蓝牙之六-A2dp代码调用流程

上图描述的是蓝牙协议栈，通过该图，查看A2dp的代码在协议栈的调用流程。其分层架构如下：

1.蓝牙的系统服务service通过JNI与bluedroid协议栈进行通信。协议栈分为两层，Bluetooth Embedded System（BTE）和Bluetooth Application Layer（BTA）。这两层和framework层应用进行通信。
2.蓝牙服务通过Binder IPC通信与应用程序交互。
3.系统服务给开发者提供了获取各种profile的接口。

Application framework

该层使用android.bluetooth APIs和Bluetooth hardware进行通信。通过Binder IPC机制和Bluetooth进程进行通信。该层代码位于frameworks/base/core/java/android/bluetooth/目录下。
如A2DP连接frameworks/base/core/java/android/bluetooth/BluetoothA2dp.java

    public boolean connect(BluetoothDevice device) {
        if (DBG) log("connect(" + device + ")");
        if (mService != null && isEnabled() &&
            isValidDevice(device)) {
            try {
                return mService.connect(device);
            } catch (RemoteException e) {
                Log.e(TAG, "Stack:" + Log.getStackTraceString(new Throwable()));
                return false;
            }
        }
        if (mService == null) Log.w(TAG, "Proxy not attached to service");
        return false;
    }

该方法通过Binder IPC通信机制，调用packages/apps/Bluetooth/src/com/android/bluetooth/a2dp/A2dpService.java下的内部私有类。通过aidl机制实现。

private static class BluetoothA2dpBinder extends IBluetoothA2dp.Stub {
        public boolean connect(BluetoothDevice device) {
            A2dpService service = getService();
            if (service == null) return false;
            return service.connect(device);
        }
}

然后调用到A2dpService的connect的方法。

public class A2dpService extends ProfileService {
        ...
        public boolean connect(BluetoothDevice device) {
        enforceCallingOrSelfPermission(BLUETOOTH_ADMIN_PERM,
                                       "Need BLUETOOTH ADMIN permission");
        if (getPriority(device) == BluetoothProfile.PRIORITY_OFF) {
            return false;
        }
        ParcelUuid[] featureUuids = device.getUuids();
        if ((BluetoothUuid.containsAnyUuid(featureUuids, A2DP_SOURCE_UUID)) &&
            !(BluetoothUuid.containsAllUuids(featureUuids ,A2DP_SOURCE_SINK_UUIDS))) {
            Log.e(TAG,"Remote does not have A2dp Sink UUID");
            return false;
         }
        int connectionState = mStateMachine.getConnectionState(device);
        if (connectionState == BluetoothProfile.STATE_CONNECTED ||
            connectionState == BluetoothProfile.STATE_CONNECTING) {
            return false;
        }
        mStateMachine.sendMessage(A2dpStateMachine.CONNECT, device);
        return true;
    }
    ....
        private static class BluetoothA2dpBinder extends IBluetoothA2dp.Stub {
            public boolean connect(BluetoothDevice device) {
                A2dpService service = getService();
                if (service == null) return false;
                return service.connect(device);
           }
       }
  }

上述过程就是Bluetooth application framework与Bluetooth process的调用过程。

Bluetooth System service

这部分代码位于packages/apps/Bluetooth下，在framework层实现蓝牙服务和各种profile，以apps形式存在。该层通过JNI调用HAL层代码。
A2dpService的connect方法会发送状态机改变消息，
mStateMachine.sendMessage(A2dpStateMachine.CONNECT, device);
这个消息会被A2dpStateMachine对象的processMessage(Message)方法接收：

            switch(message.what) {
                case CONNECT:
                    BluetoothDevice device = (BluetoothDevice) message.obj;
                    broadcastConnectionState(device, BluetoothProfile.STATE_CONNECTING,
                                   BluetoothProfile.STATE_DISCONNECTED);

                    if (!connectA2dpNative(getByteAddress(device)) ) {
                        broadcastConnectionState(device, BluetoothProfile.STATE_DISCONNECTED,
                                       BluetoothProfile.STATE_CONNECTING);
                        break;
                    }

                    synchronized (A2dpStateMachine.this) {
                        mTargetDevice = device;
                        transitionTo(mPending);
                    }
                    // TODO(BT) remove CONNECT_TIMEOUT when the stack
                    //          sends back events consistently
                    sendMessageDelayed(CONNECT_TIMEOUT, 30000);
                    break;

上述的connectA2dpNative（getByteAddress(device)）会通过JNI调用Native方法。

static jboolean connectA2dpNative(JNIEnv *env, jobject object, jbyteArray address) {
    jbyte *addr;
    bt_bdaddr_t * btAddr;
    bt_status_t status;

    ALOGI("%s: sBluetoothA2dpInterface: %p", __FUNCTION__, sBluetoothA2dpInterface);
    if (!sBluetoothA2dpInterface) return JNI_FALSE;

    addr = env->GetByteArrayElements(address, NULL);
    btAddr = (bt_bdaddr_t *) addr;
    if (!addr) {
        jniThrowIOException(env, EINVAL);
        return JNI_FALSE;
    }

    if ((status = sBluetoothA2dpInterface->connect((bt_bdaddr_t *)addr)) != BT_STATUS_SUCCESS) {
        ALOGE("Failed HF connection, status: %d", status);
    }
    env->ReleaseByteArrayElements(address, addr, 0);
    return (status == BT_STATUS_SUCCESS) ? JNI_TRUE : JNI_FALSE;
}

JNI

和JNI相关的代码在package/apps/Bluetooth/jni目录下，上一节的connectA2dpNative方法就是这个目录的代码。JNI层代码调用到HAL代码并且从HAL接收到回调。
A2dp连接中，会调用sBluetoothA2dpInterface->connect实现。

static void initNative(JNIEnv *env, jobject object) {
    const bt_interface_t* btInf;
    bt_status_t status;
...
    if ( (sBluetoothA2dpInterface = (btav_interface_t *)
          btInf->get_profile_interface(BT_PROFILE_ADVANCED_AUDIO_ID)) == NULL) {
        ALOGE("Failed to get Bluetooth A2DP Interface");
        return;
    }

    if ( (status = sBluetoothA2dpInterface->init(&sBluetoothA2dpCallbacks)) != BT_STATUS_SUCCESS) {
        ALOGE("Failed to initialize Bluetooth A2DP, status: %d", status);
        sBluetoothA2dpInterface = NULL;
        return;
    }

    mCallbacksObj = env->NewGlobalRef(object);
}

在initNative中获得sBluetoothA2dpInterface对象。

HAL

硬件抽象层定义了android.bluetooth APIs和Bluetooth process调用的接口。Bluetooth 的HAL层代码在：
hardware/libhardware/include/hardware/bluetooth.h，HAL层的以下代码也要重视：

hardware/libhardware/include/hardware/bt_common_types.h  
//A2DP profile定义
hardware/libhardware/include/hardware/bt_av.h   
//GATT profile 定义         
hardware/libhardware/include/hardware/bt_gatt_types.h  
hardware/libhardware/include/hardware/bt_gatt_client.h 
hardware/libhardware/include/hardware/bt_gatt_server.h 
hardware/libhardware/include/hardware/bt_gatt.h  
//HFP profile定义
hardware/libhardware/include/hardware/bt_hf.h  
//HDP profile 定义
hardware/libhardware/include/hardware/bt_hl.h 
//MAP profile
hardware/libhardware/include/hardware/bt_mce.h
//HFP client profile
hardware/libhardware/include/hardware/bt_hf_client.h 
//pan profile  
hardware/libhardware/include/hardware/bt_pan.h
//AVRCP profile     
hardware/libhardware/include/hardware/bt_rc.h     
hardware/libhardware/include/hardware/bt_hh.h 
//SDP profile         
hardware/libhardware/include/hardware/bt_sdp.h
//RFCOMM sockets         
hardware/libhardware/include/hardware/bt_sock.h

JNI中的sBluetoothA2dpInterface是btav_interface_t结构体，位于hardware/libhardware/include/hardware/bt_av.h中，定义为：

typedef struct {
    /** set to sizeof(btav_interface_t) */
    size_t          size;
    /**
     * Register the BtAv callbacks
     */
    bt_status_t (*init)( btav_callbacks_t* callbacks );

    /** connect to headset */
    bt_status_t (*connect)( bt_bdaddr_t *bd_addr );

    /** dis-connect from headset */
    bt_status_t (*disconnect)( bt_bdaddr_t *bd_addr );

    /** Closes the interface. */
    void  (*cleanup)( void );
} btav_interface_t;

BT stack

协议栈在/system/bt下，实现了HAL层和扩展配置等。包括BTA和BTE量大组件，BTA实现了蓝牙设备管理、状态管理及一些应用规范。而BTE则通过HCI与厂商蓝牙芯片交互以实现了蓝牙协议栈的通用功能和相关协议。另外，BTE还包括一个统一内核接口（GKI），蓝牙芯片厂商可借助GKI快速轻松得移植蓝牙协议栈到其他操作系统或手机平台上。
A2dp的连接将调用

<system/bt/btif/src/btif_av.c>
static bt_status_t connect_int(bt_bdaddr_t *bd_addr, uint16_t uuid)
{
    btif_av_connect_req_t connect_req;
    connect_req.target_bda = bd_addr;
    connect_req.uuid = uuid;
    BTIF_TRACE_EVENT("%s", __FUNCTION__);

    btif_sm_dispatch(btif_av_cb.sm_handle, BTIF_AV_CONNECT_REQ_EVT, (char*)&connect_req);

    return BT_STATUS_SUCCESS;
}

Vendor extensions

增加自定义扩展或者HCI层追踪，可以创建libbt-vendor模块并指定这些组件。