Android12蓝牙框架

参考：
https://evilpan.com/2021/07/11/android-bt/
https://source.android.com/docs/core/connect/bluetooth?hl=zh-cn
https://developer.android.com/guide/topics/connectivity/bluetooth?hl=zh-cn
https://developer.android.com/guide/components/intents-filters?hl=zh-cn

文章目录

- AIDL Server
- - - btm_ble_set_discoverability
    - btsnd_hcic_write_cur_iac_lap
- HCI 子系统
- - 接收数据
  - 发送数据

BTIF: Bluetooth Interface
BTU : Bluetooth Upper Layer
BTM: Bluetooth Manager
BTE: Bluetooth embedded system
BTA :Blueetooth application layer
CO: call out
CI: call in
HF : Handsfree Profile
HH: HID Host Profile
HL: Health Device Profile
av: audio/vidio
ag: audio gateway
ar: audio/video registration
gattc: GATT client
HIDL: HAL Interface Definition Language
Android4.2 之后采用 bluedroid 作为协议；整体由 bluetooth.apk,bluedroid ,libbt-vendor 三个部分组成。

蓝牙协议栈一方面是以系统服务的方式提供接口，另一方面也以client的方式给应用程序提供SDK，不管怎样，最终都是需要经过HCI协议去与Controller进行交互

对于BlueDroid而言，协议栈是在用户层实现的，内核只暴露出HCI(USB/UART)的接口。因此，我们可以从HCI出发，自底向上进行分析，也可以参考上面的框架图，从用户应用程序开始，自顶向下进行分析

AIDL Server

 bool SetScanMode(int scan_mode) 				system/bt/service/adapter.cc
 	SetAdapterProperty
 		hal::BluetoothInterface::Get()->GetHALInterface()->set_adapter_property
 			set_adapter_property				system/bt/btif/src/bluetooth.cc
 				do_in_main_thread 
 				btif_set_adapter_property		system/bt/btif/src/btif_core.cc
 					BTA_DmSetVisibility			system/bt/bta/dm/bta_dm_act.cc

android12/system/bt/stack/btm/btm_inq.cc

这其中涉及了几个API:
btm_ble_set_discoverability
btsnd_hcic_write_cur_iac_lap
btsnd_hcic_write_inqscan_cfg
btsnd_hcic_write_scan_enable

btm_ble_set_discoverability

第一个API是BLE相关，内部实际上最终也调用了btsnd_hcic_xxx的类似接口。IAC意为Inquiry Access Code，蓝牙baseband定义了几个固定IAC，分别是LIAC和GIAC(见baseband)。LAP是蓝牙地址的一部分，如下图所示:

BDADDR

NAP: Non-significant Address Part， NAP的值在跳频同步帧中会用到
UAP: Upper Address Part，UAP的值会参与对蓝牙协议算法的选择
LAP: Lower Address Part，由设备厂商分配，LAP的值作为Access Code的一部分，唯一确定某个蓝牙设备
SAP (significant address part) = UAP + LAP

btsnd_hcic_write_cur_iac_lap

// system/bt/stack/hcic/hcicmds.cc
void btsnd_hcic_write_cur_iac_lap(uint8_t num_cur_iac, LAP* const iac_lap) {
    btu_hcif_send_cmd(LOCAL_BR_EDR_CONTROLLER_ID, p);
}
/UINTx_TO_STREAM(pp, n)的作用是将整数以小端的形式写入p->data中，最终调用btu_hcif_send_cmd函数发送数据
    
// system/btstack/btu/btu_hcif.cc
void btu_hcif_send_cmd(UNUSED_ATTR uint8_t controller_id, BT_HDR* p_buf) {
    uint8_t* stream = p_buf->data + p_buf->offset;
    stream++;
    btu_hcif_log_command_metrics(opcode, stream,
                                 android::bluetooth::hci::STATUS_UNKNOWN, false);
    hci_layer_get_interface()->transmit_command(
        p_buf, btu_hcif_command_complete_evt, btu_hcif_command_status_evt,
        vsc_callback);
}
//可见p_buf->data中保存的就是HCI数据，前16位为opcode，其中高6字节为ogf，低10字节为ocf，也就是我们平时使用hcitool cmd时的前两个参数

HCI 子系统

继续跟踪transmit_command，就来到了HCI子系统中

transmit_command		system/bt/hci/src/hci_layer.cc
	enqueue_command

process_command_credits

其调用链路为:

BluetoothHciCallbacks::hciEventReceived 		system/bt/hci/src/hci_layer_android.cc
	hci_event_received							system/bt/hci/src/hci_layer.cc
		filter_incoming_event
			process_command_credits

接收数据

BluetoothHciCallbacks::hciEventReceived 这个函数回调是在HCI初始化的时候调用的

BluetoothHci::initialize(system/bt/vendor_libs/linux/interface/bluetooth_hci.cc):

Return<void> BluetoothHci::initialize		system/bt/vendor_libs/linux/interface/bluetooth_hci.cc
	openBtHci

fd_watcher_本质上是针对hci_fd文件句柄的读端事件监控，后者由openBtHci函数产生，该函数由厂商实现，接口文件是hardware/interfaces/bluetooth/1.0/IBluetoothHci.hal。在Linux中的参考实现如下:

发送数据

继续回头接着上节之前的内容讲，我们的任务队列是在process_command_credits中被消费的，取出来之后需要进入到hci_thread线程中执行。从接收数据一节中也能看出，hci接口本身使用的是串行总线，因此不能并发地发送数据，所有命令都是在之前的命令响应后再发送。

值得一提的是，enqueue_command实际上绑定的是函数event_command_ready，以包含我们命令内容和对应回调的类型waiting_command_t为参数:

static void enqueue_command(waiting_command_t* wait_entry) {
	base::Closure callback = base::Bind(&event_command_ready, wait_entry);
    //...
	command_queue.push(std::move(callback));
}

因此，负责执行HCI发送命令的是event_command_ready函数:

  static void event_command_ready(waiting_command_t* wait_entry) {
    {
      /// Move it to the list of commands awaiting response
      std::lock_guard<std::recursive_timed_mutex> lock(
          commands_pending_response_mutex);
      wait_entry->timestamp = std::chrono::steady_clock::now();
      list_append(commands_pending_response, wait_entry);
    }
    // Send it off
    packet_fragmenter->fragment_and_dispatch(wait_entry->command);

    update_command_response_timer();
  }

首先将command放到一个等待响应的队列里，然后分片发送:

static void fragment_and_dispatch(BT_HDR* packet) {
    CHECK(packet != NULL);

    uint16_t event = packet->event & MSG_EVT_MASK;
    uint8_t* stream = packet->data + packet->offset;

    // We only fragment ACL packets
    if (event != MSG_STACK_TO_HC_HCI_ACL) {
      callbacks->fragmented(packet, true);
      return;
    }
    // ACL/L2CAP fragment...
}

实现中只对ACL类型的HCI数据进行分片发送，不管是不是分片，都对最后一个packet调用callbacks->fragmented()，callbacks的类型是packet_fragmenter_callbacks_t，在packet_fragmenter_t->init中初始化并设置。而packet_fragmenter的初始化发生在hci_module_start_up()中，HCI层定义的回调如下:

static const packet_fragmenter_callbacks_t packet_fragmenter_callbacks = {  transmit_fragment, dispatch_reassembled, fragmenter_transmit_finished };

fragmented即对应transmit_fragment，对应定义如下:

// Callback for the fragmenter to send a fragment static void transmit_fragment(BT_HDR* packet, bool send_transmit_finished) {    btsnoop->capture(packet, false);     // HCI command packets are freed on a different thread when the matching    // event is received. Check packet->event before sending to avoid a race.    bool free_after_transmit =        (packet->event & MSG_EVT_MASK) != MSG_STACK_TO_HC_HCI_CMD &&        send_transmit_finished;     hci_transmit(packet);     if (free_after_transmit) {      buffer_allocator->free(packet);    } }

hci_transmit有不同平台的实现，分别在: