Android之binder驱动个人学习小结

本文均属自己阅读源码的点滴总结,转账请注明出处谢谢。

欢迎和大家交流。qq:1037701636 email: [email protected][email protected]

 

前言:

Read the fucking Source Code.

这段时间,大概花了两个星期(期间还偷懒了好几天),深入学习了一下Android的Binder驱动。话说上半年在看Mediaplay的源码时,就遇到过很多的IPC,当时也没有深入的去了解这块内容。这次为了对Android有一个系统级别的了解,所以较为深入的学习了一番。主要参考的内容包括:csdn的android 红人老罗,以及手里的一本杨丰盛的Android技术内幕(系统卷),作为主要的学习资料。当然我所小结的内容,也没有他们那么的详细,只是理清了整个思路而已。

注释:

SM:ServiceManager

MS:MediaPlayerService

xxx:指的是某种服务,入HelloService。

一.Binder驱动的整体架构

单从C++层宏观上看来,binder驱动的主要组成部分是:client(客户端),server(服务端),一个Service Manager和binder底层驱动。

整体的框图如下(摘自老罗的图):

Android之binder驱动个人学习小结_第1张图片

其实从图中可以清晰的发现,在Android的应用层中Client和Server所谓的IPC,其实真正的工作均由底层的Binder驱动来完成。也就是说binder驱动可以完成进程间通信,这也是Android特点之一。Service Manager做为一个守护进程,主要来处理客户端的服务请求,管理所有的服务项。

 

二.binder底层驱动核心内容。

说到底,binder底层的驱动架构和通用的linux驱动没有区别,核心的内容包括binder_init,binder_open,binder_mmap,binder_ioctl.

binder驱动在Android系统中以miscdevice完成设备的注册,作为抽象设备,他没有直接操作硬件,只是完成了内存的拷贝处理。如果要深入理解这块机制,请参考老罗的android之旅。在这里对binder_ioctl做一定的分析:

2.1 驱动核心的操作数据结构:

binder_proc和binder_thread:

每open一个binder驱动(系统允许多个进程打开binder驱动),都会有一个专门的binder_proc管理当前进程的信息,包括进程的ID,当前进程由mmap所映射出的buffer信息,以及当前进程所允许的最大线程量。同时这个binder_proc会加入到系统的全局链表binder_procs中去,方便在不同进程之间可以查找信息。

binder_thread:在当前进程下存在多线程,因此binder驱动使用binder_thread来管理对应的线程信息,主要包括线程所属的binder_proc、当前状态looper以及一个transaction_stack(我的理解是负责着实际进程间通信交互的源头和目的地)。

binder_write_read :

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驱动中,以该结构体作为信息封装的中转(可以理解为内核和用户的连接)。在驱动中为根据write_size和read_size的大小来进行处理(见ioctl的解析部分),在write_buffer和read_buffer都代表着用户空间的buffer地址。在write_buffer中,由一个cmd和binder_transaction_data组成,cmd主要告知驱动当前所要处理的内容。
binder_transaction_data:

struct binder_transaction_data {
	/* The first two are only used for bcTRANSACTION and brTRANSACTION,
	 * identifying the target and contents of the transaction.
	 */
	union {
		size_t	handle;	/* target descriptor of command transaction */
		void	*ptr;	/* target descriptor of return transaction */
	} target;
	void		*cookie;	/* target object cookie */
	unsigned int	code;		/* transaction command */

	/* General information about the transaction. */
	unsigned int	flags;
	pid_t		sender_pid;
	uid_t		sender_euid;
	size_t		data_size;	/* number of bytes of data */
	size_t		offsets_size;	/* number of bytes of offsets */

	/* If this transaction is inline, the data immediately
	 * follows here; otherwise, it ends with a pointer to
	 * the data buffer.
	 */
	union {
		struct {
			/* transaction data */
			const void	*buffer;
			/* offsets from buffer to flat_binder_object structs */
			const void	*offsets;
		} ptr;
		uint8_t	buf[8];
	} data;
};

在这里,buffer和offsets分别代表传输内容的数据量以及Binder实体的偏移量(会遇到多个Binder实体)。

binder_transaction:该结构体主要C/S即请求进程和服务进程的相关信息,方便进程间通信,以及信息的调用

binder_work:理解为binder驱动中,进程所要处理的工作项。
binder_transactionbinder_transactionbinder_transactionbinder_transaction

2.2 binder驱动之ioctl解析:

和常用的ioctl相类似,在这里我们关注BINDER_WRITE_READ命令项的内容。

binder_thread_write和binder_thread_read会根据用户传入的write_size和read_size的有无来进行处理。在这里以Mediaplayservice和ServiceManager的通信来分析,调用的cmd如下:

MS首先传入cmd=BC_TRANSACTION:

调用binder_transaction:

static void binder_transaction(struct binder_proc *proc,
			       struct binder_thread *thread,
			       struct binder_transaction_data *tr, int reply)
{
...else {//client请求service
		if (tr->target.handle) {//SM时为target.handle=0
			struct binder_ref *ref;
			ref = binder_get_ref(proc, tr->target.handle);
			if (ref == NULL) {
				binder_user_error("binder: %d:%d got "
					"transaction to invalid handle\n",
					proc->pid, thread->pid);
				return_error = BR_FAILED_REPLY;
				goto err_invalid_target_handle;
			}
			target_node = ref->node;
		} else {
			target_node = binder_context_mgr_node;//调用的是SM守护进程节点
			if (target_node == NULL) {
				return_error = BR_DEAD_REPLY;
				goto err_no_context_mgr_node;
			}
		}
		e->to_node = target_node->debug_id;
		target_proc = target_node->proc;//SM守护进程的相关信息
		if (target_proc == NULL) {
			return_error = BR_DEAD_REPLY;
			goto err_dead_binder;
		}
		if (!(tr->flags & TF_ONE_WAY) && thread->transaction_stack) {
			struct binder_transaction *tmp;
			tmp = thread->transaction_stack;
			if (tmp->to_thread != thread) {
				binder_user_error("binder: %d:%d got new "
					"transaction with bad transaction stack"
					", transaction %d has target %d:%d\n",
					proc->pid, thread->pid, tmp->debug_id,
					tmp->to_proc ? tmp->to_proc->pid : 0,
					tmp->to_thread ?
					tmp->to_thread->pid : 0);
				return_error = BR_FAILED_REPLY;
				goto err_bad_call_stack;
			}
			while (tmp) {
				if (tmp->from && tmp->from->proc == target_proc)
					target_thread = tmp->from;
				tmp = tmp->from_parent;
			}
		}
	}
	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;//SM进程binder_proc的todo
		target_wait = &target_proc->wait;//等待队列头,对应于SM
	}
	...
	if (!reply && !(tr->flags & TF_ONE_WAY))
		t->from = thread;//事务性记录from binder进程,即记录下请求进程
	else
		t->from = NULL;
	t->sender_euid = proc->tsk->cred->euid;
	t->to_proc = target_proc;
	t->to_thread = target_thread;//目的服务进程
	t->code = tr->code;
	t->flags = tr->flags;
	t->priority = task_nice(current);
	t->buffer = binder_alloc_buf(target_proc, tr->data_size,
		tr->offsets_size, !reply && (t->flags & TF_ONE_WAY));//在SM上进程上开辟一个binder_buffer
	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)
		binder_inc_node(target_node, 1, 0, NULL);//增加目标节点的引用

	offp = (size_t *)(t->buffer->data + ALIGN(tr->data_size, sizeof(void *)));//内存中的偏移量

	if (copy_from_user(t->buffer->data, tr->data.ptr.buffer, tr->data_size)) {
		binder_user_error("binder: %d:%d got transaction with invalid "
			"data ptr\n", proc->pid, thread->pid);
		return_error = BR_FAILED_REPLY;
		goto err_copy_data_failed;
	}
	if (copy_from_user(offp, tr->data.ptr.offsets, tr->offsets_size)) {
		binder_user_error("binder: %d:%d got transaction with invalid "
			"offsets ptr\n", proc->pid, thread->pid);
		return_error = BR_FAILED_REPLY;
		goto err_copy_data_failed;
	}
	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;
	}
	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 *))) {       //对buffer大小做一定的检验
			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;
		}
		fp = (struct flat_binder_object *)(t->buffer->data + *offp);//获取一个binder实体
		switch (fp->type) {
		case BINDER_TYPE_BINDER://初次调用
		case BINDER_TYPE_WEAK_BINDER: {
			struct binder_ref *ref;
			struct binder_node *node = binder_get_node(proc, fp->binder);
			if (node == NULL) {
				node = binder_new_node(proc, fp->binder, fp->cookie);//创建一个mediaservice节点
				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;
			}
			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;
			}
			if (fp->type == BINDER_TYPE_BINDER)
				fp->type = BINDER_TYPE_HANDLE;//fp->type类型改为了BINDER_TYPE_HANDLE句柄
			else
				fp->type = BINDER_TYPE_WEAK_HANDLE;
			fp->handle = ref->desc;//
			binder_inc_ref(ref, fp->type == BINDER_TYPE_HANDLE,
				       &thread->todo);//增加引用次数

			binder_debug(BINDER_DEBUG_TRANSACTION,
				     "        node %d u%p -> ref %d desc %d\n",
				     node->debug_id, node->ptr, ref->debug_id,
				     ref->desc);
		} break;
		case BINDER_TYPE_HANDLE:
		case BINDER_TYPE_WEAK_HANDLE: {
			struct binder_ref *ref = binder_get_ref(proc, fp->handle);
			if (ref == NULL) {
				binder_user_error("binder: %d:%d got "
					"transaction with invalid "
					"handle, %ld\n", proc->pid,
					thread->pid, fp->handle);
				return_error = BR_FAILED_REPLY;
				goto err_binder_get_ref_failed;
			}
			if (ref->node->proc == target_proc) {
				if (fp->type == BINDER_TYPE_HANDLE)
					fp->type = BINDER_TYPE_BINDER;
				else
					fp->type = BINDER_TYPE_WEAK_BINDER;
				fp->binder = ref->node->ptr;
				fp->cookie = ref->node->cookie;
				binder_inc_node(ref->node, fp->type == BINDER_TYPE_BINDER, 0, NULL);
				binder_debug(BINDER_DEBUG_TRANSACTION,
					     "        ref %d desc %d -> node %d u%p\n",
					     ref->debug_id, ref->desc, ref->node->debug_id,
					     ref->node->ptr);
			} else {
				struct binder_ref *new_ref;
				new_ref = binder_get_ref_for_node(target_proc, ref->node);
				if (new_ref == NULL) {
					return_error = BR_FAILED_REPLY;
					goto err_binder_get_ref_for_node_failed;
				}
				fp->handle = new_ref->desc;
				binder_inc_ref(new_ref, fp->type == BINDER_TYPE_HANDLE, NULL);
				binder_debug(BINDER_DEBUG_TRANSACTION,
					     "        ref %d desc %d -> ref %d desc %d (node %d)\n",
					     ref->debug_id, ref->desc, new_ref->debug_id,
					     new_ref->desc, ref->node->debug_id);
			}
		} break;
		default:
			binder_user_error("binder: %d:%d got transactio"
				"n with invalid object type, %lx\n",
				proc->pid, thread->pid, fp->type);
			return_error = BR_FAILED_REPLY;
			goto err_bad_object_type;
		}
	}
	if (reply) {
		BUG_ON(t->buffer->async_transaction != 0);
		binder_pop_transaction(target_thread, in_reply_to);
	} else if (!(t->flags & TF_ONE_WAY)) {
		BUG_ON(t->buffer->async_transaction != 0);
		t->need_reply = 1;
		t->from_parent = thread->transaction_stack; 
		thread->transaction_stack = t;
	} else {
		BUG_ON(target_node == NULL);
		BUG_ON(t->buffer->async_transaction != 1);
		if (target_node->has_async_transaction) {
			target_list = &target_node->async_todo;
			target_wait = NULL;
		} else
			target_node->has_async_transaction = 1;
	}
	t->work.type = BINDER_WORK_TRANSACTION;
	list_add_tail(&t->work.entry, target_list);//binder_work添加到SM进程Proc链表中
	tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE;//type设置为BINDER_WORK_TRANSACTION_COMPLETE
	list_add_tail(&tcomplete->entry, &thread->todo);//待完成的工作加入的本线程的todo链表中
	if (target_wait)
		wake_up_interruptible(target_wait);//唤醒Service Manager进程
	return;

...}

分析这个函数,可以知道和SM通信时,获取target_proc为SM进程的相关信息。然后是维护当前请求的binder实体,以免被crash。以binder_transaction t为C/S之间做为传递的信息,做初始化记录请求进程和服务进程到t中。最后做如下操作:

 t->work.type = BINDER_WORK_TRANSACTION;
 list_add_tail(&t->work.entry, target_list);//binder_work添加到SM进程Proc链表中
 tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE;//type设置为BINDER_WORK_TRANSACTION_COMPLETE
 list_add_tail(&tcomplete->entry, &thread->todo);//待完成的工作加入的本线程的todo链表中
 if (target_wait)
  wake_up_interruptible(target_wait);//唤醒Service Manager进程

可以看到,将这个t加入到了服务进程SM的链表中,将待完成的tcomplete加入到当前MS的thread中,最后唤醒SM,做相关的处理。

MS继续执行binder_thread_read如下:

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))//添加BR_NOOP
			return -EFAULT;
		ptr += sizeof(uint32_t);
	}

retry:
	wait_for_proc_work = thread->transaction_stack == NULL &&
				list_empty(&thread->todo);// false

	if (thread->return_error != BR_OK && ptr < end) {
		if (thread->return_error2 != BR_OK) {
			if (put_user(thread->return_error2, (uint32_t __user *)ptr))
				return -EFAULT;
			ptr += sizeof(uint32_t);
			if (ptr == end)
				goto done;
			thread->return_error2 = BR_OK;
		}
		if (put_user(thread->return_error, (uint32_t __user *)ptr))
			return -EFAULT;
		ptr += sizeof(uint32_t);
		thread->return_error = BR_OK;
		goto done;
	}


	thread->looper |= BINDER_LOOPER_STATE_WAITING;
	if (wait_for_proc_work)
		proc->ready_threads++;
	mutex_unlock(&binder_lock);
	if (wait_for_proc_work) {
		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) {
			if (!binder_has_proc_work(proc, thread))
				ret = -EAGAIN;
		} else
			ret = wait_event_interruptible_exclusive(proc->wait, binder_has_proc_work(proc, thread));//binder_has_proc_work为false唤醒
	} else {
		if (non_block) {
			if (!binder_has_thread_work(thread))
				ret = -EAGAIN;
		} else
			ret = wait_event_interruptible(thread->wait, binder_has_thread_work(thread));
	}
	mutex_lock(&binder_lock);
	if (wait_for_proc_work)
		proc->ready_threads--;
	thread->looper &= ~BINDER_LOOPER_STATE_WAITING;

	if (ret)
		return ret;

	while (1) {
		uint32_t cmd;
		struct binder_transaction_data tr;
		struct binder_work *w;
		struct binder_transaction *t = NULL;

		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)//在SM被唤醒时proc->todo为1且wait_for_proc_work等待进程有事情做
			w = list_first_entry(&proc->todo, struct binder_work, entry);//获取binder_work
		else {
			if (ptr - buffer == 4 && !(thread->looper & BINDER_LOOPER_STATE_NEED_RETURN)) /* no data added */
				goto retry;
			break;
		}

		if (end - ptr < sizeof(tr) + 4)
			break;

		switch (w->type) {
		case BINDER_WORK_TRANSACTION: {//SM唤醒时带调用
			t = container_of(w, struct binder_transaction, work);//通过binder_transaction的指针变量work为w,获取binder_transaction
		} break;
		case BINDER_WORK_TRANSACTION_COMPLETE: { 
			cmd = BR_TRANSACTION_COMPLETE;
			if (put_user(cmd, (uint32_t __user *)ptr))  //BR_TRANSACTION_COMPLETE命令写回
				return -EFAULT;
			ptr += sizeof(uint32_t);

			binder_stat_br(proc, thread, cmd);
			binder_debug(BINDER_DEBUG_TRANSACTION_COMPLETE,
				     "binder: %d:%d BR_TRANSACTION_COMPLETE\n",
				     proc->pid, thread->pid);

			list_del(&w->entry);//从thread->todo删除链表
			kfree(w);
			binder_stats_deleted(BINDER_STAT_TRANSACTION_COMPLETE);
		} break;

写会BR_NOOP和BR_TRANSACTION_COMPLETE给用户空间,相当于从内核读取了数据,同时也做list_del(&w->entry)的处理。

MS继续与binder交互,进入ret = wait_event_interruptible(thread->wait, binder_has_thread_work(thread));进入睡眠等待SM的唤醒。

 

SM在被MS唤醒后所做的处理如下:

SM同样在binder_thread_read时处于ret = wait_event_interruptible(thread->wait, binder_has_thread_work(thread));的睡眠当中,但是此时proc->todo已经有内容,在前面的MS write的过程进行了(list_add_tail(&t->work.entry, target_list);//binder_work添加到SM进程Proc链表中)操作,所以会执行:

 w = list_first_entry(&proc->todo, struct binder_work, entry);//获取binder_work

 t = container_of(w, struct binder_transaction, work);//通过binder_transaction的指针变量work为w,获取binder_transaction

最后获取binder_transaction t 用于SM和MS用来交互和中转信息。

有了从MS传递过来的t,将t的相关信息读取回SM的用户空间,传递给SM的命令为cmd=BR_TRANSACTION。

MS再次传递cmd=BC_REPLY,再次回到binder_thread_write

{  //reply=1,sevice回复给client
		in_reply_to = thread->transaction_stack;//获取当前事务性即原来MS传递给SM的binder_transaction变量t
		if (in_reply_to == NULL) {
			binder_user_error("binder: %d:%d got reply transaction "
					  "with no transaction stack\n",
					  proc->pid, thread->pid);
			return_error = BR_FAILED_REPLY;
			goto err_empty_call_stack;
		}
		binder_set_nice(in_reply_to->saved_priority);
		if (in_reply_to->to_thread != thread) {
			binder_user_error("binder: %d:%d got reply transaction "
				"with bad transaction stack,"
				" transaction %d has target %d:%d\n",
				proc->pid, thread->pid, in_reply_to->debug_id,
				in_reply_to->to_proc ?
				in_reply_to->to_proc->pid : 0,
				in_reply_to->to_thread ?
				in_reply_to->to_thread->pid : 0);
			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;//获取请求的线程
		if (target_thread == NULL) {
			return_error = BR_DEAD_REPLY;
			goto err_dead_binder;
		}
		if (target_thread->transaction_stack != in_reply_to) {
			binder_user_error("binder: %d:%d got reply transaction "
				"with bad target transaction stack %d, "
				"expected %d\n",
				proc->pid, thread->pid,
				target_thread->transaction_stack ?
				target_thread->transaction_stack->debug_id : 0,
				in_reply_to->debug_id);
			return_error = BR_FAILED_REPLY;
			in_reply_to = NULL;
			target_thread = NULL;
			goto err_dead_binder;
		}
		target_proc = target_thread->proc;//请教进程的相关信息
	}

前期MS在执行时,将MS自己的thread信息记录在了t当中。

	if (!reply && !(tr->flags & TF_ONE_WAY))
		t->from = thread;//事务性记录from binder进程,即记录下请求进程

因此SM在执行binder_thread_write时,会获取到请求进程的thread,最终和前面MS唤醒SM一样,唤醒SM,只是现在的目标进程target_proc换成了MS的内容。

最终SM回互用户空间BR_TRANSACTION_COMPLETE,SM随后再次进行LOOP循环,睡眠等待其他请求进程的唤醒。

MS被唤醒后,所做的处理和SM被唤醒时相类似,在这里写会的cmd=BR_REPLY,以此完成了一次SM和MS的IPC.

 

2.3 binder 驱动C++层的机制简单介绍

可以简单的理解Binder IPC 实际就是C/S通过Linux的机制,对各自线程的信息进行维护,使SM和MS的用户空间不断和内核空间以ioctl进行读写的交互。服务端对信息进行解析完成相应的操作。客户度实际只需发送命令即可。作为应用程序的开发,Android很好的为我们做了各种封装,包括C++层次的binder和Java层次的binder驱动。

核心类:BpBinder(远程BinderProxy),BBinder(Native 本地Binder)

基于Binder C++层的机制,以SM和MS为例,在MS如果要和SM通信,就需要获得SM在MS进程中的一个Proxy,这里称之为BpServiceManager,BpServiceManager的操作函数分为addservice和getservice,需要的参量为一个Bpbinder(在这里就是SM远程的binder对象,相当于一个句柄,由于其特殊性,句柄数值为0)。

在2.2中分析的binder底层部分内容,就是基于用户空间的addservice开始的。在这里引用罗老师的UML图,方便自己的理解。

Android之binder驱动个人学习小结_第2张图片

在这里只对BpServiceManager的addservice做解析,该类最终的实现其实还是调用BpBinder的transact来完成,而该函数的实现最终调用的是IPCThreadState的transact,在该transact代码如下:

status_t IPCThreadState::transact(int32_t handle,
                                  uint32_t code, const Parcel& data,
                                  Parcel* reply, uint32_t flags)  //handle=0,flags=0
{
    status_t err = data.errorCheck();

    flags |= TF_ACCEPT_FDS;

    IF_LOG_TRANSACTIONS() {
        TextOutput::Bundle _b(alog);
        alog << "BC_TRANSACTION thr " << (void*)pthread_self() << " / hand "
            << handle << " / code " << TypeCode(code) << ": "
            << indent << data << dedent << endl;
    }
    
    if (err == NO_ERROR) {
        LOG_ONEWAY(">>>> SEND from pid %d uid %d %s", getpid(), getuid(),
            (flags & TF_ONE_WAY) == 0 ? "READ REPLY" : "ONE WAY");
        err = writeTransactionData(BC_TRANSACTION, flags, handle, code, data, NULL);//将要发送的数据整理成一个binder_transaction_data
    }
    
    if (err != NO_ERROR) {
        if (reply) reply->setError(err);
        return (mLastError = err);
    }
    
    if ((flags & TF_ONE_WAY) == 0) {
        #if 0
        if (code == 4) { // relayout
            LOGI(">>>>>> CALLING transaction 4");
        } else {
            LOGI(">>>>>> CALLING transaction %d", code);
        }
        #endif
        if (reply) {
            err = waitForResponse(reply);
        } else {
            Parcel fakeReply;
            err = waitForResponse(&fakeReply);
        }
        #if 0
        if (code == 4) { // relayout
            LOGI("<<<<<< RETURNING transaction 4");
        } else {
            LOGI("<<<<<< RETURNING transaction %d", code);
        }
        #endif
        
        IF_LOG_TRANSACTIONS() {
            TextOutput::Bundle _b(alog);
            alog << "BR_REPLY thr " << (void*)pthread_self() << " / hand "
                << handle << ": ";
            if (reply) alog << indent << *reply << dedent << endl;
            else alog << "(none requested)" << endl;
        }
    } else {
        err = waitForResponse(NULL, NULL);
    }
    
    return err;
}

在这里真正实现ioctl的内容在waitForResponse的talkWithDriver中实现。

SM作为Android系统中特殊的一部分,他即可用当做服务端,也管理着系统的所有Service。新的服务需要向他完成注册才可以正常的使用。因此在这里的addservice就是在远程通过Binder驱动和SM交互,完成了MS的注册,注册传入的是一个BBinder的实体BnMediaPlayService,name=MediaPlay。

在C++的binder机制中,Bpxxx对应的Bnxxx(Bpxxx继承自BpBinder,Bnxxx继承自BBinder),简单理解就是Bnxxx在向SM完成注册后,会自动启动一个线程来等待客户端的请求,而在客户端如果要请求服务,需要获取一个Bpxxx远程代理来完成。Bpxxx在getservice时还回xxx服务的binder句柄,存放在Bpxxx对应的BpBinder的mHandle中。在binder驱动的底层会根据这个mHandle,查找到对应的target服务进程,同理根据2.2中MS唤醒SM的过程,进行命令的处理。

因此总结出在客户端需要服务时,首先获得Bpxxx(new BpBinder(mHandle))。然后是最终调用BpBinder的remote()->transact。而在用户端以BBinder->ontransact完成命令的解析。

 

2.4 Binder驱动的Java机制

简单的说一下Java层的binder驱动,其实这部分的难点还是在于Java 中Native函数在JNI的转换,感谢Google的开发人员,实现了Java和C++层的Binder函数的转换。

简单的总结3个小点:

1.Java层拥有一个SM的远程接口SMProxy,句柄为0 的BinderProxy对象,BinderProxy相当于BpBinder,在JNI实现转换。

2,Ixxx接口定义一个stub和proxy,Stub(存根):理解为本地服务。proxy:远程的代理。与C++相对应的前者就是Bnxxx,后者就是Bpxxx。

3. xxx需要继承了Ixxx的Stub,才可以完成请求的处理。

 

2.5 总结

上面的内容,基本是自己的阅读和学习的感受,Binder驱动的复杂程度是难以想象的,源码量大。写完本文也没有全部读通,但是这也为深入的去了解整个android系统开辟了基础。其中有些内容都是参考罗老师的Android之旅来完成的,在这里表示感谢。在接下去的一端时间将在Android4.0.3 ICS上学习Android系统整个系统过程,主要关心的是3个开机画面,继续给力,最近身体不是很舒服,对着电脑头老是晕,效率下降了很多,但是依旧在继续努力,给自己加油。

 

 

 

  
 

 
 


 

 

 


 

 

 

 

 

 

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