// kernel/drivers/android/binder.c
// allocate = 1 当为 0 时表示释放物理内存
// start = proc->buffer
// end = proc->buffer + PAGE_SIZE
static int binder_update_page_range(struct binder_proc *proc, int allocate,
            void *start, void *end,
            struct vm_area_struct *vma)
{
  void *page_addr;
  unsigned long user_page_addr;
  struct vm_struct tmp_area;
  struct page **page;
  // 描述进程的虚拟地址空间，位于用户空间
  struct mm_struct *mm;

  if (end <= start)
    return 0;

  if (vma)
    mm = NULL;
  else
    mm = get_task_mm(proc->tsk);

  if (mm) {
    down_write(&mm->mmap_sem);
    vma = proc->vma;
  }

  if (allocate == 0)
    goto free_range;

  if (vma == NULL) {
    printk(KERN_ERR "binder: %d: binder_alloc_buf failed to "
           "map pages in userspace, no vma\n", proc->pid);
    goto err_no_vma;
  }

  for (page_addr = start; page_addr < end; page_addr += PAGE_SIZE) {
    int ret;
    struct page **page_array_ptr;
    // 这里取出第 0 个页面
    page = &proc->pages[(page_addr - proc->buffer) / PAGE_SIZE];

    BUG_ON(*page);
    *page = alloc_page(GFP_KERNEL | __GFP_ZERO);
    if (*page == NULL) {
      printk(KERN_ERR "binder: %d: binder_alloc_buf failed "
             "for page at %p\n", proc->pid, page_addr);
      goto err_alloc_page_failed;
    }
    // 内核虚拟内存页面起始和大小描述
    tmp_area.addr = page_addr;
    tmp_area.size = PAGE_SIZE + PAGE_SIZE /* guard page? */;
    // 物理内存页面描述
    page_array_ptr = page;
    // 将内核虚拟内存页面和物理内存页面进行映射
    ret = map_vm_area(&tmp_area, PAGE_KERNEL, &page_array_ptr);
    if (ret) {
      printk(KERN_ERR "binder: %d: binder_alloc_buf failed "
             "to map page at %p in kernel\n",
             proc->pid, page_addr);
      goto err_map_kernel_failed;
    }
    // 用户虚拟内存页面起始地址
    user_page_addr =
      (uintptr_t)page_addr + proc->user_buffer_offset;
    // 将用户虚拟内存页面和物理内存页面进行映射
    ret = vm_insert_page(vma, user_page_addr, page[0]);
    if (ret) {
      printk(KERN_ERR "binder: %d: binder_alloc_buf failed "
             "to map page at %lx in userspace\n",
             proc->pid, user_page_addr);
      goto err_vm_insert_page_failed;
    }
    /* vm_insert_page does not seem to increment the refcount */
  }
  if (mm) {
    up_write(&mm->mmap_sem);
    mmput(mm);
  }
  return 0;

free_range:
  for (page_addr = end - PAGE_SIZE; page_addr >= start;
       page_addr -= PAGE_SIZE) {
    page = &proc->pages[(page_addr - proc->buffer) / PAGE_SIZE];
    if (vma)
      // 仅仅释放用户空间指定的页面???
      zap_page_range(vma, (uintptr_t)page_addr +
        proc->user_buffer_offset, PAGE_SIZE, NULL);
err_vm_insert_page_failed:
    unmap_kernel_range((unsigned long)page_addr, PAGE_SIZE);
err_map_kernel_failed:
    __free_page(*page);
    *page = NULL;
err_alloc_page_failed:
    ;
  }
err_no_vma:
  if (mm) {
    up_write(&mm->mmap_sem);
    mmput(mm);
  }
  return -ENOMEM;
}

// kernel/drivers/android/binder.c
// buffer = 指向用户空间的 4 字节数组，[BC_ENTER_LOOPER, ... ]
// size = 32 * 4
// *consumed = 0
// non_block = 0
static int binder_thread_read(struct binder_proc *proc,
            struct binder_thread *thread,
            void  __user *buffer, int size,
            signed long *consumed, int non_block)
{
  void __user *ptr = buffer + *consumed;
  void __user *end = buffer + size;

  int ret = 0;
  int wait_for_proc_work;

  if (*consumed == 0) {
    // 由于 consumed 位 0，这里向 buffer 里面写入一个 32 位整数 BR_NOOP
    if (put_user(BR_NOOP, (uint32_t __user *)ptr))
      return -EFAULT;
    ptr += sizeof(uint32_t);
  }

retry:
  // 还没有任务可处理，这里为 true
  wait_for_proc_work = thread->transaction_stack == NULL &&
        list_empty(&thread->todo);
  ...
  // 将当前线程的状态置为等待，稍后如果发现有任务会解除该状态
  thread->looper |= BINDER_LOOPER_STATE_WAITING;
  // 如果当前线程自己没有任务可处理，说明已经准备处理进程的任务，于是这里将 ready_threads 加 1
  if (wait_for_proc_work)
    proc->ready_threads++;
  mutex_unlock(&binder_lock);
  if (wait_for_proc_work) {
    // 只有 binder 线程能到这里来取任务，如果不是则记录异常
    if (!(thread->looper & (BINDER_LOOPER_STATE_REGISTERED |
          BINDER_LOOPER_STATE_ENTERED))) {
      binder_user_error("binder: %d:%d ERROR: Thread waiting "
        "for process work before calling BC_REGISTER_"
        "LOOPER or BC_ENTER_LOOPER (state %x)\n",
        proc->pid, thread->pid, thread->looper);
      wait_event_interruptible(binder_user_error_wait,
             binder_stop_on_user_error < 2);
    }
    binder_set_nice(proc->default_priority);
    if (non_block) {
      // 对于非阻塞情况，如果没有任务则返回
      if (!binder_has_proc_work(proc, thread))
        ret = -EAGAIN;
    } else
      // 对于阻塞情况，则等待唤醒，当 binder_has_proc_work 为 true 时才往后面执行
      // binder_has_proc_work 会检查 proc 的 todo 是否有任务。

      // 目前我们就停在这里等待唤醒。
      ret = wait_event_interruptible_exclusive(proc->wait, binder_has_proc_work(proc, thread));
  } else {
    // 此时不会走这里。
    // binder_has_thread_work 会检测 thread 的 todo 是否有任务
    if (non_block) {
      if (!binder_has_thread_work(thread))
        ret = -EAGAIN;
    } else
      ret = wait_event_interruptible(thread->wait, binder_has_thread_work(thread));
  }
  // 当线程被唤醒后会获取锁，在释放之前，其他线程即使被唤醒了也阻塞在这里。
  // 只有当获取锁的线程重新进入上面 retry 释放锁后，其他线程方有机会获取锁。
  mutex_lock(&binder_lock);
  // 如果此前线程没有自己的任务，这里准备执行进程任务了，于是可用的线程需要减 1，并
  // 删除线程的等待标识。
  if (wait_for_proc_work)
    proc->ready_threads--;
  thread->looper &= ~BINDER_LOOPER_STATE_WAITING;
  ...
}

// android_util_binder.cpp
static jobject android_os_BinderInternal_getContextObject(JNIEnv* env, jobject clazz)
{
   // 见 2.1.1.1
    sp<IBinder> b = ProcessState::self()->getContextObject(NULL);
  // 见 2.1.1.2
    return javaObjectForIBinder(env, b);
}

// ProcessState.cpp
sp<ProcessState> ProcessState::self()
{
    Mutex::Autolock _l(gProcessMutex);
    if (gProcess != NULL) {
        return gProcess;
    }
    gProcess = new ProcessState;
    return gProcess;
}

// ProcessState.cpp
sp<IBinder> ProcessState::getContextObject(const sp<IBinder>& /*caller*/)
{
  return getStrongProxyForHandle(0);
}

// handle = 0
sp<IBinder> ProcessState::getStrongProxyForHandle(int32_t handle)
{
  sp<IBinder> result;

  AutoMutex _l(mLock);

  // 根据 handle 在一个 vector 中查找，找不到则创建一个 handle_entry
  handle_entry* e = lookupHandleLocked(handle);

  if (e != NULL) {
    // b 为 NULL
    IBinder* b = e->binder;
    if (b == NULL || !e->refs->attemptIncWeak(this)) {
      if (handle == 0) {
        // 检测 context manager 是否已经注册
        Parcel data;
        status_t status = IPCThreadState::self()->transact(
            0, IBinder::PING_TRANSACTION, data, NULL, 0);
        if (status == DEAD_OBJECT)
          return NULL;
      }

      b = new BpBinder(handle); 
      e->binder = b;
      if (b) e->refs = b->getWeakRefs();
      result = b;
    } else {
      // This little bit of nastyness is to allow us to add a primary
      // reference to the remote proxy when this team doesn't have one
      // but another team is sending the handle to us.
      result.force_set(b);
      e->refs->decWeak(this);
    }
  }

  return result;
}

// android_util_Binder.cpp
// val = new BpBinder(0)
jobject javaObjectForIBinder(JNIEnv* env, const sp<IBinder>& val)
{
  if (val == NULL) return NULL;

  // 检测是否为 android.os.Binder 的子类
  if (val->checkSubclass(&gBinderOffsets)) {
    jobject object = static_cast<JavaBBinder*>(val.get())->object();
    return object;
  }

  // For the rest of the function we will hold this lock, to serialize
  // looking/creation of Java proxies for native Binder proxies.
  AutoMutex _l(mProxyLock);

  // Someone else's...  do we know about it?
  // BpBinder 是刚刚创建的 C++对象，不走这里，下一次就会走这里
  jobject object = (jobject)val->findObject(&gBinderProxyOffsets);
  if (object != NULL) {
    ...
  }

  // 创建一个 android.os.BinderProxy 的 Java 对象
  object = env->NewObject(gBinderProxyOffsets.mClass, gBinderProxyOffsets.mConstructor);
  if (object != NULL) {
    LOGDEATH("objectForBinder %p: created new proxy %p !\n", val.get(), object);
    // 将 BpBinder 的地址赋给 BinderProxy 的 mObject 字段
    env->SetLongField(object, gBinderProxyOffsets.mObject, (jlong)val.get());
    val->incStrong((void*)javaObjectForIBinder);

    // The native object needs to hold a weak reference back to the
    // proxy, so we can retrieve the same proxy if it is still active.
    jobject refObject = env->NewGlobalRef(
        env->GetObjectField(object, gBinderProxyOffsets.mSelf));
    val->attachObject(&gBinderProxyOffsets, refObject,
        jnienv_to_javavm(env), proxy_cleanup);

    // Also remember the death recipients registered on this proxy
    sp<DeathRecipientList> drl = new DeathRecipientList;
    drl->incStrong((void*)javaObjectForIBinder);
    env->SetLongField(object, gBinderProxyOffsets.mOrgue, reinterpret_cast<jlong>(drl.get()));

    // Note that a new object reference has been created.
    android_atomic_inc(&gNumProxyRefs);
    incRefsCreated(env);
  }

  // BinderProxy 对象
  return object;
}

// android_os_Parcel.cpp
static jlong android_os_Parcel_create(JNIEnv* env, jclass clazz)
{
    Parcel* parcel = new Parcel();
    return reinterpret_cast<jlong>(parcel);
}

// android_os_Parcel.cpp
// name = "android.os.IServiceManager"
static void android_os_Parcel_writeInterfaceToken(JNIEnv* env, jclass clazz, jlong nativePtr, jstring name){
  Parcel* parcel = reinterpret_cast<Parcel*>(nativePtr);
  if (parcel != NULL) {
    // In the current implementation, the token is just the serialized interface name that
    // the caller expects to be invoking
    const jchar* str = env->GetStringCritical(name, 0);
    if (str != NULL) {
      parcel->writeInterfaceToken(String16(
         reinterpret_cast<const char16_t*>(str),
         env->GetStringLength(name)));
      env->ReleaseStringCritical(name, str);
    }
  }
 }

// Parcel.cpp
// interface = "android.os.IServiceManager"
status_t Parcel::writeInterfaceToken(const String16& interface)
{
  // 写入一个整数标识严格模式 
  writeInt32(IPCThreadState::self()->getStrictModePolicy() |
        STRICT_MODE_PENALTY_GATHER);
  // 写入传入的字符串
  return writeString16(interface);
}

// android_os_Parcel.cpp
// object = ActivityManagerService 对象
static void android_os_Parcel_writeStrongBinder(JNIEnv* env, jclass clazz, jlong nativePtr, jobject object)
{
  Parcel* parcel = reinterpret_cast<Parcel*>(nativePtr);
  if (parcel != NULL) {
  // ibinderForJavaObject 见 2.1.4.1
  // writeStrongBinder 见 2.1.4.2
    const status_t err = parcel->writeStrongBinder(ibinderForJavaObject(env, object));
    if (err != NO_ERROR) {
      signalExceptionForError(env, clazz, err);
    }
  }
}

// android_os_Parcel.cpp
// obj = ActivityManagerService 对象
sp<IBinder> ibinderForJavaObject(JNIEnv* env, jobject obj)
{
  if (obj == NULL) return NULL;

  // 由于 ActivityManagerService 继承 Binder 对象，进入该分支
  if (env->IsInstanceOf(obj, gBinderOffsets.mClass)) {
     // Java 端的 Binder 对象会持有一个 mObject 字段，指向
    // native 端的 JavaBBinderHolder 对象，见 2.1.4.1.1
    JavaBBinderHolder* jbh = (JavaBBinderHolder*)
      env->GetLongField(obj, gBinderOffsets.mObject);
    // jbh->get 返回的是一个 JavaBBinder 对象，见 2.1.4.1.2
    return jbh != NULL ? jbh->get(env, obj) : NULL;
  }

  // 对于非服务类，则继承 BinderProxy，前面已经讲过
  if (env->IsInstanceOf(obj, gBinderProxyOffsets.mClass)) {
    return (IBinder*)
      env->GetLongField(obj, gBinderProxyOffsets.mObject);
  }

  ALOGW("ibinderForJavaObject: %p is not a Binder object", obj);
  return NULL;
}

// android_util_Binder.cpp
static void android_os_Binder_init(JNIEnv* env, jobject obj)
{
  JavaBBinderHolder* jbh = new JavaBBinderHolder();
  if (jbh == NULL) {
    jniThrowException(env, "java/lang/OutOfMemoryError", NULL);
    return;
  }
  ALOGV("Java Binder %p: acquiring first ref on holder %p", obj, jbh);
  jbh->incStrong((void*)android_os_Binder_init);
  env->SetLongField(obj, gBinderOffsets.mObject, (jlong)jbh);
}

// android_util_Binder.cpp#JavaBBinderHolder

wp<JavaBBinder> mBinder;

// obj = ActivityManagerService 对象
sp<JavaBBinder> get(JNIEnv* env, jobject obj)
{
  AutoMutex _l(mLock);
  // mBinder 是一个弱引用，第一次调用返回 NULL
  sp<JavaBBinder> b = mBinder.promote();
  if (b == NULL) {
    b = new JavaBBinder(env, obj);
    mBinder = b;
  }

  return b;
}

// Parcel.cpp
// val = JavaBBinder 对象
status_t Parcel::writeStrongBinder(const sp<IBinder>& val)
{
 // 见 2.1.4.2.1
  return flatten_binder(ProcessState::self(), val, this);
}

// Parcel.cpp
// binder = JavaBBinder 对象
status_t flatten_binder(const sp<ProcessState>& /*proc*/,
    const sp<IBinder>& binder, Parcel* out)
{
  flat_binder_object obj;

  obj.flags = 0x7f | FLAT_BINDER_FLAG_ACCEPTS_FDS;
  if (binder != NULL) {
    // JavaBBinder 继承 BBinder，所以 localBinder 返回自身
    IBinder *local = binder->localBinder();
    if (!local) {
      BpBinder *proxy = binder->remoteBinder();
      if (proxy == NULL) {
        ALOGE("null proxy");
      }
      const int32_t handle = proxy ? proxy->handle() : 0;
      obj.type = BINDER_TYPE_HANDLE;
      obj.binder = 0; /* Don't pass uninitialized stack data to a remote process */
      obj.handle = handle;
      obj.cookie = 0;
    } else {
      // 走这里   
      obj.type = BINDER_TYPE_BINDER;
      obj.binder = reinterpret_cast<uintptr_t>(local->getWeakRefs());
      obj.cookie = reinterpret_cast<uintptr_t>(local);
    }
  } else {
    obj.type = BINDER_TYPE_BINDER;
    obj.binder = 0;
    obj.cookie = 0;
  }

  return finish_flatten_binder(binder, obj, out);
}

inline static status_t finish_flatten_binder(
    const sp<IBinder>& /*binder*/, const flat_binder_object& flat, Parcel* out)
{
  return out->writeObject(flat, false);
}

// Parcel.cpp
/*
val = {
 flags = 0x7f | FLAT_BINDER_FLAG_ACCEPTS_FDS,
 type: BINDER_TYPE_BINDER,
 binder: 弱引用,
 cookie: 指向 Binder 对象指针,
}
nullMetaData = false
*/
status_t Parcel::writeObject(const flat_binder_object& val, bool nullMetaData)
{
  const bool enoughData = (mDataPos+sizeof(val)) <= mDataCapacity;
  const bool enoughObjects = mObjectsSize < mObjectsCapacity;
  if (enoughData && enoughObjects) {
 restart_write:
    *reinterpret_cast<flat_binder_object*>(mData+mDataPos) = val;
    ...
    if (nullMetaData || val.binder != 0) {
      // 记录 flat_binder_object 的在 buffer 中的位置
      mObjects[mObjectsSize] = mDataPos;
      acquire_object(ProcessState::self(), val, this, &mOpenAshmemSize);
      mObjectsSize++;
    }

    return finishWrite(sizeof(flat_binder_object));
  }

  if (!enoughData) {
  ...
  }
  if (!enoughObjects) {
  ...
  }

  goto restart_write;
}

// android_util_Binder.cpp
// code =  IServiceManager.ADD_SERVICE_TRANSACTION
// dataObj = 写入的 Parcel，见 2.1.2 注释
// flags = 0
static jboolean android_os_BinderProxy_transact(JNIEnv* env, jobject obj,
        jint code, jobject dataObj, jobject replyObj, jint flags) // throws RemoteException
{
  if (dataObj == NULL) {
    jniThrowNullPointerException(env, NULL);
    return JNI_FALSE;
  }
  // 从 Java 对象中取出 native 对象
  Parcel* data = parcelForJavaObject(env, dataObj);
  if (data == NULL) {
    return JNI_FALSE;
  }
  Parcel* reply = parcelForJavaObject(env, replyObj);
  if (reply == NULL && replyObj != NULL) {
    return JNI_FALSE;
  }
  
 // mObject = BpBinder(0)
  IBinder* target = (IBinder*)
    env->GetLongField(obj, gBinderProxyOffsets.mObject);
  if (target == NULL) {
    jniThrowException(env, "java/lang/IllegalStateException", "Binder has been finalized!");
    return JNI_FALSE;
  }
  ...
 // 见 2.1.5.2
  status_t err = target->transact(code, *data, reply, flags);
  ...
  if (err == NO_ERROR) {
    return JNI_TRUE;
  } else if (err == UNKNOWN_TRANSACTION) {
    return JNI_FALSE;
  }

  signalExceptionForError(env, obj, err, true /*canThrowRemoteException*/, data->dataSize());
  return JNI_FALSE;
}

// BpBinder.cpp
// code =  IServiceManager.ADD_SERVICE_TRANSACTION
// data = 写入的 Parcel，见 2.1.2 注释
// reply = 待写入的 Parcel
// flags = 0
status_t BpBinder::transact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
  if (mAlive) {
  // 见 2.2
    status_t status = IPCThreadState::self()->transact(
      mHandle, code, data, reply, flags);
    if (status == DEAD_OBJECT) mAlive = 0;
    return status;
  }

  return DEAD_OBJECT;
}

// IPCThreadState.cpp
// handle = 0
// code =  IServiceManager.ADD_SERVICE_TRANSACTION
// data = 写入的 Parcel，见 2.1.2 注释
// reply = 待写入的 Parcel
// flags = 0
status_t IPCThreadState::transact(int32_t handle,
                                  uint32_t code, const Parcel& data,
                                  Parcel* reply, uint32_t flags)
{
  flags |= TF_ACCEPT_FDS;
 if (err == NO_ERROR) {
    // 见 2.2.1
    err = writeTransactionData(BC_TRANSACTION, flags, handle, code, data, NULL);
 } 
  ...
 // 不是非阻塞模式，进入该分支
  if ((flags & TF_ONE_WAY) == 0) {
    // reply 不为 NULL
    if (reply) {
      // 见 2.2.2   
      err = waitForResponse(reply);
    } else {
      Parcel fakeReply;
      err = waitForResponse(&fakeReply);
    }
    ...
  } else {
    err = waitForResponse(NULL, NULL);
  }
  
  return err;
}

// IPCThreadState.cpp
// cmd = BC_TRANSACTION
// binderFlags = TF_ACCEPT_FDS
// handle = 0
// code = IServiceManager.ADD_SERVICE_TRANSACTION
// data = 写入的 Parcel，见 2.1.2 注释
// statusBuffer = null
status_t IPCThreadState::writeTransactionData(int32_t cmd, uint32_t binderFlags,
    int32_t handle, uint32_t code, const Parcel& data, status_t* statusBuffer)
{
  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();
  // 走该分支
  if (err == NO_ERROR) {
    tr.data_size = data.ipcDataSize();
    tr.data.ptr.buffer = data.ipcData();
    // 存储 binder 对象位置的数组字节大小
    tr.offsets_size = data.ipcObjectsCount()*sizeof(binder_size_t);
  // binder 对象的对象位置的数组
    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 是发给 binder 驱动的数据  
  mOut.writeInt32(cmd);
  mOut.write(&tr, sizeof(tr));
  
  return NO_ERROR;
 }

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

  while (1) {
  // 见 2.2.3
    if ((err=talkWithDriver()) < NO_ERROR) break;
    if (mIn.dataAvail() == 0) continue;
    cmd = (uint32_t)mIn.readInt32();
    switch (cmd) {
    // 第二个指令 BR_TRANSACTION_COMPLETE  
    case BR_TRANSACTION_COMPLETE:
      if (!reply && !acquireResult) goto finish;
      // 跳出 switch，继续循环
      break;
      ...
    default:
      // 第一个指令 BR_NOOP，executeCommand 不会进行任何操作
      err = executeCommand(cmd);
      if (err != NO_ERROR) goto finish;
      break;
    }
  }
 ...
  return err;
}

// IPCThreadState.cpp
// doReceive = true
status_t IPCThreadState::talkWithDriver(bool doReceive)
{
  // mProcess 即为 ProcessState
  if (mProcess->mDriverFD <= 0) {
    return -EBADF;
  }
  
  binder_write_read bwr;
  
  // Is the read buffer empty?
  // 如果为 true 说明驱动写入的数据已经读取完了，或者根本就没数据
  // 当前情况为 true
  const bool needRead = mIn.dataPosition() >= mIn.dataSize();
  
  // outAvail = mOut.dataSize()
  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) {
    // 初始化 IPCThreadState 时，mIn 和 mOut 容量都为 256
    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 {
    // 系统调用，见 2.2.4
    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);
    }
    // 8
    if (bwr.read_consumed > 0) {
      // 当读完后，position 就和 size 相同了，发起第二次 talkWithDriver 时 needRead 就为 true
      mIn.setDataSize(bwr.read_consumed);
      mIn.setDataPosition(0);
    }
    return NO_ERROR;
  }
  
  return err;
}

// kernel/drivers/android/binder.c
/*
cmd = BINDER_WRITE_READ
arg = 指向用户空间的 binder_write_read 结构体：
{
  write_size: write_buffer 大小,
  write_consumed: 0,
  write_buffer: [BC_TRANSACTION, binder_transaction_data 对象], // 见 2.2.1
  read_size: 256,
  read_consumed: 0,
  read_buffer: 指向用户空间
}
*/
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);
  void __user *ubuf = (void __user *)arg;
  ...
  mutex_lock(&binder_lock);
  thread = binder_get_thread(proc);
  if (thread == NULL) {
    ret = -ENOMEM;
    goto err;
  }

  switch (cmd) {
  case BINDER_WRITE_READ: {
    struct binder_write_read bwr;
    if (size != sizeof(struct binder_write_read)) {
      ret = -EINVAL;
      goto err;
    }
    if (copy_from_user(&bwr, ubuf, sizeof(bwr))) {
      ret = -EFAULT;
      goto err;
    }
    // 不为 0，见 2.2.5
    if (bwr.write_size > 0) {
      ret = binder_thread_write(proc, thread, (void __user *)bwr.write_buffer, bwr.write_size, &bwr.write_consumed);
      if (ret < 0) {
        bwr.read_consumed = 0;
        if (copy_to_user(ubuf, &bwr, sizeof(bwr)))
          ret = -EFAULT;
        goto err;
      }
    }
    // 不为 0，见 2.2.7
    if (bwr.read_size > 0) {
      ret = binder_thread_read(proc, thread, (void __user *)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) {
        if (copy_to_user(ubuf, &bwr, sizeof(bwr)))
          ret = -EFAULT;
        goto err;
      }
    }
    binder_debug(BINDER_DEBUG_READ_WRITE,
           "binder: %d:%d wrote %ld of %ld, read return %ld of %ld\n",
           proc->pid, thread->pid, bwr.write_consumed, bwr.write_size,
           bwr.read_consumed, bwr.read_size);
    if (copy_to_user(ubuf, &bwr, sizeof(bwr))) {
      ret = -EFAULT;
      goto err;
    }
    break;
  }
  }
  ret = 0;
err:
  ...
  return ret;
}

// kernel/drivers/android/binder.c
// *buffer = [BC_TRANSACTION, binder_transaction_data 对象]// 见 2.2.1
// size = buffer 大小
// *consumed = 0
int binder_thread_write(struct binder_proc *proc, struct binder_thread *thread,
      void __user *buffer, int size, signed long *consumed)
{
  uint32_t cmd;
  void __user *ptr = buffer + *consumed;
  void __user *end = buffer + size;

  while (ptr < end && thread->return_error == BR_OK) {
    //取出 cmd 为 BC_TRANSACTION
    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;
      // 从用户空间取出数据，浅拷贝，数据中的指针仍指向用户空间
      if (copy_from_user(&tr, ptr, sizeof(tr)))
        return -EFAULT;
      // ptr 将指向 buffer 的末尾        
      ptr += sizeof(tr);
      // 见 2.2.6      
      binder_transaction(proc, thread, &tr, cmd == BC_REPLY);
      break;
    ...
    }
    }
    // 数据全部消耗，*consumed 等于 buffer 所指向数据的大小
    *consumed = ptr - buffer;
  }
  return 0;
}

// kernel/drivers/android/binder.c
// tr = 指向 binder_transaction_data 对象 // 见 2.2.1
// reply = false
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;
  size_t *offp, *off_end;
  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;
  ...
  if (reply) {
    ...
  } else {
    // tr->target.handle = 0    
    if (tr->target.handle) {
      ...
    } else {
      // 第 1 阶段已经介绍了
      target_node = binder_context_mgr_node;
      if (target_node == NULL) {
        return_error = BR_DEAD_REPLY;
        goto err_no_context_mgr_node;
      }
    }
    // Service Manager 进程的 proc
    target_proc = target_node->proc;
    if (target_proc == NULL) {
      return_error = BR_DEAD_REPLY;
      goto err_dead_binder;
    }
    if (security_binder_transaction(proc->tsk, target_proc->tsk) < 0) {
      return_error = BR_FAILED_REPLY;
      goto err_invalid_target_handle;
    }
    // tr->flags = TF_ACCEPT_FDS，并不是非阻塞调用
    // thread->transaction_stack = null
    if (!(tr->flags & TF_ONE_WAY) && thread->transaction_stack) {
      ...
    }
  }
  // target_thread = null，没有指定线程，那么后续操作的是目标进程的 todo  
  if (target_thread) {
    target_list = &target_thread->todo;
    target_wait = &target_thread->wait;
  } else {
    target_list = &target_proc->todo;
    target_wait = &target_proc->wait;
  }

  /* TODO: reuse incoming transaction for reply */
  // 为当前的 binder 事务分配内存
  t = kzalloc(sizeof(*t), GFP_KERNEL);
  if (t == NULL) {
    return_error = BR_FAILED_REPLY;
    goto err_alloc_t_failed;
  }
  binder_stats_created(BINDER_STAT_TRANSACTION);

  // 为待完成的任务分配内存
  tcomplete = kzalloc(sizeof(*tcomplete), GFP_KERNEL);
  if (tcomplete == NULL) {
    return_error = BR_FAILED_REPLY;
    goto err_alloc_tcomplete_failed;
  }
  binder_stats_created(BINDER_STAT_TRANSACTION_COMPLETE);

  t->debug_id = ++binder_last_id;

  if (!reply && !(tr->flags & TF_ONE_WAY))
    // 走这里，这里记下发起调用的线程，后面会将数据写入该线程  
    t->from = thread;
  else
    t->from = NULL;
  t->sender_euid = proc->tsk->cred->euid;
  // Service Manager 的 proc
  t->to_proc = target_proc;
  // null
  t->to_thread = target_thread;
  // IServiceManager.ADD_SERVICE_TRANSACTION
  t->code = tr->code;
  // TF_ACCEPT_FDS
  t->flags = tr->flags;
  t->priority = task_nice(current);
  // 分配内存，准备将 tr->data.ptr->buffer 和 tr->data.ptr.offsets
  // 指向的用户数据取出来，见 2.2.6.1
  t->buffer = binder_alloc_buf(target_proc, tr->data_size,
    tr->offsets_size, !reply && (t->flags & TF_ONE_WAY));
  if (t->buffer == NULL) {
    return_error = BR_FAILED_REPLY;
    goto err_binder_alloc_buf_failed;
  }
  t->buffer->allow_user_free = 0;
  t->buffer->debug_id = t->debug_id;
  t->buffer->transaction = t;
  t->buffer->target_node = target_node;
  if (target_node)
    // 增加引用计数???  
    binder_inc_node(target_node, 1, 0, NULL);
  // buffer 存储了两部分数据，这里计算存储 offsets 的起始地址
  offp = (size_t *)(t->buffer->data + ALIGN(tr->data_size, sizeof(void *)));
  // 将 buffer 数据取出，具体内容见 2.1.2 注释
  if (copy_from_user(t->buffer->data, tr->data.ptr.buffer, tr->data_size)) {
    ...
  }
  // 将 buffer 中所包含的 Binder 对象位置信息数组 offsets 取出
  if (copy_from_user(offp, tr->data.ptr.offsets, tr->offsets_size)) {
    ...
  }
  if (!IS_ALIGNED(tr->offsets_size, sizeof(size_t))) {
    binder_user_error("binder: %d:%d got transaction with "
      "invalid offsets size, %zd\n",
      proc->pid, thread->pid, tr->offsets_size);
    return_error = BR_FAILED_REPLY;
    goto err_bad_offset;
  }
  // 指向 offsets 的末尾，也是整个 buffer 的末尾  
  off_end = (void *)offp + tr->offsets_size;
  for (; offp < off_end; offp++) {
    struct flat_binder_object *fp;
    if (*offp > t->buffer->data_size - sizeof(*fp) ||
        t->buffer->data_size < sizeof(*fp) ||
        !IS_ALIGNED(*offp, sizeof(void *))) {
      binder_user_error("binder: %d:%d got transaction with "
        "invalid offset, %zd\n",
        proc->pid, thread->pid, *offp);
      return_error = BR_FAILED_REPLY;
      goto err_bad_offset;
    }
    // 取出 Binder 对象，即起始位置加上一个偏移，这里只有一个对象，内容为：
    // {
    //   flags: 0x7f | FLAT_BINDER_FLAG_ACCEPTS_FDS,
    //   type: BINDER_TYPE_BINDER,
    //   binder: 弱引用,
    //   cookie: 指向 Binder 对象指针,
    // }
    fp = (struct flat_binder_object *)(t->buffer->data + *offp);
    switch (fp->type) {
    case BINDER_TYPE_BINDER:
    case BINDER_TYPE_WEAK_BINDER: {
      struct binder_ref *ref;
      // 每个进程都在 proc 的 nodes 字段中维护当前进程的 binder_node，binder_node
      // 保存了 Service 的信息，例如地址。驱动会将一个 binder_node 的引用传递给其他
      // 进程。
      struct binder_node *node = binder_get_node(proc, fp->binder);
      if (node == NULL) {
        // 没有查到则新建一个并插入到 proc.nodes 中，这里的 fp->cookie 便是 AMS
        // 在进程中的实际地址。
        node = binder_new_node(proc, fp->binder, fp->cookie);
        if (node == NULL) {
          return_error = BR_FAILED_REPLY;
          goto err_binder_new_node_failed;
        }
        node->min_priority = fp->flags & FLAT_BINDER_FLAG_PRIORITY_MASK;
        node->accept_fds = !!(fp->flags & FLAT_BINDER_FLAG_ACCEPTS_FDS);
      }
      if (fp->cookie != node->cookie) {
        binder_user_error("binder: %d:%d sending u%p "
          "node %d, cookie mismatch %p != %p\n",
          proc->pid, thread->pid,
          fp->binder, node->debug_id,
          fp->cookie, node->cookie);
        goto err_binder_get_ref_for_node_failed;
      }
      ...
      // 为 binder_node 创建一个引用，见 2.2.6.2
      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;
      }
      // 将类型改为 BINDER_TYPE_HANDLE，记住，其他进程持有的都是 handle
      if (fp->type == BINDER_TYPE_BINDER)
        fp->type = BINDER_TYPE_HANDLE;
      else
        fp->type = BINDER_TYPE_WEAK_HANDLE;
      fp->handle = ref->desc;
      binder_inc_ref(ref, fp->type == BINDER_TYPE_HANDLE,
               &thread->todo);
    } break;
  }
  if (reply) {
    ...
  } else if (!(t->flags & TF_ONE_WAY)) {
    BUG_ON(t->buffer->async_transaction != 0);
    // 将事务放在发起调用的线程的事务栈的栈顶    
    t->need_reply = 1;
    t->from_parent = thread->transaction_stack;
    thread->transaction_stack = t;
  } else {
    ...
  }
  t->work.type = BINDER_WORK_TRANSACTION;
  // 将事务的工作添加到目标任务列表中
  list_add_tail(&t->work.entry, target_list);
  // 将标识写完成的工作添加到当前线程的任务列表中
  tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE;
  list_add_tail(&tcomplete->entry, &thread->todo);
  // 唤醒 Service Manager
  if (target_wait)
    // 见 2.3
    wake_up_interruptible(target_wait);
  return;
  ...// 错误处理
}