Binder驱动之最简单的通讯
在Android系统中能够所是无处不Binder，Binder传输在每时每刻都发生着。不少状况下，一个进程中都不会只存在一个独立的Binder传输，常常是并发多个Binder传输，并且会存在Binder嵌套。尤为像system_server这种重要的进程Binder传输会更多。在系统发生问题时，若是追踪到system_server，会发现大部分状况都是在Binder传输中。但不管有多少Binder传输或多复杂的Binder嵌套，最终都是经过两种Binder传输实现的：同步传输和异步传输。这里试图经过最简单的传输来解释Binder通讯流程。node
Binder同步传输
Binder传输中最多见的就是同步传输。同步传输中，IPC通讯的发起端须要等到对端处理完消息才能继续。一个完整的同步传输以下图所示。
跳过Binder设备初始化的过程，直接看传输过程。客户端经过ioctl的BINDER_WRITE_READ发送BC_TRANSACTION命令到Binder驱动。android
drivers/staging/android/binder.c
static long binder_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
......
switch (cmd) {
case BINDER_WRITE_READ: {
struct binder_write_read bwr;
......
// 须要写数据
if (bwr.write_size > 0) {
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 err;
}
}
// 须要读数据
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);
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;
}
}
......
break;
}
......
1.BC_TRANSACTION
发起Binder传输时，须要写入BC_TRANSACTION命令，而后等待命令返回。cookie
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)
{
......
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_TRANSACTION和BC_REPLY都会调用binder_transaction()，区别在因而否设置reply。binder_transaction()也是写数据的核心函数。函数很长，逻辑不少，尽可能分析一下。并发
drivers/staging/android/binder.c
static void binder_transaction(struct binder_proc *proc,
struct binder_thread *thread,
struct binder_transaction_data *tr, int reply)
{
......
if (reply) {
......
} else {
if (tr->target.handle) {
// 根据handle找到相应的binder实体
struct binder_ref *ref;
ref = binder_get_ref(proc, tr->target.handle, true);
......
target_node = ref->node;
} else {
// handle为0时为service manager的binder实体
target_node = binder_context_mgr_node;
......
}
e->to_node = target_node->debug_id;
// binder实体的binder_proc
target_proc = target_node->proc;
......
if (!(tr->flags & TF_ONE_WAY) && thread->transaction_stack) {
struct binder_transaction *tmp;
tmp = thread->transaction_stack;
......
// 若是是同步传输，寻找是否传输栈中是否有来自对端的传输，若是有就使用对端线程处理传输
while (tmp) {
if (tmp->from && tmp->from->proc == target_proc)
target_thread = tmp->from;
tmp = tmp->from_parent;
}
}
}
// 找到对端线程这使用线程todo list，不然使用进程todo list
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;
}
e->to_proc = target_proc->pid;
// 分配binder transaction
t = kzalloc(sizeof(*t), GFP_KERNEL);
......
// 分配binder_work用于处理传输完成
tcomplete = kzalloc(sizeof(*tcomplete), GFP_KERNEL);
......
// 同步的非reply传输，设置当前线程为from
if (!reply && !(tr->flags & TF_ONE_WAY))
t->from = thread;
else
t->from = NULL;
t->sender_euid = proc->tsk->cred->euid;
// 设置传输的目标进程和线程
t->to_proc = target_proc;
t->to_thread = target_thread;
t->code = tr->code;
t->flags = tr->flags;
t->priority = task_nice(current);
// 从目标进程中分配传输空间
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;
// 增长binder实体的引用计数
if (target_node)
binder_inc_node(target_node, 1, 0, NULL);
offp = (binder_size_t *)(t->buffer->data +
ALIGN(tr->data_size, sizeof(void *)));
// 拷贝用户数据到binder实体的传输空间中
if (copy_from_user(t->buffer->data, (const void __user *)(uintptr_t)
tr->data.ptr.buffer, tr->data_size)) {
......
}
// 拷贝用户数据的flat_binder_object对象信息
if (copy_from_user(offp, (const void __user *)(uintptr_t)
tr->data.ptr.offsets, tr->offsets_size)) {
......
}
......
off_end = (void *)offp + tr->offsets_size;
off_min = 0;
// 处理flat_binder_object对象信息
for (; offp < off_end; offp++) {
struct flat_binder_object *fp;
......
fp = (struct flat_binder_object *)(t->buffer->data + *offp);
off_min = *offp + sizeof(struct flat_binder_object);
switch (fp->type) {
// 类型为binder实体，用于server注册
case BINDER_TYPE_BINDER:
case BINDER_TYPE_WEAK_BINDER: {
struct binder_ref *ref;
// 若是找不到binder实体就建立一个
struct binder_node *node = binder_get_node(proc, fp->binder);
if (node == NULL) {
node = binder_new_node(proc, fp->binder, fp->cookie);
......
}
......
// 在目标进程中建立引用
ref = binder_get_ref_for_node(target_proc, node);
......
// 修改binder对象的类型为handle
if (fp->type == BINDER_TYPE_BINDER)
fp->type = BINDER_TYPE_HANDLE;
else
fp->type = BINDER_TYPE_WEAK_HANDLE;
fp->binder = 0;
// 将引用的handle赋值给对象
fp->handle = ref->desc;
fp->cookie = 0;
// 增长引用计数
binder_inc_ref(ref, fp->type == BINDER_TYPE_HANDLE,
&thread->todo);
......
} break;
// 类型为binder引用，client向server传输
case BINDER_TYPE_HANDLE:
case BINDER_TYPE_WEAK_HANDLE: {
// 获取当前进程中的binder引用
struct binder_ref *ref = binder_get_ref(
proc, fp->handle,
fp->type == BINDER_TYPE_HANDLE);
......
if (ref->node->proc == target_proc) {
// 若是binder传输发生在同一进程中则直接使用binder实体
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);
......
} else {
struct binder_ref *new_ref;
// 在目标进程中建立binder引用
new_ref = binder_get_ref_for_node(target_proc, ref->node);
......
fp->binder = 0;
fp->handle = new_ref->desc;
fp->cookie = 0;
binder_inc_ref(new_ref, fp->type == BINDER_TYPE_HANDLE, NULL);
......
}
} break;
// 类型为文件描述符，用于共享文件或内存
case BINDER_TYPE_FD: {
......
} break;
......
}
}
if (reply) {
......
} else if (!(t->flags & TF_ONE_WAY)) {
// 当前线程的传输入栈
t->need_reply = 1;
t->from_parent = thread->transaction_stack;
thread->transaction_stack = t;
} else {
// 异步传输使用aync todo list
if (target_node->has_async_transaction) {
target_list = &target_node->async_todo;
target_wait = NULL;
} else
target_node->has_async_transaction = 1;
}
// 将传输添加到目标队列中
t->work.type = BINDER_WORK_TRANSACTION;
list_add_tail(&t->work.entry, target_list);
// 将传输完成添加到当前线程todo队列中
tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE;
list_add_tail(&tcomplete->entry, &thread->todo);
// 唤醒目标线程或进程
if (target_wait)
wake_up_interruptible(target_wait);
return;
......
}
BC_TRANSACTION简单来讲流程以下，异步
- 找到目标进程或线程。
- 将用户空间的数据拷贝到目前进程空间，并解析flat_binder_object。
- 将传输入栈到当前线程中。
- 将BINDER_WORK_TRANSACTION加入到目标队列，将BINDER_WORK_TRANSACTION_COMPLETE加入到当前线程队列。
- 唤醒目标进程或线程进行处理。
2.BR_TRANSACTION_COMPLETE
Client在执行BINDER_WRITE_READ时，先经过binder_thread_write()写数据，将BINDER_WORK_TRANSACTION_COMPLETE放入工做队列。紧接着就执行binder_thread_read()读取返回数据。这里会将命令BR_TRANSACTION_COMPLETE返回给Client线程。async
drivers/staging/android/binder.c
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)
{
......
// 第一次读时，插入命令BR_NOOP返回给用户
if (*consumed == 0) {
if (put_user(BR_NOOP, (uint32_t __user *)ptr))
return -EFAULT;
ptr += sizeof(uint32_t);
}
retry:
// 当前线程没有传输而且todo队列为空时，处理进程的工做队列
wait_for_proc_work = thread->transaction_stack == NULL &&
list_empty(&thread->todo);
......
thread->looper |= BINDER_LOOPER_STATE_WAITING;
// 若是处理进程工做队列，则当前线程为空闲线程
if (wait_for_proc_work)
proc->ready_threads++;
......
// 等待进程或线程工做队列被唤醒
if (wait_for_proc_work) {
......
ret = wait_event_freezable_exclusive(proc->wait, binder_has_proc_work(proc, thread));
} else {
......
ret = wait_event_freezable(thread->wait, binder_has_thread_work(thread));
}
......
// 唤醒后，开始处理传输，空闲线程减1
if (wait_for_proc_work)
proc->ready_threads--;
thread->looper &= ~BINDER_LOOPER_STATE_WAITING;
......
while (1) {
......
// 优先处理线程工做队列，再处理进程工做队列
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 {
if (ptr - buffer == 4 && !(thread->looper & BINDER_LOOPER_STATE_NEED_RETURN)) /* no data added */
goto retry;
break;
}
......
switch (w->type) {
......
case BINDER_WORK_TRANSACTION_COMPLETE: {
// 发送命令BR_TRANSACTION_COMPLETE给用户
cmd = BR_TRANSACTION_COMPLETE;
if (put_user(cmd, (uint32_t __user *)ptr))
return -EFAULT;
ptr += sizeof(uint32_t);
......
list_del(&w->entry);
kfree(w);
binder_stats_deleted(BINDER_STAT_TRANSACTION_COMPLETE);
} break;
......
if (!t)
continue;
......
}
3.BR_TRANSACTION
Server端线程启动后就调用talkWithDriver()等待读取数据。Binder驱动处理Client发送的BC_TRANSACTION命令后，会唤醒Server线程。Server线程读取数据进行处理一样是在binder_thread_read()中完成的。函数
drivers/staging/android/binder.c
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)
{
......
while (1) {
switch (w->type) {
// binder_transaction()将工做BINDER_WORK_TRANSACTION加入队列后唤醒目标进程
case BINDER_WORK_TRANSACTION: {
t = container_of(w, struct binder_transaction, work);
} break;
......
// 只有BINDER_WORK_TRANSACTION取出传输事件，因此能够继续执行
if (!t)
continue;
BUG_ON(t->buffer == NUL);
// target_node存在时代表是BC_TRANSACTION产生的工做事件，须要回复BR_TRANSACTION。
// 不然是BC_REPLY产生的工做事件，回复BR_REPLY
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;
t->saved_priority = task_nice(current);
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 {
tr.target.ptr = 0;
tr.cookie = 0;
cmd = BR_REPLY;
}
tr.code = t->code;
tr.flags = t->flags;
tr.sender_euid = from_kuid(current_user_ns(), t->sender_euid);
// 同步传输时，sender_pid为调用进程的pid。异步传输时为0。
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 {
tr.sender_pid = 0;
}
......
// 将数据拷贝到用户空间
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);
......
// 从队列中移除当前工做事件
list_del(&t->work.entry);
t->buffer->allow_user_free = 1;
if (cmd == BR_TRANSACTION && !(t->flags & TF_ONE_WAY)) {
// 同步传输时，命令为BR_TRANSACTION的状况下，将工做事件入栈
t->to_parent = thread->transaction_stack;
t->to_thread = thread;
thread->transaction_stack = t;
} else {
// 其余状况下，代表传输已经完成，释放工做事件
t->buffer->transaction = NULL;
kfree(t);
binder_stats_deleted(BINDER_STAT_TRANSACTION);
}
break;
}
......
}
BR_REPLY也是一样的流程，区别在于发送BR_REPLY意味着传输已经完成，能够释放工做事件。oop
4.BC_REPLY
Server端接收到BR_TRANSACTION命令后，取出buffer进行处理，完成后会发送BC_REPLY给Binder驱动。ui
frameworks/native/libs/binder/IPCThreadState.cpp
status_t IPCThreadState::executeCommand(int32_t cmd)
{
......
case BR_TRANSACTION:
{
// 取出传输数据
binder_transaction_data tr;
result = mIn.read(&tr, sizeof(tr));
......
Parcel reply;
......
// BBinder对数据进行解析
if (tr.target.ptr) {
sp<BBinder> b((BBinder*)tr.cookie);
error = b->transact(tr.code, buffer, &reply, tr.flags);
} else {
error = the_context_object->transact(tr.code, buffer, &reply, tr.flags);
}
if ((tr.flags & TF_ONE_WAY) == 0) {
LOG_ONEWAY("Sending reply to %d!", mCallingPid);
if (error < NO_ERROR) reply.setError(error);
// 同步传输须要发送BC_REPLY
sendReply(reply, 0);
} else {
LOG_ONEWAY("NOT sending reply to %d!", mCallingPid);
}
......
}
break;
......
}
BC_REPLY也是经过binder_transaction()处理，只是须要设置参数reply。下面只分析与以前不一样的地方。this
drivers/staging/android/binder.c
static void binder_transaction(struct binder_proc *proc,
struct binder_thread *thread,
struct binder_transaction_data *tr, int reply)
{
......
if (reply) {
// 从当前线程中出栈
in_reply_to = thread->transaction_stack;
......
thread->transaction_stack = in_reply_to->to_parent;
// 目标线程为发起端线程
target_thread = in_reply_to->from;
......
target_proc = target_thread->proc;
} else {
......
}
if (target_thread) {
e->to_thread = target_thread->pid;
target_list = &target_thread->todo;
target_wait = &target_thread->wait;
} else {
......
}
......
// reply传输的from为空
if (!reply && !(tr->flags & TF_ONE_WAY))
t->from = thread;
else
t->from = NULL;
......
if (reply) {
// 从目标线程中出栈
binder_pop_transaction(target_thread, in_reply_to);
} else if (!(t->flags & TF_ONE_WAY)) {
......
} else {
......
}
t->work.type = BINDER_WORK_TRANSACTION;
list_add_tail(&t->work.entry, target_list);
tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE;
list_add_tail(&tcomplete->entry, &thread->todo);
if (target_wait)
wake_up_interruptible(target_wait);
return;
......
}
binder_transaction()执行完BC_REPLY后一样是加入工做队列，唤醒target。BINDER_WORK_TRANSACTION_COMPLETE会将BR_TRANSACTION_COMPLETE返回给当前线程，也就是Server端。BINDER_WORK_TRANSACTION由target处理，这时的target为Client端。根据上面分析，驱动将返回BR_REPLY给Client端。
5.BC_FREE_BUFFER
Binder的每一次传输，不管是从Client到Sever仍是Server到Client，在对端接收到数据并处理完成后，都会经过BC_FREE_BUFFER来释放传输空间。在同步传输中会包含两次传输，由Client发出的BC_TRANSACTION和由Server发出的BC_REPLY。
在BC_TRANSACTION中，Server端接收到BR_TRANSACTION命令开始处理Binder数据，处理完成后就会发出BC_FREE_BUFFER来释放buffer。这个释放命令不是直接发出的，是经过Parcel的释放函数完成的。将freeBuffer设置为Parcel实例buffer的释放函数，在buffer析构时会调用释放函数freeBuffer。
frameworks/native/libs/binder/IPCThreadState.cpp
status_t IPCThreadState::executeCommand(int32_t cmd)
{
......
case BR_TRANSACTION:
{
binder_transaction_data tr;
result = mIn.read(&tr, sizeof(tr));
......
Parcel buffer;
// 设置buffer的释放函数为freeBuffer
buffer.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);
......
sp<BBinder> b((BBinder*)tr.cookie);
error = b->transact(tr.code, buffer, &reply, tr.flags);
......
}
break;
......
}
......
void IPCThreadState::freeBuffer(Parcel* parcel, const uint8_t* data,
size_t /*dataSize*/,
const binder_size_t* /*objects*/,
size_t /*objectsSize*/, void* /*cookie*/)
{
......
if (parcel != NULL) parcel->closeFileDescriptors();
// 发送BC_FREE_BUFFER命令
IPCThreadState* state = self();
state->mOut.writeInt32(BC_FREE_BUFFER);
state->mOut.writePointer((uintptr_t)data);
}
在BC_REPLY中，Client端接收到BR_REPLY会将freeBuffer设置为释放函数或直接调用freeBuffer。
frameworks/native/libs/binder/IPCThreadState.cpp
status_t IPCThreadState::waitForResponse(Parcel *reply, status_t *acquireResult)
{
int32_t cmd;
int32_t err;
while (1) {
......
case BR_REPLY:
{
binder_transaction_data tr;
err = mIn.read(&tr, sizeof(tr));
ALOG_ASSERT(err == NO_ERROR, "Not enough command data for brREPLY");
if (err != NO_ERROR) goto finish;
if (reply) {
if ((tr.flags & TF_STATUS_CODE) == 0) {
// 设置freeBuffer为释放函数
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 {
// 发生错误时直接调用freeBuffer
err = *reinterpret_cast<const status_t*>(tr.data.ptr.buffer);
freeBuffer(NULL,
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), this);
}
} else {
freeBuffer(NULL,
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), this);
continue;
}
}
goto finish;
......
}
......
FreeBuffer()发送BC_FREE_BUFFER命令给Binder驱动。
drivers/staging/android/binder.c
static void binder_free_buf(struct binder_proc *proc,
struct binder_buffer *buffer)
{
size_t size, buffer_size;
// 获取buffer的大小
buffer_size = binder_buffer_size(proc, buffer);
size = ALIGN(buffer->data_size, sizeof(void *)) +
ALIGN(buffer->offsets_size, sizeof(void *));
......
// 更新异步传输的free_async_space
if (buffer->async_transaction) {
proc->free_async_space += size + sizeof(struct binder_buffer);
......
}
// 释放物理内存
binder_update_page_range(proc, 0,
(void *)PAGE_ALIGN((uintptr_t)buffer->data),
(void *)(((uintptr_t)buffer->data + buffer_size) & PAGE_MASK),
NULL);
// 将buffer从allocated_buffers树上擦除
rb_erase(&buffer->rb_node, &proc->allocated_buffers);
buffer->free = 1;
// 向后合并空闲buffer
if (!list_is_last(&buffer->entry, &proc->buffers)) {
struct binder_buffer *next = list_entry(buffer->entry.next,
struct binder_buffer, entry);
if (next->free) {
rb_erase(&next->rb_node, &proc->free_buffers);
binder_delete_free_buffer(proc, next);
}
}
// 向前合并空闲buffer
if (proc->buffers.next != &buffer->entry) {
struct binder_buffer *prev = list_entry(buffer->entry.prev,
struct binder_buffer, entry);
if (prev->free) {
binder_delete_free_buffer(proc, buffer);
rb_erase(&prev->rb_node, &proc->free_buffers);
buffer = prev;
}
}
// 将合并后的buffer插入到free_buffers上
binder_insert_free_buffer(proc, buffer);
}
......
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)
{
......
case BC_FREE_BUFFER: {
binder_uintptr_t data_ptr;
struct binder_buffer *buffer;
// 获取用户空间数据
if (get_user(data_ptr, (binder_uintptr_t __user *)ptr))
return -EFAULT;
ptr += sizeof(binder_uintptr_t);
// 从buffer树中找到相应的binder_buffer
buffer = binder_buffer_lookup(proc, data_ptr);
......
if (buffer->transaction) {
buffer->transaction->buffer = NULL;
buffer->transaction = NULL;
}
// 异步传输在释放buffer时将未完成的async_todo工做移动到线程的todo队列上
if (buffer->async_transaction && buffer->target_node) {
BUG_ON(!buffer->target_node->has_async_transaction);
if (list_empty(&buffer->target_node->async_todo))
buffer->target_node->has_async_transaction = 0;
else
list_move_tail(buffer->target_node->async_todo.next, &thread->todo);
}
trace_binder_transaction_buffer_release(buffer);
// 减小binder引用计数
binder_transaction_buffer_release(proc, buffer, NULL);
// 释放buffer内存空间
binder_free_buf(proc, buffer);
break;
}
......
}
Binder异步传输
Binder通讯中，若是Client端只但愿发送数据而无论Server端的执行结果，可使用异步传输。异步传输须要在传输数据的flag中设置TF_ONE_WAY位，简单的传输流程以下图。
异步传输在Binder驱动中的处理流程与同步传输同样，咱们重点看一下对TF_ONE_WAY标志的处理流程。
drivers/staging/android/binder.c
static struct binder_buffer *binder_alloc_buf(struct binder_proc *proc,
size_t data_size,
size_t offsets_size, int is_async)
{
......
// 异步传输须要考虑free_async_space
if (is_async &&
proc->free_async_space < size + sizeof(struct binder_buffer)) {
binder_debug(BINDER_DEBUG_BUFFER_ALLOC,
"%d: binder_alloc_buf size %zd failed, no async space left\n",
proc->pid, size);
return NULL;
}
......
buffer->data_size = data_size;
buffer->offsets_size = offsets_size;
// buffer中设置is_async标志
buffer->async_transaction = is_async;
if (is_async) {
// 更新free_async_space
proc->free_async_space -= size + sizeof(struct binder_buffer);
binder_debug(BINDER_DEBUG_BUFFER_ALLOC_ASYNC,
"%d: binder_alloc_buf size %zd async free %zd\n",
proc->pid, size, proc->free_async_space);
}
return buffer;
}
......
static void binder_transaction(struct binder_proc *proc,
struct binder_thread *thread,
struct binder_transaction_data *tr, int reply)
{
......
// 异步传输时须要将传输事件的from设置为空
if (!reply && !(tr->flags & TF_ONE_WAY))
t->from = thread;
else
t->from = NULL;
......
// 分配buffer时带有异步标志位
t->buffer = binder_alloc_buf(target_proc, tr->data_size,
tr->offsets_size, !reply && (t->flags & TF_ONE_WAY));
......
if (reply) {
......
} else if (!(t->flags & TF_ONE_WAY)) {
......
} else {
// 异步传输使用async_todo队列
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);
tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE;
list_add_tail(&tcomplete->entry, &thread->todo);
if (target_wait)
wake_up_interruptible(target_wait);
return;
......
}
......
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)
{
......
if (cmd == BR_TRANSACTION && !(t->flags & TF_ONE_WAY)) {
......
} else {
// 异步传输是单向的，不须要回复。
t->buffer->transaction = NULL;
kfree(t);
binder_stats_deleted(BINDER_STAT_TRANSACTION);
}
break;
......
}
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