这里来分析一下ServiceManager的启动过程，ServiceManager是管理系统所有服务的进程，用于提供API给用户注册以及查找相应的服务。ServiceManager直接与binder驱动打交道去实现跨进程的IPC。下面首先来看一下ServiceManager启动的地方，它是在init.rc脚本，这个脚本会被init程序解析并执行其中不同的服务。首先来看ServiceManager的启动命令：
service servicemanager /system/bin/servicemanager class core user system group system critical onrestart restart healthd onrestart restart zygote onrestart restart media onrestart restart surfaceflinger onrestart restart drm
上面命令会启动路径/system/bin/servicemanager下面的servicemanager可执行程序，并且这个service的名字也是servicemanager，后面会设置这个service的一些属性。可执行程序servicemanager的源文件是service_manager.c，我们来分析它的main函数：
int main(int argc, char **argv){ struct binder_state *bs; void *svcmgr = BINDER_SERVICE_MANAGER; bs = binder_open(128*1024); if (binder_become_context_manager(bs)) { ALOGE("cannot become context manager (%s)\n", strerror(errno)); return -1; } svcmgr_handle = svcmgr; binder_loop(bs, svcmgr_handler); return 0;}binder_state结构如下：
struct binder_state{ int fd; void *mapped; unsigned mapsize;};fd描述打开的/dev/binder文件句柄；mapped是映射到用户空间的内存起始地址；mapsize为映射内存区域的大小。BINDER_SERVICE_MANAGER在binder.h中定义为(void*) 0，因为ServiceManager在binder驱动中的hander ID为0，后面分析中我们会看到。接着调用binder_open去打开binder设备文件并映射到用户空间：
struct binder_state *binder_open(unsigned mapsize){ struct binder_state *bs; bs = malloc(sizeof(*bs)); if (!bs) { errno = ENOMEM; return 0; } bs->fd = open("/dev/binder", O_RDWR); if (bs->fd < 0) { fprintf(stderr,"binder: cannot open device (%s)\n", strerror(errno)); goto fail_open; } bs->mapsize = mapsize; bs->mapped = mmap(NULL, mapsize, PROT_READ, MAP_PRIVATE, bs->fd, 0); if (bs->mapped == MAP_FAILED) { fprintf(stderr,"binder: cannot map device (%s)\n", strerror(errno)); goto fail_map; } return bs;fail_map: close(bs->fd);fail_open: free(bs); return 0;}首先调用open方法去打开设备文件，我们到binder驱动文件中去分析，代码在kernel-3.4/drivers/staging/android/binder.c：
static const struct file_operations binder_fops = { .owner = THIS_MODULE, .poll = binder_poll, .unlocked_ioctl = binder_ioctl, .mmap = binder_mmap, .open = binder_open, .flush = binder_flush, .release = binder_release,};static struct miscdevice binder_miscdev = { .minor = MISC_DYNAMIC_MINOR, .name = "binder", .fops = &binder_fops};根据上面的定义，open和mmap方法最终会调用到file_operations里面的binder_open和binder_mmap两个函数指针。首先来看binder驱动中的binder_open方法：
static int binder_open(struct inode *nodp, struct file *filp){ struct binder_proc *proc; proc = kzalloc(sizeof(*proc), GFP_KERNEL); if (proc == NULL) return -ENOMEM; get_task_struct(current); proc->tsk = current; INIT_LIST_HEAD(&proc->todo); init_waitqueue_head(&proc->wait); proc->default_priority = task_nice(current); binder_lock(__func__); binder_stats_created(BINDER_STAT_PROC); hlist_add_head(&proc->proc_node, &binder_procs); proc->pid = current->group_leader->pid; INIT_LIST_HEAD(&proc->delivered_death); filp->private_data = proc; binder_unlock(__func__); return 0;}binder_open首先构造一个binder_proc数据结构，binder_proc保存了打开/dev/binder设备进程的上下文信息，先来总体看一下binder_proc的结构：
struct binder_proc { struct hlist_node proc_node; //用来链接所有的binder_proc到binder_procs的节点 struct rb_root threads; //binder threads红黑树根节点，链接当前进程上所有的binder thread struct rb_root nodes; //nodes红黑树根节点，存放当前进程上所有的binder实体 struct rb_root refs_by_desc; //引用binder的红黑树根节点，通过decs id号来索引 struct rb_root refs_by_node; //引用binder的红黑树根节点，通过node来索引 int pid; //当前进程的group leader的进程号 struct vm_area_struct *vma; //用户空间内存映射地址 struct mm_struct *vma_vm_mm; //内核空间内存映射地址 struct task_struct *tsk; //保存当前进程的task struck struct files_struct *files; //保存打开的文件 struct hlist_node deferred_work_node; int deferred_work; void *buffer; //内核虚拟空间起始地址 ptrdiff_t user_buffer_offset; //用户映射地址和内核虚拟空间地址之间的偏移 struct list_head buffers; struct rb_root free_buffers; //free buffer的红黑树根节点 struct rb_root allocated_buffers; size_t free_async_space; struct page **pages; //实际物理内存页面 size_t buffer_size; //分配的内存大小 uint32_t buffer_free; //剩下的free buffer struct list_head todo; //待完成的事务 wait_queue_head_t wait; //等待信号 struct binder_stats stats; //当前binder的状态记录 struct list_head delivered_death; int max_threads; int requested_threads; int requested_threads_started; int ready_threads; long default_priority; struct dentry *debugfs_entry;};上面的binder_open方法会去初始化binder_proc中的todo、wait等链表，并把当前binder_proc保存在/dev/binder打开文件filp的private_data中，方便以后访问。接着来看binder_mmap函数：
static int binder_mmap(struct file *filp, struct vm_area_struct *vma){ int ret; struct vm_struct *area; struct binder_proc *proc = filp->private_data; const char *failure_string; struct binder_buffer *buffer; if ((vma->vm_end - vma->vm_start) > SZ_4M) vma->vm_end = vma->vm_start + SZ_4M; if (vma->vm_flags & FORBIDDEN_MMAP_FLAGS) { ret = -EPERM; failure_string = "bad vm_flags"; goto err_bad_arg; } vma->vm_flags = (vma->vm_flags | VM_DONTCOPY) & ~VM_MAYWRITE; mutex_lock(&binder_mmap_lock); if (proc->buffer) { ret = -EBUSY; failure_string = "already mapped"; goto err_already_mapped; } area = get_vm_area(vma->vm_end - vma->vm_start, VM_IOREMAP); if (area == NULL) { ret = -ENOMEM; failure_string = "get_vm_area"; goto err_get_vm_area_failed; } proc->buffer = area->addr; proc->user_buffer_offset = vma->vm_start - (uintptr_t)proc->buffer; mutex_unlock(&binder_mmap_lock); proc->pages = kzalloc(sizeof(proc->pages[0]) * ((vma->vm_end - vma->vm_start) / PAGE_SIZE), GFP_KERNEL); if (proc->pages == NULL) { ret = -ENOMEM; failure_string = "alloc page array"; goto err_alloc_pages_failed; } proc->buffer_size = vma->vm_end - vma->vm_start; vma->vm_ops = &binder_vm_ops; vma->vm_private_data = proc; if (binder_update_page_range(proc, 1, proc->buffer, proc->buffer + PAGE_SIZE, vma)) { ret = -ENOMEM; failure_string = "alloc small buf"; goto err_alloc_small_buf_failed; } buffer = proc->buffer; INIT_LIST_HEAD(&proc->buffers); list_add(&buffer->entry, &proc->buffers); buffer->free = 1; binder_insert_free_buffer(proc, buffer); proc->free_async_space = proc->buffer_size / 2; barrier(); proc->files = get_files_struct(proc->tsk); proc->vma = vma; proc->vma_vm_mm = vma->vm_mm; return 0;}首先从打开文件private_data取出当前的binder_proc结构，然后检查内存映射区域是否大于4M，由前面的在service_manager中binder_open(128*1024)，我们知道，这里的内存映射大小是128K。接着调用get_vm_area为内核去分配内存虚拟空间。Linux内核中，关于虚存管理的最基本的管理单元应该是struct vm_area_struct了，它描述的是一段连续的、具有相同访问属性的虚存空间，该虚存空间的大小为物理内存页面的整数倍。而vm_struct与vm_area_struct类似，只是vm_struct给内核使用，vm_area_struct主要给用户空间访问。这样他们两者之间就有一个差值，我们通过这个差值可以很方便的通过用户空间地址计算出内核空间地址，或者从内核空间地址计算出用户空间地址，这个差值就保存在binder_proc的user_buffer_offset中。
接下来就调用binder_update_page_range分配实际的物理内存页面了并映射到用户和内核空间：
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; struct mm_struct *mm; if (end <= start) return 0; trace_binder_update_page_range(proc, allocate, start, end); if (vma) mm = NULL; else mm = get_task_mm(proc->tsk); if (mm) { down_write(&mm->mmap_sem); vma = proc->vma; if (vma && mm != proc->vma_vm_mm) { pr_err("binder: %d: vma mm and task mm mismatch\n", proc->pid); vma = NULL; } } 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; page = &proc->pages[(page_addr - proc->buffer) / PAGE_SIZE]; BUG_ON(*page); *page = alloc_page(GFP_KERNEL | __GFP_HIGHMEM | __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; } tmp_area.addr = page_addr; tmp_area.size = PAGE_SIZE + PAGE_SIZE /* guard page? */; page_array_ptr = page; 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; } user_page_addr = (uintptr_t)page_addr + proc->user_buffer_offset; 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;}这里的allocate等于1，表示要分配内存。首先调用alloc_page分配一个页面，并把这个页面插入到tmp_area所描述的内核虚拟空间地址和大小；然后根据user_buffer_offset计算出用户虚拟地址并映射用户空间地址。
回到service_manager中会调用binder_become_context_manager让serviceManager成为binder的管理者：
int binder_become_context_manager(struct binder_state *bs){ return ioctl(bs->fd, BINDER_SET_CONTEXT_MGR, 0);}这里会通过ioctrl向上面打开的/dev/binder句柄中发送BINDER_SET_CONTEXT_MGR命令，我们到binder驱动中的binder_ioctl来分析：
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; thread = binder_get_thread(proc); if (thread == NULL) { ret = -ENOMEM; goto err; } case BINDER_SET_CONTEXT_MGR: if (binder_context_mgr_node != NULL) { printk(KERN_ERR "binder: BINDER_SET_CONTEXT_MGR already set\n"); ret = -EBUSY; goto err; } ret = security_binder_set_context_mgr(proc->tsk); if (ret < 0) goto err; if (binder_context_mgr_uid != -1) { if (binder_context_mgr_uid != current->cred->euid) { printk(KERN_ERR "binder: BINDER_SET_" "CONTEXT_MGR bad uid %d != %d\n", current->cred->euid, binder_context_mgr_uid); ret = -EPERM; goto err; } } else binder_context_mgr_uid = current->cred->euid; binder_context_mgr_node = binder_new_node(proc, NULL, NULL); if (binder_context_mgr_node == NULL) { ret = -ENOMEM; goto err; } binder_context_mgr_node->local_weak_refs++; binder_context_mgr_node->local_strong_refs++; binder_context_mgr_node->has_strong_ref = 1; binder_context_mgr_node->has_weak_ref = 1; break; if (thread) thread->looper &= ~BINDER_LOOPER_STATE_NEED_RETURN;binder_ioctl函数首先调用binder_get_thread获取当前调用操作的binder thread：
static struct binder_thread *binder_get_thread(struct binder_proc *proc){ struct binder_thread *thread = NULL; struct rb_node *parent = NULL; struct rb_node **p = &proc->threads.rb_node; while (*p) { parent = *p; thread = rb_entry(parent, struct binder_thread, rb_node); if (current->pid < thread->pid) p = &(*p)->rb_left; else if (current->pid > thread->pid) p = &(*p)->rb_right; else break; } if (*p == NULL) { thread = kzalloc(sizeof(*thread), GFP_KERNEL); if (thread == NULL) return NULL; binder_stats_created(BINDER_STAT_THREAD); thread->proc = proc; thread->pid = current->pid; init_waitqueue_head(&thread->wait); INIT_LIST_HEAD(&thread->todo); rb_link_node(&thread->rb_node, parent, p); rb_insert_color(&thread->rb_node, &proc->threads); thread->looper |= BINDER_LOOPER_STATE_NEED_RETURN; thread->return_error = BR_OK; thread->return_error2 = BR_OK; } return thread;}因为这里是第一次调用，所以会新建一个binder_thread结构，将binder_thread的proc设为当前binder_proc结构，pid设置为当前进程的pid，并初始化wait、todo链表，并通过binder_thread结构中的rb_node将这个binder_thread加入到binder_proc中的threads红黑树中；然后将looper设置为BINDER_LOOPER_STATE_NEED_RETURN，表示这个binder_thread处理完后需要返回。下面是binder_thread的数据结构：
struct binder_thread { struct binder_proc *proc; //进程的binder_proc结构 struct rb_node rb_node; //通过rb_node链接到binder_proc中的threads红黑树 int pid; //进程的pid int looper; //binder_thread的状态 struct binder_transaction *transaction_stack; // struct list_head todo; //待处理事务列表 uint32_t return_error; /* Write failed, return error code in read buf */ uint32_t return_error2; /* Write failed, return error code in read */ /* buffer. Used when sending a reply to a dead process that */ /* we are also waiting on */ wait_queue_head_t wait; struct binder_stats stats;};接着来看处理BINDER_SET_CONTEXT_MGR命令的代码，binder_context_mgr_node为记录serviceManage的binder_node，binder_context_mgr_uid记录serviceManager的uid信息，这里将它置为serviceManger进程的uid；并调用binder_new_node为binder_context_mgr_node分配binder_node结构：
static struct binder_node *binder_new_node(struct binder_proc *proc, void __user *ptr, void __user *cookie){ struct rb_node **p = &proc->nodes.rb_node; struct rb_node *parent = NULL; struct binder_node *node; while (*p) { parent = *p; node = rb_entry(parent, struct binder_node, rb_node); if (ptr < node->ptr) p = &(*p)->rb_left; else if (ptr > node->ptr) p = &(*p)->rb_right; else return NULL; } node = kzalloc(sizeof(*node), GFP_KERNEL); if (node == NULL) return NULL; binder_stats_created(BINDER_STAT_NODE); rb_link_node(&node->rb_node, parent, p); rb_insert_color(&node->rb_node, &proc->nodes); node->debug_id = ++binder_last_id; node->proc = proc; node->ptr = ptr; node->cookie = cookie; node->work.type = BINDER_WORK_NODE; INIT_LIST_HEAD(&node->work.entry); INIT_LIST_HEAD(&node->async_todo); binder_debug(BINDER_DEBUG_INTERNAL_REFS, "binder: %d:%d node %d u%p c%p created\n", proc->pid, current->pid, node->debug_id, node->ptr, node->cookie); return node;}因为service_manager进程是第一次创建binder_node，所以在binder_proc上的node红黑树初始化时是空的。然后就创建一个新的binder_node结构，先来看一下数据结构：
struct binder_node { int debug_id; struct binder_work work; //表示binder_node的type union { struct rb_node rb_node; //链接所有的binder_node到binder_proc的nodes红黑树中 struct hlist_node dead_node; //链接所有的dead node到全局binder_dead_nodes }; struct binder_proc *proc; //属于的binder_proc结构 struct hlist_head refs; // int internal_strong_refs; int local_weak_refs; int local_strong_refs; void __user *ptr; //指向binder的weak refs void __user *cookie; //指向binder本身 unsigned has_strong_ref:1; unsigned pending_strong_ref:1; unsigned has_weak_ref:1; unsigned pending_weak_ref:1; unsigned has_async_transaction:1; unsigned accept_fds:1; unsigned min_priority:8; struct list_head async_todo;};首先初始化binder_node的一些信息，然后将这个binder_node通过rb_node链接到binder_proc结构的nodes红黑树中。接着在处理BINDER_SET_CONTEXT_MGR命令中增加binder_context_mgr_node的强弱引用计数。在处理完BINDER_SET_CONTEXT_MGR命令后，又将binder_thread中的looper置为0。回到service_manager中，接着会调用binder_loop去循环的处理客户端的请求：
void binder_loop(struct binder_state *bs, binder_handler func){ int res; struct binder_write_read bwr; unsigned readbuf[32]; bwr.write_size = 0; bwr.write_consumed = 0; bwr.write_buffer = 0; readbuf[0] = BC_ENTER_LOOPER; binder_write(bs, readbuf, sizeof(unsigned)); for (;;) { bwr.read_size = sizeof(readbuf); bwr.read_consumed = 0; bwr.read_buffer = (unsigned) readbuf; res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr); if (res < 0) { ALOGE("binder_loop: ioctl failed (%s)\n", strerror(errno)); break; } res = binder_parse(bs, 0, readbuf, bwr.read_consumed, func); if (res == 0) { ALOGE("binder_loop: unexpected reply?!\n"); break; } if (res < 0) { ALOGE("binder_loop: io error %d %s\n", res, strerror(errno)); break; } }}func是处理请求的函数指针，也就是svcmgr_handler。这里调用binder_write向binder驱动发送BC_ENTER_LOOPER命令，我们先来看binder_write的实现：
int binder_write(struct binder_state *bs, void *data, unsigned len){ struct binder_write_read bwr; int res; bwr.write_size = len; bwr.write_consumed = 0; bwr.write_buffer = (unsigned) 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;}首先声明一个binder_write_read结构，binder_write_read是在用户空间和内核空间传递数据的结构，定义如下：
struct binder_write_read { signed long write_size; /* bytes to write */ signed long write_consumed; /* bytes consumed by driver */ unsigned long write_buffer; signed long read_size; /* bytes to read */ signed long read_consumed; /* bytes consumed by driver */ unsigned long read_buffer;};在binder_write中，向binder驱动发送一个BINDER_WRITE_READ指令，带有一个binder_write_read数据结构，它里面只有write_buffer和write_size，read_size和read_buffer都为空，来看binder驱动如何处理这个请求：
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; binder_lock(__func__); 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; } if (bwr.write_size > 0) { ret = binder_thread_write(proc, thread, (void __user *)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 err; } } 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); trace_binder_read_done(ret); 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; } } if (copy_to_user(ubuf, &bwr, sizeof(bwr))) { ret = -EFAULT; goto err; } break; }err: if (thread) thread->looper &= ~BINDER_LOOPER_STATE_NEED_RETURN;首先还是通过binder_get_thread去查找是否有处理的binder_thread，通过前面处理BINDER_SET_CONTEXT_MGR命令，我们知道，这里已经创建了一个binder_thread，所以会获取到它并返回。然后通过copy_from_user将用户空间的binder_write_read结构cope到内核空间的bwr变量中，然后判断它的write_size和read_size，如果它们都不为0，就分别调用binder_thread_write和binder_thread_read去分别处理写请求和读请求。从前面binder_writer函数我们知道，这里的write_szie不为0，所以会调用binder_thread_write来处理BC_ENTER_LOOP命令：
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) { if (get_user(cmd, (uint32_t __user *)ptr)) return -EFAULT; ptr += sizeof(uint32_t); trace_binder_command(cmd); switch (cmd) { case BC_ENTER_LOOPER: if (thread->looper & BINDER_LOOPER_STATE_REGISTERED) { thread->looper |= BINDER_LOOPER_STATE_INVALID; binder_user_error("binder: %d:%d ERROR:" " BC_ENTER_LOOPER called after " "BC_REGISTER_LOOPER\n", proc->pid, thread->pid); } thread->looper |= BINDER_LOOPER_STATE_ENTERED; break;首先从binder_write_read数据结构中的writer_buffer取出BC_ENTER_LOOPER命令，然后将binder_thread的looper设置位BINDER_LOOPER_STATE_ENTERED，表示进入到looper循环当中了。处理完BC_ENTER_LOOPER命令后，在binder_looper方法中接着向binder驱动发送BINDER_WRITE_READ，这次带有的binder_write_read参数中，只有read_size不为0，write_size为0，通过上面的知识我们知道，这里会调用binder_thread_read来处理：
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) { if (put_user(BR_NOOP, (uint32_t __user *)ptr)) return -EFAULT; ptr += sizeof(uint32_t); }retry: wait_for_proc_work = thread->transaction_stack == NULL && list_empty(&thread->todo); if (thread->return_error != BR_OK && ptr < end) { } thread->looper |= BINDER_LOOPER_STATE_WAITING; if (wait_for_proc_work) proc->ready_threads++; binder_unlock(__func__); if (wait_for_proc_work) { if (!(thread->looper & (BINDER_LOOPER_STATE_REGISTERED | BINDER_LOOPER_STATE_ENTERED))) { } binder_set_nice(proc->default_priority); if (non_block) { } else ret = wait_event_freezable_exclusive(proc->wait, binder_has_proc_work(proc, thread)); } else { }因为在binder_loop中设置的read_consumed等于0，所以这里会先往read_buffer写入一个BR_NOOP命令。由于刚之前创建的binder_thread中的transaction_stack和todo列表都是空，所以这里的wait_for_proc_work为true，表示需要等待客户端的请求，并将binder_thread的looper置或上BINDER_LOOPER_STATE_WAITING，由于之前执行BC_ENTER_LOOPER命令，所以现在looper的值BINDER_LOOPER_STATE_ENTERED | BINDER_LOOPER_STATE_WAITING。由于在打开/dev/binder中，并没有设置O_NONBLOCK标志，所以这里的non_block为false。最后这里调用wait_event_freezable_exclusive等待客户端的请求。流程如下：
