cast<IServiceManager>(
                ProcessState::self()->getContextObject(NULL));
                ...
        }
    }

    return gDefaultServiceManager;
}
...

• gDefaultServiceManager就是一个IServiceManger的指针
• 首次获取时候一定是通过interface_cast(ProcessState::self()->getContextObject(NULL))这一系列method调用获取的，一步步分析
• 先看ProcessState::self()

/frameworks/native/libs/binder/ProcessState
...
sp<ProcessState> ProcessState::self()
{
    Mutex::Autolock _l(gProcessMutex);
    if (gProcess != NULL) {
        return gProcess;
    }
    //注意这里的文件name
    gProcess = new ProcessState(\"/dev/binder\");
    return gProcess;
}
...

• 这个类构造一定持有打开/dev/binder的句柄，不信你看构造声明

//这里是cpp构造函数形式，就是成员变量赋值
ProcessState::ProcessState(const char *driver)
    : mDriverName(String8(driver))
    , mDriverFD(open_driver(driver))
    , mVMStart(MAP_FAILED)
    , mThreadCountLock(PTHREAD_MUTEX_INITIALIZER)
    , mThreadCountDecrement(PTHREAD_COND_INITIALIZER)
    , mExecutingThreadsCount(0)
    , mMaxThreads(DEFAULT_MAX_BINDER_THREADS)
    , mStarvationStartTimeMs(0)
    , mManagesContexts(false)
    , mBinderContextCheckFunc(NULL)
    , mBinderContextUserData(NULL)
    , mThreadPoolStarted(false)
    , mThreadPoolSeq(1)
{
if (mDriverFD >= 0) {
        // mmap the binder, providing a chunk of virtual address space to receive transactions.
        //注意这里分配的BINDER_VM_SIZE为1016kb，具体见宏定义处
        mVMStart = mmap(0, BINDER_VM_SIZE, PROT_READ, MAP_PRIVATE | MAP_NORESERVE, mDriverFD, 0);
        if (mVMStart == MAP_FAILED) {
            ...
            close(mDriverFD);
            mDriverFD = -1;
            mDriverName.clear();
        }
    }
    ...
}

• 再来看getContextObject

sp<IBinder> ProcessState::getContextObject(const sp<IBinder>& /*caller*/)
{
    //注意这里传入的值为0
    return getStrongProxyForHandle(0);
}

...
sp<IBinder> ProcessState::getStrongProxyForHandle(int32_t handle)
{
    sp<IBinder> result;
    ...
    handle_entry* e = lookupHandleLocked(handle);
    if (e != NULL) {
        ...
        b = BpBinder::create(handle);
        e->binder = b;
        if (b) e->refs = b->getWeakRefs();
        result = b;
    }else{
        ...
    }
    return result;
}

• getContextObject()内部是调用getStrongProxyForHandle(0)来获取一个IBinder指针(这里0其实就是为了后续查找之前ServiceManager启动注册那个特殊的Service组件)，可以看到其实这里创建的是一个BpBinder对象，并且他的句柄值是0
• 我们来看BpBinder的相关调用

frameworks/native/libs/binder/BpBinder.cpp
//handle句柄值 与binder驱动中的binder引用对象进行关联
BpBinder::BpBinder(int32_t handle, int32_t trackedUid)
    //注意这里传入的是0
    : mHandle(handle)
    //注意这个值为1
    , mAlive(1)
    , mObitsSent(0)
    , mObituaries(NULL)
    , mTrackedUid(trackedUid)
{
    ...
    IPCThreadState::self()->incWeakHandle(handle, this);
}

BpBinder* BpBinder::create(int32_t handle) {
    ...
    return new BpBinder(handle, trackedUid);
}

• 回到defaultServiceManager()，最后interface_cast(BpBinder(0))实际上转换成了这个，这个在哪里定义呢？

/frameworks/native/include/binder/IInterface.h
template<typename INTERFACE>
inline sp<INTERFACE> interface_cast(const sp<IBinder>& obj)
{
    return INTERFACE::asInterface(obj);
}

• 发现他是个内联模板函数，实际上调用的是IServiceManager::asInterface(BpBinder(0))，那么IServiceManager::asInterface在哪里定义呢？

/frameworks/native/libs/binder/IServiceManager.cpp
IMPLEMENT_META_INTERFACE(ServiceManager, \"android.os.IServiceManager\");

看上面这个宏定义，实际定义在
/frameworks/native/include/binder/IInterface.h

#define IMPLEMENT_META_INTERFACE(INTERFACE, NAME)                       
    const ::android::String16 I##INTERFACE::descriptor(NAME);           
    const ::android::String16&                                          
        I##INTERFACE::getInterfaceDescriptor() const {              
        return I##INTERFACE::descriptor;                                
    }                                                                   
    ::android::sp<I##INTERFACE> I##INTERFACE::asInterface(              
            const ::android::sp<::android::IBinder>& obj)               
    {                                                                   
        ::android::sp<I##INTERFACE> intr;                               
        if (obj != NULL) {                                              
            intr = static_cast<I##INTERFACE*>(                          
                obj->queryLocalInterface(                               
                        I##INTERFACE::descriptor).get());               
            if (intr == NULL) {                                         
                intr = new Bp##INTERFACE(obj);                          
            }                                                           
        }                                                               
        return intr;                                                    
    }  

• 带入转换下，发现我们拿到的其实就是一个BpServiceManager
• 接下来我们来看后续的调用

BpServiceManager#addService

virtual status_t addService(const String16& name, const sp<IBinder>& service,bool allowIsolated, int dumpsysPriority) {
        //这里data表示要写入的数据，reply表示返回的数据
        Parcel data, reply;
        //存储描述符\"android.os.IServiceManager\"    
        data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor());
        //存储服务名字\"media.drm\"     
        data.writeString16(name);
        //存储服务MediaDrmService
        data.writeStrongBinder(service);
        data.writeInt32(allowIsolated ? 1 : 0);
        data.writeInt32(dumpsysPriority);
        status_t err = remote()->transact(ADD_SERVICE_TRANSACTION, data, &reply);
        return err == NO_ERROR ? reply.readExceptionCode() : err;
    }

• 这里实际上把上一步传入的数据通过Parcel存起来，然后调用remote()获取一个BpBinder然后调用其transact，传入的code为ADD_SERVICE_TRANSACTION，经过包装后data中的数据长这样子
• 
• 记下来我们去看BpBinder#transact

status_t BpBinder::transact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    // Once a binder has died, it will never come back to life.
    //构造中mAlive=1，所以这里会走进if
    if (mAlive) {
        //这里调用的是IPCThreadState#transact
        status_t status = IPCThreadState::self()->transact(
            mHandle, code, data, reply, flags);
        if (status == DEAD_OBJECT) mAlive = 0;
        return status;
    }

    return DEAD_OBJECT;
}

• 前面ServiceManager代理对象获取BpBinder时候，mHandle为0，code为ADD_SERVICE_TRANSACTION，data存储binder信息，reply存储要返回的信息，flags查看method定义默认参数为0
• 接下来分析IPCThreadState#transact
  /frameworks/native/libs/binder/IPCThreadState.cpp

status_t IPCThreadState::transact(int32_t handle,
                                  uint32_t code, const Parcel& data,
                                  Parcel* reply, uint32_t flags)
{
    status_t err;

    //允许返回中携带文件描述符
    flags |= TF_ACCEPT_FDS;
    ...
    //封装BC_TRANSACTION通信请求
    err = writeTransactionData(BC_TRANSACTION, flags, handle, code, data, NULL);
    ...
    //同步or异步，这里是同步调用
    if ((flags & TF_ONE_WAY) == 0) {
        ...
        if (reply) {
            //向Binder驱动发起BC_TRANSACTION
            err = waitForResponse(reply);
        } else {
            Parcel fakeReply;
            err = waitForResponse(&fakeReply);
            ...
        }
        ...
    }else{
        err = waitForResponse(NULL, NULL);
    }
    return err;
}

• 上面首先将flags与TF_ACCEPT_FDS做或操作，表示接收文件描述符
• 之后调用writeTransactionData将数据封装成一个BC_TRANSACTION命令协议,这里是同步调用切需要返回，所以执行到的是waitForResponse(reply)，接下来分别来看writeTransactionData、waitForResponse的实现

status_t IPCThreadState::writeTransactionData(int32_t cmd, uint32_t binderFlags,
    int32_t handle, uint32_t code, const Parcel& data, status_t* statusBuffer)
    {
    //对应内核中要求io携带binder_transaction_data结构体
    binder_transaction_data tr;

    tr.target.ptr = 0; /* Don\'t pass uninitialized stack data to a remote process */
    tr.target.handle = handle;
    tr.code = code;
    tr.flags = binderFlags;
    tr.cookie = 0;
    tr.sender_pid = 0;
    tr.sender_euid = 0;

    const status_t err = data.errorCheck();
    if (err == NO_ERROR) {
        tr.data_size = data.ipcDataSize();
        tr.data.ptr.buffer = data.ipcData();
        tr.offsets_size = data.ipcObjectsCount()*sizeof(binder_size_t);
        tr.data.ptr.offsets = data.ipcObjects();
    } else if (statusBuffer) {
        tr.flags |= TF_STATUS_CODE;
        *statusBuffer = err;
        tr.data_size = sizeof(status_t);
        tr.data.ptr.buffer = reinterpret_cast<uintptr_t>(statusBuffer);
        tr.offsets_size = 0;
        tr.data.ptr.offsets = 0;
    } else {
        return (mLastError = err);
    }

    mOut.writeInt32(cmd);
    mOut.write(&tr, sizeof(tr));

    return NO_ERROR;
}

• 这里主要是创建一个binder_transaction_data，初其始化后将要传输的数据写入到mOut缓冲区中，最终整个数据长这样子
• 
• 数据封装完成以后，我们来看waitForResponse的实现

status_t IPCThreadState::waitForResponse(Parcel *reply, status_t *acquireResult)
{
    uint32_t cmd;
    int32_t err;
    while (1) {
        if ((err=talkWithDriver()) < NO_ERROR) break;
        ...
        //从输出缓冲区取出返回协议命令
        cmd = (uint32_t)mIn.readInt32();
        ...
    }
    ...
    return err;
}

• 这里可以是一个死循环，不断的通过talkWithDriver去跟binder驱动通信，然后从输出缓冲区中取出返回协议然后处理返回协议，这里先不看具体的协议处理，先来看talkWithDriver的实现

//默认doReceive = true
status_t IPCThreadState::talkWithDriver(bool doReceive)
{
    ...
    binder_write_read bwr;

    // Is the read buffer empty?
    const bool needRead = mIn.dataPosition() >= mIn.dataSize();

    // We don\'t want to write anything if we are still reading
    // from data left in the input buffer and the caller
    // has requested to read the next data.
    const size_t outAvail = (!doReceive || needRead) ? mOut.dataSize() : 0;

    bwr.write_size = outAvail;
    bwr.write_buffer = (uintptr_t)mOut.data();

    // This is what we\'ll read.
    if (doReceive && needRead) {
        bwr.read_size = mIn.dataCapacity();
        bwr.read_buffer = (uintptr_t)mIn.data();
    } else {
        bwr.read_size = 0;
        bwr.read_buffer = 0;
    }
    ...
    // Return immediately if there is nothing to do.
    if ((bwr.write_size == 0) && (bwr.read_size == 0)) return NO_ERROR;

    bwr.write_consumed = 0;
    bwr.read_consumed = 0;
    status_t err;
    do{
        ...
        //这里通过io想驱动写入一个binder_write_read结构体
        if (ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr) >= 0)
            err = NO_ERROR;
        else
            err = -errno;
    } while (err == -EINTR);
    
    if (err >= NO_ERROR) {
        if (bwr.write_consumed > 0) {
            if (bwr.write_consumed < mOut.dataSize())
                mOut.remove(0, bwr.write_consumed);
            else {
                mOut.setDataSize(0);
                processPostWriteDerefs();
            }
        }
        if (bwr.read_consumed > 0) {
            mIn.setDataSize(bwr.read_consumed);
            mIn.setDataPosition(0);
        }
        ...
        return NO_ERROR;
    }

    return err;
}

• 这里主要通过io控制命令向Binder驱动写入一个type为BINDER_WRITE_READ，data为binder_write_read，其输出缓冲区为前面mOut中写入的数据
• 接下来的操作就转到Binder驱动中进行了，需要记住，Clinet进程此时执行到的位置

/platform/drivers/staging/android/binder.c
static long binder_ioctl(struct file *filp, unsigned int cmd, 
    unsigned long arg)
{
    int ret;
    struct binder_proc *proc = filp->private_data;
    struct binder_thread *thread;
    unsigned int size = _IOC_SIZE(cmd);
    ...
    //这里根据进程找到对应的thread，如果没找到就创建一个
    thread = binder_get_thread(proc);
    ...
    switch (cmd) {
    case BINDER_WRITE_READ:
        ret = binder_ioctl_write_read(filp, cmd, arg, thread);
        if (ret)
            goto err;
        break;
    ...
}

• Binder驱动中对应的binder_ioctl()会调用，之后会处理cmd为BINDER_WRITE_READ的分支，之后会调用到binder_ioctl_write_read()

static int binder_ioctl_write_read(struct file *filp,
                unsigned int cmd, unsigned long arg,
                struct binder_thread *thread)
{
    int ret = 0;
    //从文件描述符中取出进程地址
    struct binder_proc *proc = filp->private_data;
    //cmd信息
    unsigned int size = _IOC_SIZE(cmd);
    void __user *ubuf = (void __user *)arg;
    struct binder_write_read bwr;
    //cmd校验
    if (size != sizeof(struct binder_write_read)) {
        ret = -EINVAL;
        goto out;
    }
    //从用户空间中取出binder_write_read结构体
    if (copy_from_user(&bwr, ubuf, sizeof(bwr))) {
        ret = -EFAULT;
        goto out;
    }
    ...
    //输出缓冲区有数据就处理输出缓冲区
    if (bwr.write_size > 0) {
        //这里是真正处理输出缓冲数据的func
        ret = binder_thread_write(proc, thread,
                      bwr.write_buffer,
                      bwr.write_size,
                      &bwr.write_consumed);
        trace_binder_write_done(ret);
        if (ret < 0) {
            bwr.read_consumed = 0;
            if (copy_to_user(ubuf, &bwr, sizeof(bwr)))
                ret = -EFAULT;
            goto out;
        }
    }
    //输入缓冲区有数据就处理输入缓冲区
    if (bwr.read_size > 0) {
        //这里是真正处理输出缓冲数据的func
        ret = binder_thread_read(proc, thread, bwr.read_buffer,
                     bwr.read_size,
                     &bwr.read_consumed,
                     filp->f_flags & O_NONBLOCK);
        trace_binder_read_done(ret);
        //如果进程todo队里不为空，说明有事务正在处理，需要等待处理
        if (!list_empty(&proc->todo))
            wake_up_interruptible(&proc->wait);
        if (ret < 0) {
            if (copy_to_user(ubuf, &bwr, sizeof(bwr)))
                ret = -EFAULT;
            goto out;
        }
    }
    ...
    //将数据copy回用户空间
    if (copy_to_user(ubuf, &bwr, sizeof(bwr))) {
        ret = -EFAULT;
        goto out;
    }
out:
    return ret;
}

• 前面调用可知，输出缓冲区是有数据，输入缓冲区是没有数据的，所以上面方法执行流程应该是，先调用binder_thread_write去处理输出缓冲区

static int binder_thread_write(struct binder_proc *proc,
            struct binder_thread *thread,
            binder_uintptr_t binder_buffer, size_t size,
            binder_size_t *consumed)
{
    uint32_t cmd;
    void __user *buffer = (void __user *)(uintptr_t)binder_buffer;
    void __user *ptr = buffer + *consumed;
    void __user *end = buffer + size;
    
    while (ptr < end && thread->return_error == BR_OK) {
        //根据cmd取出消费的数据偏移地址
        if (get_user(cmd, (uint32_t __user *)ptr))
            return -EFAULT;
        ptr += sizeof(uint32_t);
        ...
        switch (cmd) {
            ...
            case BC_TRANSACTION:
            case BC_REPLY: {
                struct binder_transaction_data tr;
    
                if (copy_from_user(&tr, ptr, sizeof(tr)))
                    return -EFAULT;
                ptr += sizeof(tr);
                binder_transaction(proc, thread, &tr, cmd == BC_REPLY);
            break;
            ...
        }
        *consumed = ptr - buffer;
    }
    return 0;   
}   

• 这里是取出cmd，然后处理BC_TRANSACTION时候，再讲binder_transaction_data取出，之后交由binder_transaction去处理
• binder_transaction比较长，其实主要分为三个部分
• 1.初始化目标线程进程
• 2.封装返回数据binder_transaction_data
• 3.为binder_transaction_data分配合适的线程or进程

static void binder_transaction(struct binder_proc *proc,
                   struct binder_thread *thread,
                   struct binder_transaction_data *tr, int reply)
{
    struct binder_transaction *t;
    struct binder_work *tcomplete;
    binder_size_t *offp, *off_end;
    binder_size_t off_min;
    struct binder_proc *target_proc;
    struct binder_thread *target_thread = NULL;
    struct binder_node *target_node = NULL;
    struct list_head *target_list;
    wait_queue_head_t *target_wait;
    struct binder_transaction *in_reply_to = NULL;
    ...
    uint32_t return_error;
    ...
    //如果是reply走if分支，否则走else分支，这里走else
    if (reply) {
        ...
    }else{
        //根据句柄找对应的binder_ref、binder_node
        if (tr->target.handle) {
            struct binder_ref *ref;
            ref = binder_get_ref(proc, tr->target.handle);
            ...
        else{
            //handle=0，则需要找指向service_manager的binder_node
            target_node = binder_context_mgr_node;
            ...
        }
        ...
        //根据binder_node找到对应的进程binder_proc，这里也就是service_manager
        target_proc = target_node->proc;
        ...
        //如果是同步请求，尝试寻找一个在等待其他事物执行的线程，tips优化调度
        if (!(tr->flags & TF_ONE_WAY) && thread->transaction_stack) {
            struct binder_transaction *tmp;
            tmp = thread->transaction_stack;
            ...
            //找到一个等待别的事务完成的依赖线程
            while (tmp) {
                if (tmp->from && tmp->from->proc == target_proc)
                    target_thread = tmp->from;
                tmp = tmp->from_parent;
            }
        }
    }
    //有空闲线程就是用空闲线程的等待队列，否则使用进程的事物队列
    if (target_thread) {
        e->to_thread = target_thread->pid;
        target_list = &target_thread->todo;
        target_wait = &target_thread->wait;
    } else {
        target_list = &target_proc->todo;
        target_wait = &target_proc->wait;
    }
    ...
    //创建binder_transaction结构体，BINDER_WORK_TRANSACTION
    //用于向目标进程发送数据
    t = kzalloc(sizeof(*t), GFP_KERNEL);
    ...
    binder_stats_created(BINDER_STAT_TRANSACTION);
    ...
    //创建binder_work结构体,BINDER_STAT_TRANSACTION_COMPLETE
    //便于向源进程返回数据处理结果
    tcomplete = kzalloc(sizeof(*tcomplete), GFP_KERNEL);
    ...
    binder_stats_created(BINDER_STAT_TRANSACTION_COMPLETE);
    ...
    //初始化t
    if (!reply && !(tr->flags & TF_ONE_WAY))
        t->from = thread;
    else
        t->from = NULL;
    t->sender_euid = task_euid(proc->tsk);
    //目标进程
    t->to_proc = target_proc;
    //目标线程
    t->to_thread = target_thread;
    //code ADD_SERVICE_TRANSACTION
    t->code = tr->code;
    //flag = TF_ACCEPT_FLAGS
    t->flags = tr->flags;
    //优先级
    t->priority = task_nice(current);
    ...
    //缓冲区
    t->buffer = binder_alloc_buf(target_proc, tr->data_size,
        tr->offsets_size, !reply && (t->flags & TF_ONE_WAY));
    ...
    t->buffer->allow_user_free = 0;
    t->buffer->debug_id = t->debug_id;
    t->buffer->transaction = t;
    t->buffer->target_node = target_node;
    ...
    //为t分配内核缓冲区
    if (target_node)
        binder_inc_node(target_node, 1, 0, NULL);
    
    offp = (binder_size_t *)(t->buffer->data +
                 ALIGN(tr->data_size, sizeof(void *)));
     //数据copy
    if (copy_from_user(t->buffer->data, (const void __user *)(uintptr_t)
               tr->data.ptr.buffer, tr->data_size)) {
        ...
        goto err_copy_data_failed;
    }
    if (copy_from_user(offp, (const void __user *)(uintptr_t)
               tr->data.ptr.offsets, tr->offsets_size)) {
        ...
        goto err_copy_data_failed;
    }
    ...
    
    //处理Binder请求，内核中很多都是地址起止位置操作
    off_end = (void *)offp + tr->offsets_size;
    off_min = 0;
    for (; offp < off_end; offp++) {
        struct flat_binder_object *fp;
        ...
        //前面存储的MediaDrmService信息
        fp = (struct flat_binder_object *)(t->buffer->data + *offp);
        off_min = *offp + sizeof(struct flat_binder_object);
        switch(fp->type){
            case BINDER_TYPE_BINDER:
            case BINDER_TYPE_WEAK_BINDER: {
                struct binder_ref *ref;
                struct binder_node *node = 
                    binder_get_node(proc, fp->binder);
                //如果是首次就创建新的binder_node
                if (node == NULL) {
                node = binder_new_node(proc, fp->binder, fp->cookie);
                ...
                //设定线程优先级
                node->min_priority = fp->flags & FLAT_BINDER_FLAG_PRIORITY_MASK;
                //设置是否接收文件描述符
                node->accept_fds = !!(fp->flags & FLAT_BINDER_FLAG_ACCEPTS_FDS);
                }
                ...
                //如果是首次就创建对应的binder_ref对象
                ref = binder_get_ref_for_node(target_proc, node);
                if (ref == NULL) {
                    return_error = BR_FAILED_REPLY;
                    goto err_binder_get_ref_for_node_failed;
                }
                //修改flat_binder_objec.type
                if (fp->type == BINDER_TYPE_BINDER)
                    fp->type = BINDER_TYPE_HANDLE;
                else
                    fp->type = BINDER_TYPE_WEAK_HANDLE;
                //设置句柄
                fp->handle = ref->desc;
                //增加binder_ref的引用计数
                binder_inc_ref(ref, fp->type == BINDER_TYPE_HANDLE,
                           &thread->todo);
               ...
            }break;
            ...
        }
    }
    
    //分配事务t的要进入那个栈
    if (reply) {
        ...
        binder_pop_transaction(target_thread, in_reply_to);
    } else if (!(t->flags & TF_ONE_WAY)) {
        ...
        t->need_reply = 1;
        t->from_parent = thread->transaction_stack;
        thread->transaction_stack = t;
    } else {
        ...
        if (target_node->has_async_transaction) {
            target_list = &target_node->async_todo;
            target_wait = NULL;
        } else
            target_node->has_async_transaction = 1;
    }
    //将binder_transaction_data指针t的类型修改为BINDER_WORK_TRANSACTION
    t->work.type = BINDER_WORK_TRANSACTION;
    //添加到target_list队列尾部
    list_add_tail(&t->work.entry, target_list);
    //将binder_work指针tcomplete.type置为BINDER_WORK_TRANSACTION_COMPLETE
    tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE;
    list_add_tail(&tcomplete->entry, &thread->todo);
    //这里有两个执行分支
    //1.处理类型为BINDER_WORK_TRANSACTION的binder_transaction_data
    //2.处理类型为BINDER_WORK_TRANSACTION_COMPLETE的binder_work
    if (target_wait)
        wake_up_interruptible(target_wait);
    return;
    ...
}

• 经过上面func以后，binder驱动中就会为MediaDrmService创建对应的binder_node并加入到整个Binder实体对象的红黑树中，接着会分别向Client进程、ServiceManager发送一个BINDER_WORK_TRANSACTION_COMPLETE的binder_work及BINDER_WORK_TRANSACTION的binder_transaction
• 至此，源线程thread、target_proc或者target_thread会并发的去执行各自todo队列中的任务
• 先来看源线程,回到binder_ioctl_write_read中，接下来要处理输入缓冲区，对应的调用binder_thread_read

static int binder_thread_read(struct binder_proc *proc,
                  struct binder_thread *thread,
                  binder_uintptr_t binder_buffer, size_t size,
                  binder_size_t *consumed, int non_block)
{
    void __user *buffer = (void __user *)(uintptr_t)binder_buffer;
    void __user *ptr = buffer + *consumed;
    void __user *end = buffer + size;

    int ret = 0;
    int wait_for_proc_work;
    ...
    //线程唤醒
    while (1) {
        uint32_t cmd;
        struct binder_transaction_data tr;
        struct binder_work *w;
        struct binder_transaction *t = NULL;
        //检查工作队列，并将待处理项赋值给w
        if (!list_empty(&thread->todo)) {
            w = list_first_entry(&thread->todo, struct binder_work,
                         entry);
        } else if (!list_empty(&proc->todo) && wait_for_proc_work) {
            w = list_first_entry(&proc->todo, struct binder_work,
                         entry);
        } else {
            /* no data added */
            if (ptr - buffer == 4 &&
                !(thread->looper & BINDER_LOOPER_STATE_NEED_RETURN))
                goto retry;
            break;
        }
        switch (w->type) {
            ...
            case BINDER_WORK_TRANSACTION_COMPLETE: {
                cmd = BR_TRANSACTION_COMPLETE;
                if (put_user(cmd, (uint32_t __user *)ptr))
                    return -EFAULT;
                ptr += sizeof(uint32_t);
    
                binder_stat_br(proc, thread, cmd);
                ...
                //删除binder_work，并释放资源
                list_del(&w->entry);
                kfree(w);
                binder_stats_deleted(BINDER_STAT_TRANSACTION_COMPLETE);
            } break;
        }
done:
    *consumed = ptr - buffer;
    ...
    return 0;
}

• 这里返回一个BR_TRANSACTION_COMPLETE协议，之后会经过一些列调用会回到IPCThreadState#waitForResponse中

status_t IPCThreadState::waitForResponse(Parcel *reply, status_t *acquireResult)
{
    ...
    uint32_t cmd;
    int32_t err;

    while (1) {
        ...
        cmd = (uint32_t)mIn.readInt32();
        ...
        switch (cmd) {
        case BR_TRANSACTION_COMPLETE:
            if (!reply && !acquireResult) goto finish;
            break;
        }
        ...
    }
finish:
    ...
    return error;
}

• IPCThreadState::waitForResponse对于BR_TRANSACTION_COMPLETE处理比较简单，就直接返回了
• 我们来看下target_proc即ServiceManager是怎么接收处理BINDER_WORK_TRANSACTION类型的binder_transaction的，假设ServiceManager之前没有通信，那么他就在Binder驱动中一直等待事务的到来，现在有事务了那么对应的就会调用binder_read_thread

static int binder_thread_read(struct binder_proc *proc,
                  struct binder_thread *thread,
                  binder_uintptr_t binder_buffer, size_t size,
                  binder_size_t *consumed, int non_block)
{
    void __user *buffer = (void __user *)(uintptr_t)binder_buffer;
    void __user *ptr = buffer + *consumed;
    void __user *end = buffer + size;
    ...
    //线程唤醒
    while (1) {
        uint32_t cmd;
        struct binder_transaction_data tr;
        struct binder_work *w;
        struct binder_transaction *t = NULL;

        //检查工作队列，并将待处理项赋值给w
        if (!list_empty(&thread->todo)) {
            w = list_first_entry(&thread->todo, struct binder_work,
                         entry);
        } else if (!list_empty(&proc->todo) && wait_for_proc_work) {
            w = list_first_entry(&proc->todo, struct binder_work,
                         entry);
        } else {
            /* no data added */
            if (ptr - buffer == 4 &&
                !(thread->looper & BINDER_LOOPER_STATE_NEED_RETURN))
                goto retry;
            break;
        }
        ...
        switch (w->type) {
        case BINDER_WORK_TRANSACTION: {
            t = container_of(w, struct binder_transaction, work);
        } break;
        ...
        }
    //返回协议的处理，现在target_node指向的是binder_context_mgr_node
        if (t->buffer->target_node) {
            struct binder_node *target_node = t->buffer->target_node;

            tr.target.ptr = target_node->ptr;
            tr.cookie =  target_node->cookie;
            //保存原本的线程优先级，便于后续恢复
            t->saved_priority = task_nice(current);
            //修改binder驱动中对应proc的线程优先级（模拟Client进程的线程优先级）
            if (t->priority < target_node->min_priority &&
                !(t->flags & TF_ONE_WAY))
                binder_set_nice(t->priority);
            else if (!(t->flags & TF_ONE_WAY) ||
                 t->saved_priority > target_node->min_priority)
                binder_set_nice(target_node->min_priority);
            cmd = BR_TRANSACTION;
        } else {
            ...
        }
        //拷贝code与flags，
        //注册服务过程中这里是ADD_SERVICE_TRANSACTION、TF_ACCEPT_FDS
        tr.code = t->code;
        tr.flags = t->flags;
        if (t->from) {
            struct task_struct *sender = t->from->proc->tsk;
            tr.sender_pid = task_tgid_nr_ns(sender,
                            task_active_pid_ns(current));
        } else {
            ...
        }
        //Binder驱动程序分配给进程的内核缓冲区同时，
        //映射了用户的内核地址、用户空间地址
        tr.data_size = t->buffer->data_size;
        tr.offsets_size = t->buffer->offsets_size;
        tr.data.ptr.buffer = (binder_uintptr_t)(
                    (uintptr_t)t->buffer->data +
                    proc->user_buffer_offset);
        //直接操作offsets
        tr.data.ptr.offsets = tr.data.ptr.buffer +
                    ALIGN(t->buffer->data_size,
                        sizeof(void *));

        //提供一个返回协议数据的缓冲区
        if (put_user(cmd, (uint32_t __user *)ptr))
            return -EFAULT;
        ptr += sizeof(uint32_t);
        if (copy_to_user(ptr, &tr, sizeof(tr)))
            return -EFAULT;
        ptr += sizeof(tr);
        ...
        list_del(&t->work.entry);
        t->buffer->allow_user_free = 1;
        //cmd = BR_TRANSACTION && 不是正在处理异步通信请求
        //就需要等待该同步进程通信完以后再进行下一步的操作
        if (cmd == BR_TRANSACTION && !(t->flags & TF_ONE_WAY)) {
            t->to_parent = thread->transaction_stack;
            t->to_thread = thread;
            thread->transaction_stack = t;
        } else {
            t->buffer->transaction = NULL;
            kfree(t);
            binder_stats_deleted(BINDER_STAT_TRANSACTION);
        }
        break;
    }
    ...
    return 0
}       

• 这里实际上是返回一个BR_TRANSACTION协议，并且将之前通过binder_transaction传输的数据封装到binder_transaction_data中，由于在service_manager启动中，进入binder_loop时候指定的函数引用为svcmgr_handler,binder_loop中会循环通过ioctl控制命令去与内核交互数据，binder_parse用于解析数据

platform/frameworks/native/cmds/servicemanager/binder.c
int binder_parse(struct binder_state *bs, struct binder_io *bio,
                 uintptr_t ptr, size_t size, binder_handler func)
{
    int r = 1;
    uintptr_t end = ptr + (uintptr_t) size;
    while (ptr < end) {
        uint32_t cmd = *(uint32_t *) ptr;
        ptr += sizeof(uint32_t);
        ...
        case BR_TRANSACTION: {
            struct binder_transaction_data *txn = 
                (struct binder_transaction_data *) ptr;
            ...
            if (func) {
                unsigned rdata[256/4];
                struct binder_io msg;
                struct binder_io reply;
                int res;
                bio_init(&reply, rdata, sizeof(rdata), 4);
                bio_init_from_txn(&msg, txn);
                //调用svcmgr_handler去处理
                res = func(bs, txn, &msg, &reply);
                if (txn->flags & TF_ONE_WAY) {
                    binder_free_buffer(bs, txn->data.ptr.buffer);
                } else {
                    //返回注册的结果给binder驱动
                    binder_send_reply(bs, &reply, 
                            txn->data.ptr.buffer, res);
                }
            }
            break;
        }   
}

• 这其中有三个结构体binder_io主要存储数据传输，定义如下:

struct binder_io
{
    char *data;            /* pointer to read/write from */
    binder_size_t *offs;   /* array of offsets */
    size_t data_avail;     /* bytes available in data buffer */
    size_t offs_avail;     /* entries available in offsets array */

    char *data0;           /* start of data buffer */
    binder_size_t *offs0;  /* start of offsets buffer */
    uint32_t flags;
    uint32_t unused;
};

• binder_parse中先通过bio_init去初始化reply，然后通过bio_init_from_txn去初始化msg，就是数据对齐的过程及flag设置，这里不再细述

• 我们重点关注func,也就是svcmgr_handler，先来回顾前面数据那张图

  
  
  

uint16_t svcmgr_id[] = {
    \'a\',\'n\',\'d\',\'r\',\'o\',\'i\',\'d\',\'.\',\'o\',\'s\',\'.\',
    \'I\',\'S\',\'e\',\'r\',\'v\',\'i\',\'c\',\'e\',\'M\',\'a\',\'n\',\'a\',\'g\',\'e\',\'r\'
};
...
int svcmgr_handler(struct binder_state *bs,
                   struct binder_transaction_data *txn,
                   struct binder_io *msg,
                   struct binder_io *reply)
{
    struct svcinfo *si;
    uint16_t *s;
    size_t len;
    uint32_t handle;
    uint32_t strict_policy;
    int allow_isolated;
    ...
    strict_policy = bio_get_uint32(msg);
    s = bio_get_string16(msg, &len);
    ...
    //svcmgr_id校验,是否为“android.os.IServiceManager”
    if ((len != (sizeof(svcmgr_id) / 2)) ||
        memcmp(svcmgr_id, s, sizeof(svcmgr_id))) {
        fprintf(stderr,\"invalid id %s\\n\", str8(s, len));
        return -1;
    }
    ...
    switch(txn->code) {
        ...
        //枚举值对应ADD_SERVICE_TRANSACTION
        case SVC_MGR_ADD_SERVICE:
        s = bio_get_string16(msg, &len);
        if (s == NULL) {
            return -1;
        }
        //取出binder_引用对象的句柄值
        handle = bio_get_ref(msg);
        allow_isolated = bio_get_uint32(msg) ? 1 : 0;
        dumpsys_priority = bio_get_uint32(msg);
        if (do_add_service(bs, s, len, handle, 
            txn->sender_euid, allow_isolated, dumpsys_priority,
                           txn->sender_pid))
            return -1;
        break;
   }
   ...
   bio_put_uint32(reply, 0);
   return 0; 
}

• 从msg中取出对应的Service的handle、name，然后调用do_add__service去执行后续的操作
• 看do_add_service之前我们先来看一个结构体svcinfo

struct svcinfo
{
    //指向下一个引用
    struct svcinfo *next;
    //句柄值
    uint32_t handle;
    //死亡代理通知
    struct binder_death death;
    int allow_isolated;
    uint32_t dumpsys_priority;
    size_t len;
    //服务名称
    uint16_t name[0];
};

• 在ServiceManager中每一个服务对应一个svcinfo结构体
• 接下来我们看do_add_service的实现

int do_add_service(struct binder_state *bs, const uint16_t *s, 
    size_t len, uint32_t handle, uid_t uid, int allow_isolated, 
    uint32_t dumpsys_priority, pid_t spid) {
    //存储要注册的服务信息
    struct svcinfo *si;
    ...
    //检查权限
    if (!svc_can_register(s, len, spid, uid)) {
        ALOGE(\"add_service(\'%s\',%x) uid=%d - PERMISSION DENIED\\n\",
             str8(s, len), handle, uid);
        return -1;
    }
    /先去找这个服务
    si = find_svc(s, len);
    if (si) {
        if (si->handle) {
            ...
            svcinfo_death(bs, si);
        }
        si->handle = handle;
    } else {
        //注册服务
        si = malloc(sizeof(*si) + (len + 1) * sizeof(uint16_t));
        ...
        si->handle = handle;
        si->len = len;
        memcpy(si->name, s, (len + 1) * sizeof(uint16_t));
        si->name[len] = \'\\0\';
        si->death.func = (void*) svcinfo_death;
        si->death.ptr = si;
        si->allow_isolated = allow_isolated;
        si->dumpsys_priority = dumpsys_priority;
        //绑定到svclist中
        si->next = svclist;
        svclist = si;
    }

    //增加引用，避免被销毁
    binder_acquire(bs, handle);
    //绑定死亡通知
    binder_link_to_death(bs, handle, &si->death);
    return 0;
}

• 这里先检查Service是否有注册权限(不同版本内核加固调用不同，感兴趣可以查看selinux)，然后先去尝试查找这个服务存在不，如果不存在就分配一个新的struct svcinfo，并将其挂到svclist中，由此可见在service_manager中是维护这一个所有Service组件信息的svclist的
• 回到binder_parse中,接下来会调用binder_send_reply向Binder驱动发送一个BC_REPLY

void binder_send_reply(struct binder_state *bs,
                       struct binder_io *reply,
                       binder_uintptr_t buffer_to_free,
                       int status)
{
    //匿名结构体
    struct {
        uint32_t cmd_free;
        binder_uintptr_t buffer;
        uint32_t cmd_reply;
        struct binder_transaction_data txn;
    } __attribute__((packed)) data;

    data.cmd_free = BC_FREE_BUFFER;
    data.buffer = buffer_to_free;
    data.cmd_reply = BC_REPLY;
    data.txn.target.ptr = 0;
    data.txn.cookie = 0;
    data.txn.code = 0;
    if (status) {
        //通信中产生了错误
        data.txn.flags = TF_STATUS_CODE;
        data.txn.data_size = sizeof(int);
        data.txn.offsets_size = 0;
        data.txn.data.ptr.buffer = (uintptr_t)&status;
        data.txn.data.ptr.offsets = 0;
    } else {
        //成功处理的一次通信请求
        data.txn.flags = 0;
        data.txn.data_size = reply->data - reply->data0;
        data.txn.offsets_size = ((char*) reply->offs) - ((char*) reply->offs0);
        data.txn.data.ptr.buffer = (uintptr_t)reply->data0;
        data.txn.data.ptr.offsets = (uintptr_t)reply->offs0;
    }
    //向Binder驱动写入数据
    binder_write(bs, &data, sizeof(data));
}

• 这里会将进程通信结果写入到匿名struct data中，然后调用binder_write去向内核写入BC_FREE_BUFFER\\BC_REPLY命令协议

int binder_write(struct binder_state *bs, void *data, size_t len)
{
    struct binder_write_read bwr;
    int res;

    bwr.write_size = len;
    bwr.write_consumed = 0;
    bwr.write_buffer = (uintptr_t) data;
    bwr.read_size = 0;
    bwr.read_consumed = 0;
    bwr.read_buffer = 0;
    res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr);
    if (res < 0) {
        fprintf(stderr,\"binder_write: ioctl failed (%s)\\n\",
                strerror(errno));
    }
    return res;
}

• binder_write实际上还是通过IO控制命令写入一个binder_write_read结构体，注意这个结构体输入缓冲区是没有数据的，也就是说不需要处理返回协议
• 略过各种调用我们来看内核中binder_thread_write对于BC_FREE_BUFFER\\BC_REPLY的处理

platform/drivers/staging/android/binder.c
static int binder_thread_write(struct binder_proc *proc,
            struct binder_thread *thread,
            binder_uintptr_t binder_buffer, size_t size,
            binder_size_t *consumed)
{
    uint32_t cmd;
    void __user *buffer = (void __user *)(uintptr_t)binder_buffer;
    void __user *ptr = buffer + *consumed;
    void __user *end = buffer + size;

    while (ptr < end && thread->return_error == BR_OK) {
        if (get_user(cmd, (uint32_t __user *)ptr))
            return -EFAULT;
        ptr += sizeof(uint32_t);
        ...
        switch (cmd) {
            ...
            case BC_TRANSACTION:
            case BC_REPLY: {
            struct binder_transaction_data tr;

            if (copy_from_user(&tr, ptr, sizeof(tr)))
                return -EFAULT;
            ptr += sizeof(tr);
            binder_transaction(proc, thread, &tr, cmd == BC_REPLY);
            break;
            }
        ...
    }

• BC_FREE_BUFFER主要是做一些资源释放的操作，感兴趣可以自己看这里不再细看
• 重点看BC_REPLY,查看binder_transaction的处理

static void binder_transaction(struct binder_proc *proc,
                   struct binder_thread *thread,
                   struct binder_transaction_data *tr, int reply)
{
    ...
    struct binder_transaction *t;
    struct binder_work *tcomplete;
    binder_size_t *offp, *off_end;
    binder_size_t off_min;
    struct binder_proc *target_proc;
    struct binder_thread *target_thread = NULL;
    struct binder_node *target_node = NULL;
    struct list_head *target_list;
    wait_queue_head_t *target_wait;
    struct binder_transaction *in_reply_to = NULL;
    struct binder_transaction_log_entry *e;
    uint32_t return_error;
    ...
    if (reply) {
        //寻找请求通信的thread
        //从之前的thread中取出通信的binder_transaction_data
        in_reply_to = thread->transaction_stack;
        if (in_reply_to == NULL) {
            ...
            return_error = BR_FAILED_REPLY;
            goto err_empty_call_stack;
        }
        //恢复线程优先级
        binder_set_nice(in_reply_to->saved_priority);
        if (in_reply_to->to_thread != thread) {
            ...
            return_error = BR_FAILED_REPLY;
            in_reply_to = NULL;
            goto err_bad_call_stack;
        }
        thread->transaction_stack = in_reply_to->to_parent;
        target_thread = in_reply_to->from;
        if (target_thread == NULL) {
            return_error = BR_DEAD_REPLY;
            goto err_dead_binder;
        }
        if (target_thread->transaction_stack != in_reply_to) {
            ...
            return_error = BR_FAILED_REPLY;
            in_reply_to = NULL;
            target_thread = NULL;
            goto err_dead_binder;
        }
        //目标进程
        target_proc = target_thread->proc;
    }else{
        ...
    }
    //有空闲线程就是用空间线程的等待队列，否则使用进程的
    if (target_thread) {
        ...
        target_list = &target_thread->todo;
        target_wait = &target_thread->wait;
    } else {
        ...
    }
    ...
    //创建binder_transaction结构体，BINDER_WORK_TRANSACTION
    t = kzalloc(sizeof(*t), GFP_KERNEL);
    ...
    //创建binder_work结构体,BINDER_STAT_TRANSACTION_COMPLETE
    tcomplete = kzalloc(sizeof(*tcomplete), GFP_KERNEL);
    ...
    //分配事务t的要进入那个栈
    if (reply) {
        BUG_ON(t->buffer->async_transaction != 0);
        //将事务弹出todo栈
        binder_pop_transaction(target_thread, in_reply_to);
    } else if (!(t->flags & TF_ONE_WAY)) {
        ...
    }else{
        ...
    }
    //将binder_transaction_data指针t的类型修改为BINDER_WORK_TRANSACTION
    t->work.type = BINDER_WORK_TRANSACTION;
    //添加到target_list队列尾部
    list_add_tail(&t->work.entry, target_list);
    //将binder_work指针tcomplete.type置为BINDER_WORK_TRANSACTION_COMPLETE
    tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE;
    list_add_tail(&tcomplete->entry, &thread->todo);
    //这里有两个执行分支
    //1.处理类型为BINDER_WORK_TRANSACTION的binder_transaction_data
    //2.处理类型为BINDER_WORK_TRANSACTION_COMPLETE的binder_work
    if (target_wait)
        wake_up_interruptible(target_wait);
    return;
    ...
}   

• 这里跟只爱去哪调用不同的地方在于走的是if分支，需要查找到之前通信的目标线程及进程，然后将上次通信的binder_transaction弹栈，然后回想之前通信的进程发送一个type为BINDER_WORK_TRANSACTION的binder_work，之前那个进程对应的binder_thread_read处理如下:

static int binder_thread_read(struct binder_proc *proc,
                  struct binder_thread *thread,
                  binder_uintptr_t binder_buffer, size_t size,
                  binder_size_t *consumed, int non_block)
{
    void __user *buffer = (void __user *)(uintptr_t)binder_buffer;
    void __user *ptr = buffer + *consumed;
    void __user *end = buffer + size;

    int ret = 0;
    ...
    //线程唤醒
    while (1) {
        uint32_t cmd;
        struct binder_transaction_data tr;
        struct binder_work *w;
        struct binder_transaction *t = NULL;
        //检查工作队列，并将待处理项赋值给w
        if (!list_empty(&thread->todo)) {
            w = list_first_entry(&thread->todo, struct binder_work,
                         entry);
        } else if (!list_empty(&proc->todo) && wait_for_proc_work) {
            w = list_first_entry(&proc->todo, struct binder_work,
                         entry);
        } else {
            ...
        }
        ...
        switch (w->type) {
            case BINDER_WORK_TRANSACTION: {
                t = container_of(w, struct binder_transaction, work);
            } break;
            ...
        }
        //返回协议的处理，现在target_node指向的是binder_context_mgr_node
        if (t->buffer->target_node) {
            ...
        }else{
            tr.target.ptr = 0;
            tr.cookie = 0;
            cmd = BR_REPLY;
        }
        
        //Binder驱动程序分配给进程的内核缓冲区同时，映射了用户的内核地址、用户空间地址
        tr.data_size = t->buffer->data_size;
        tr.offsets_size = t->buffer->offsets_size;
        tr.data.ptr.buffer = (binder_uintptr_t)(
                    (uintptr_t)t->buffer->data +
                    proc->user_buffer_offset);
        //直接操作offsets
        tr.data.ptr.offsets = tr.data.ptr.buffer +
                    ALIGN(t->buffer->data_size,
                        sizeof(void *));

        //提供一个返回协议数据的缓冲区
        if (put_user(cmd, (uint32_t __user *)ptr))
            return -EFAULT;
        ptr += sizeof(uint32_t);
        if (copy_to_user(ptr, &tr, sizeof(tr)))
            return -EFAULT;
        ptr += sizeof(tr);
        ...
        //cmd = BR_TRANSACTION && 不是正在处理异步通信请求
        //就需要等待该同步进程通信完以后再进行下一步的操作
        if (cmd == BR_TRANSACTION && !(t->flags & TF_ONE_WAY)) {
            ...
        } else {
            t->buffer->transaction = NULL;
            kfree(t);
            binder_stats_deleted(BINDER_STAT_TRANSACTION);
        }
        break;
    }
    ...
}

• 这里就会封装一个BR_REPLY返回协议，然后返回到IPCThreadState::waitForResponse中
• 接下来看IPCThreadState::waitForResponse对于BR_REPLY的处理

status_t IPCThreadState::waitForResponse(Parcel *reply, status_t *acquireResult)
{
    uint32_t cmd;
    ...
    while (1) {
        ...
        cmd = (uint32_t)mIn.readInt32();
        ...
        switch (cmd) {
            case BR_REPLY:
            {
                binder_transaction_data tr;
                err = mIn.read(&tr, sizeof(tr));
                ...
                if (reply) {
                  if ((tr.flags & TF_STATUS_CODE) == 0) {
                     //重置Parcel对象内部数据缓冲区，并指定释放函数为freeBuffer
                     reply->ipcSetDataReference(
                            reinterpret_cast
                            <const uint8_t*>(tr.data.ptr.buffer),
                            tr.data_size,
                            reinterpret_cast
                       <const binder_size_t*>(tr.data.ptr.offsets),
                            tr.offsets_size/sizeof(binder_size_t),
                            freeBuffer, this);
                  } else {
                       ...
                } else {
                    ...
                }
            }
            goto finish;
            ...
        }
    }
    ...
}

• 实际上会调用reply->ipcSetDataReference去重置数据缓冲区，这里不再细述，整个Service注册就大致完成了，后续还有Binder线程的启动感兴趣可以自行查看

ServiceManager查找服务

完整的通信流程