简介

px4与地面站的通信协议是mavlink，对于其消息格式的介绍看这里和这里。
需要注意几点：

不光是px4与qgroundcontrol通信通过mavlink，有一些sensor也支持mavlink。
mavlink在github中是一个单独的仓库，通过git submodule的方式集成到了px4中。
mavlink协议本身并不复杂，用户也不同关心底层的实现（发送、接收、校验、打包、解包）等，只需要提供消息格式（在mavlink中选择现成的or自定义）、通信周期就可以了。

我主要关心几个问题：

mavlink是如何编译的？
mavlink有哪些参数？
mavlink是如何启动的？
px4中都有哪些现有的mavlink消息？px4中都有哪些现有的mavlink消息？
如何定义新的mavlink消息？
mavlink是怎么配置硬件设备的？
mavlink是怎么把uorb的消息打包的？
mavlink是怎么定时发送消息的？
mavlink是怎么接收消息的？
mavlink是怎么解析接收消息的？

mavlink是如何编译的？

在linux下，看posix_sitl_default.cmake中的modules/mavlink

mavlink有哪些参数？

在ROMFS/px4fmu_common/init.d/rcS中：

set MAVLINK_F default
set MAVLINK_COMPANION_DEVICE /dev/ttyS2
set MAV_TYPE none

mavlink是如何启动的？

在nuttx下
见ROMFS/px4fmu_common/init.d/rcS

# Start mavlink streams.
#
sh /etc/init.d/rc.mavlink ...
#
# Boot is complete, inform MAVLink app(s) that the system is now fully up and running.
#
mavlink boot_complete

rc.mavlink中的内容：

if ! ver hwcmp AEROFC_V1
then# Start MAVLinkmavlink start -r 800000 -d /dev/ttyACM0 -m config -x
fi
...
if [ "x${MAVLINK_F}" == xdefault ]
then# Normal mode, use baudrate 57600 (default) and data rate 1000 bytes/sset MAVLINK_F "-r 1200 -f"# Use ttyS1 for MAVLink on FMUv4 in addition to ttyS0 (debug)if ver hwcmp PX4FMU_V4thenset MAVLINK_F "-r 1200 -d /dev/ttyS1"# Start MAVLink on Wifi (ESP8266 port)mavlink start -r 20000 -b 921600 -d /dev/ttyS0fi# Use ttyS3 (TELEM4) for MAVLink on FMUv5 in addition to ttyS1 (TELEM1) and ttyS2 (TELEM2)if ver hwcmp PX4FMU_V5thenmavlink start -r 2000 -b 57600 -d /dev/ttyS3fi
fi

其中，ver是px4的指令，介绍见这里。Source: systemcmds/ver，
Tool to print various version information。
mavlink start后面的-r表示rate（通信周期），-b表示波特率，-d表示端口。

在linux下
见init.d-posix/rcS

# 启动mavlink，和GCS连接
# GCS link
# -r表示rate，-s表示stream_name，-u表示network_port
mavlink start -x -u $udp_gcs_port_local -r 4000000
mavlink stream -r 50 -s POSITION_TARGET_LOCAL_NED -u $udp_gcs_port_local
mavlink stream -r 50 -s LOCAL_POSITION_NED -u $udp_gcs_port_local
mavlink stream -r 50 -s GLOBAL_POSITION_INT -u $udp_gcs_port_local
mavlink stream -r 50 -s ATTITUDE -u $udp_gcs_port_local
mavlink stream -r 50 -s ATTITUDE_QUATERNION -u $udp_gcs_port_local
mavlink stream -r 50 -s ATTITUDE_TARGET -u $udp_gcs_port_local
mavlink stream -r 50 -s SERVO_OUTPUT_RAW_0 -u $udp_gcs_port_local
mavlink stream -r 20 -s RC_CHANNELS -u $udp_gcs_port_local
mavlink stream -r 10 -s OPTICAL_FLOW_RAD -u $udp_gcs_port_local# API/Offboard link
mavlink start -x -u $udp_offboard_port_local -r 4000000 -m onboard -o $udp_offboard_port_remote
...
mavlink boot_complete

上面用到了两个脚本：

mavlink start：启动mavlink，执行mavlink::start。
mavlink stream：添加mavlink command，执行mavlink::stream_command。POSITION_TARGET_LOCAL_NED等是common.xml中定义的消息。

px4中都有哪些现有的mavlink消息？

在px4的工程中，在Firmware/mavlink/include/mavlink/v2.0/message_definitions目录下，有如下的xml消息：
├── ardupilotmega.xml
├── ASLUAV.xml
├── autoquad.xml
├── common.xml
├── icarous.xml
├── matrixpilot.xml
├── minimal.xml
├── paparazzi.xml
├── python_array_test.xml
├── slugs.xml
├── standard.xml
├── test.xml
├── ualberta.xml
└── uAvionix.xml
且在目录的上一级，每个xml都有对应的文件夹，包括各自的.h文件（都是MAVLink comm protocol generated from xml）。
关于上述xml的解释，见https://mavlink.gitbooks.io/mavlink-devguide/en/messages/

默认的消息
在这些消息中，只有heartbeat是必须的（qgc需要据此判断连接是否正常），其他都是可选的。
mavlink_main.cpp中_mode默认是MAVLINK_MODE_NORMAL。在mavlin_main.cpp中，有

 /* add default streams depending on mode */if (_mode != MAVLINK_MODE_IRIDIUM) {    //这里的Iridium应该是铱星通信系统/* HEARTBEAT is constant rate stream, rate never adjusted */configure_stream("HEARTBEAT", 1.0f);/* STATUSTEXT stream is like normal stream but gets messages from logbuffer instead of uORB */configure_stream("STATUSTEXT", 20.0f);/* COMMAND_LONG stream: use unlimited rate to send all commands */configure_stream("COMMAND_LONG");}

默认的其他消息：
在configure_streams_to_default中有：

 switch (_mode) {case MAVLINK_MODE_NORMAL:configure_stream_local("ADSB_VEHICLE", unlimited_rate);configure_stream_local("ALTITUDE", 1.0f);configure_stream_local("ATTITUDE", 20.0f);configure_stream_local("ATTITUDE_TARGET", 2.0f);configure_stream_local("CAMERA_IMAGE_CAPTURED", unlimited_rate);configure_stream_local("COLLISION", unlimited_rate);configure_stream_local("DEBUG", 1.0f);configure_stream_local("DEBUG_VECT", 1.0f);configure_stream_local("DISTANCE_SENSOR", 0.5f);configure_stream_local("ESTIMATOR_STATUS", 0.5f);configure_stream_local("EXTENDED_SYS_STATE", 1.0f);configure_stream_local("GLOBAL_POSITION_INT", 5.0f);configure_stream_local("GPS_RAW_INT", 1.0f);configure_stream_local("GPS2_RAW", 1.0f);configure_stream_local("HIGHRES_IMU", 1.5f);configure_stream_local("HOME_POSITION", 0.5f);configure_stream_local("LOCAL_POSITION_NED", 1.0f);configure_stream_local("NAMED_VALUE_FLOAT", 1.0f);configure_stream_local("NAV_CONTROLLER_OUTPUT", 1.5f);configure_stream_local("OPTICAL_FLOW_RAD", 1.0f);configure_stream_local("PING", 0.1f);configure_stream_local("POSITION_TARGET_LOCAL_NED", 1.5f);configure_stream_local("POSITION_TARGET_GLOBAL_INT", 1.5f);configure_stream_local("RC_CHANNELS", 5.0f);configure_stream_local("SERVO_OUTPUT_RAW_0", 1.0f);configure_stream_local("SYS_STATUS", 1.0f);configure_stream_local("TRAJECTORY_REPRESENTATION_WAYPOINTS", 5.0f);configure_stream_local("VFR_HUD", 4.0f);configure_stream_local("VISION_POSITION_ESTIMATE", 1.0f);configure_stream_local("WIND_COV", 1.0f);break;//...default:ret = -1;break;}

如何定义新的mavlink消息？

首先要安装mavlink generator
定义一个数据结构，比如说msg/ca_trajectory.msg
根据上面的数据结构写对应的xml
用mavlink generator生成mavlink消息（一个包含.h的文件夹）

后两个过程在这里。

mavlink是怎么配置硬件设备的？

见mavlink在rcS中的启动脚本（上面论述过）。

mavlink是怎么把uorb的消息打包的？

最底层。添加mavlink的头文件和uorb消息到mavlink_messages.cpp，比如自己添加的一个ca_trajectory消息：

#include <uORB/topics/ca_trajectory.h>
#include <v1.0/custom_messages/mavlink_msg_ca_trajectory.h>

或者一个mavlink中的common消息：

mavlink_msg_attitude_target.h  //被common.h所include，common.h被mavlink.h所include

这个.h文件就是打包过的数据结构

MAVPACKED(
typedef struct __mavlink_attitude_target_t {uint32_t time_boot_ms; /*< [ms] Timestamp (time since system boot).*/float q[4]; /*<  Attitude quaternion (w, x, y, z order, zero-rotation is 1, 0, 0, 0)*/float body_roll_rate; /*< [rad/s] Body roll rate*/float body_pitch_rate; /*< [rad/s] Body pitch rate*/float body_yaw_rate; /*< [rad/s] Body yaw rate*/float thrust; /*<  Collective thrust, normalized to 0 .. 1 (-1 .. 1 for vehicles capable of reverse trust)*/uint8_t type_mask; /*<  Mappings: If any of these bits are set, the corresponding input should be ignored: bit 1: body roll rate, bit 2: body pitch rate, bit 3: body yaw rate. bit 4-bit 7: reserved, bit 8: attitude*/
}) mavlink_attitude_target_t;

而且还包含了打包函数（mavlink_msg_attitude_target_pack）、encode函数（调用pack函数）、发送函数（mavlink_msg_attitude_target_send_struct）、decode函数（mavlink_msg_attitude_target_decode，调用unpack函数）、upack函数等。
2. 在mavlink_messages.cpp中创建一个新的类（可以看到其他消息也是这样做的），继承自MavlinkStream。这个类包括send函数

bool send(const hrt_abstime t){vehicle_attitude_setpoint_s att_sp;if (_att_sp_sub->update(&_att_sp_time, &att_sp)) {vehicle_rates_setpoint_s att_rates_sp = {};_att_rates_sp_sub->update(&att_rates_sp);mavlink_attitude_target_t msg = {};msg.time_boot_ms = att_sp.timestamp / 1000;if (att_sp.q_d_valid) {memcpy(&msg.q[0], &att_sp.q_d[0], sizeof(msg.q));} else {matrix::Quatf q = matrix::Eulerf(att_sp.roll_body, att_sp.pitch_body, att_sp.yaw_body);memcpy(&msg.q[0], q.data(), sizeof(msg.q));}msg.body_roll_rate = att_rates_sp.roll;msg.body_pitch_rate = att_rates_sp.pitch;msg.body_yaw_rate = att_rates_sp.yaw;msg.thrust = att_sp.thrust;mavlink_msg_attitude_target_send_struct(_mavlink->get_channel(), &msg);return true;}return false;}

可以看出，send函数等待uorb更新，并把uorb数据转为mavlink数据，并通过mavlink_msg_attitude_target_send_struct发送出去。
3. 附加流类streams_list的到mavlink_messages.cpp底部，这是因为streams_list包括了所有预先定义的mavlink消息，在执行create_mavlink_stream时，需要到streams_list中检查是否有定义。

static const StreamListItem streams_list[] = {StreamListItem(&MavlinkStreamHeartbeat::new_instance, &MavlinkStreamHeartbeat::get_name_static, &MavlinkStreamHeartbeat::get_id_static),StreamListItem(&MavlinkStreamStatustext::new_instance, &MavlinkStreamStatustext::get_name_static, &MavlinkStreamStatustext::get_id_static),StreamListItem(&MavlinkStreamCommandLong::new_instance, &MavlinkStreamCommandLong::get_name_static, &MavlinkStreamCommandLong::get_id_static),StreamListItem(&MavlinkStreamSysStatus::new_instance, &MavlinkStreamSysStatus::get_name_static, &MavlinkStreamSysStatus::get_id_static),StreamListItem(&MavlinkStreamHighresIMU::new_instance, &MavlinkStreamHighresIMU::get_name_static, &MavlinkStreamHighresIMU::get_id_static),StreamListItem(&MavlinkStreamScaledIMU::new_instance, &MavlinkStreamScaledIMU::get_name_static, &MavlinkStreamScaledIMU::get_id_static),StreamListItem(&MavlinkStreamScaledIMU2::new_instance, &MavlinkStreamScaledIMU2::get_name_static, &MavlinkStreamScaledIMU2::get_id_static),StreamListItem(&MavlinkStreamScaledIMU3::new_instance, &MavlinkStreamScaledIMU3::get_name_static, &MavlinkStreamScaledIMU3::get_id_static),StreamListItem(&MavlinkStreamAttitude::new_instance, &MavlinkStreamAttitude::get_name_static, &MavlinkStreamAttitude::get_id_static),StreamListItem(&MavlinkStreamAttitudeQuaternion::new_instance, &MavlinkStreamAttitudeQuaternion::get_name_static, &MavlinkStreamAttitudeQuaternion::get_id_static),StreamListItem(&MavlinkStreamVFRHUD::new_instance, &MavlinkStreamVFRHUD::get_name_static, &MavlinkStreamVFRHUD::get_id_static),StreamListItem(&MavlinkStreamGPSRawInt::new_instance, &MavlinkStreamGPSRawInt::get_name_static, &MavlinkStreamGPSRawInt::get_id_static),StreamListItem(&MavlinkStreamGPS2Raw::new_instance, &MavlinkStreamGPS2Raw::get_name_static, &MavlinkStreamGPS2Raw::get_id_static),StreamListItem(&MavlinkStreamSystemTime::new_instance, &MavlinkStreamSystemTime::get_name_static, &MavlinkStreamSystemTime::get_id_static),StreamListItem(&MavlinkStreamTimesync::new_instance, &MavlinkStreamTimesync::get_name_static, &MavlinkStreamTimesync::get_id_static),StreamListItem(&MavlinkStreamGlobalPositionInt::new_instance, &MavlinkStreamGlobalPositionInt::get_name_static, &MavlinkStreamGlobalPositionInt::get_id_static),StreamListItem(&MavlinkStreamLocalPositionNED::new_instance, &MavlinkStreamLocalPositionNED::get_name_static, &MavlinkStreamLocalPositionNED::get_id_static),StreamListItem(&MavlinkStreamVisionPositionEstimate::new_instance, &MavlinkStreamVisionPositionEstimate::get_name_static, &MavlinkStreamVisionPositionEstimate::get_id_static),StreamListItem(&MavlinkStreamLocalPositionNEDCOV::new_instance, &MavlinkStreamLocalPositionNEDCOV::get_name_static, &MavlinkStreamLocalPositionNEDCOV::get_id_static),StreamListItem(&MavlinkStreamEstimatorStatus::new_instance, &MavlinkStreamEstimatorStatus::get_name_static, &MavlinkStreamEstimatorStatus::get_id_static),StreamListItem(&MavlinkStreamAttPosMocap::new_instance, &MavlinkStreamAttPosMocap::get_name_static, &MavlinkStreamAttPosMocap::get_id_static),StreamListItem(&MavlinkStreamHomePosition::new_instance, &MavlinkStreamHomePosition::get_name_static, &MavlinkStreamHomePosition::get_id_static),StreamListItem(&MavlinkStreamServoOutputRaw<0>::new_instance, &MavlinkStreamServoOutputRaw<0>::get_name_static, &MavlinkStreamServoOutputRaw<0>::get_id_static),StreamListItem(&MavlinkStreamServoOutputRaw<1>::new_instance, &MavlinkStreamServoOutputRaw<1>::get_name_static, &MavlinkStreamServoOutputRaw<1>::get_id_static),StreamListItem(&MavlinkStreamServoOutputRaw<2>::new_instance, &MavlinkStreamServoOutputRaw<2>::get_name_static, &MavlinkStreamServoOutputRaw<2>::get_id_static),StreamListItem(&MavlinkStreamServoOutputRaw<3>::new_instance, &MavlinkStreamServoOutputRaw<3>::get_name_static, &MavlinkStreamServoOutputRaw<3>::get_id_static),StreamListItem(&MavlinkStreamHILActuatorControls::new_instance, &MavlinkStreamHILActuatorControls::get_name_static, &MavlinkStreamHILActuatorControls::get_id_static),StreamListItem(&MavlinkStreamPositionTargetGlobalInt::new_instance, &MavlinkStreamPositionTargetGlobalInt::get_name_static, &MavlinkStreamPositionTargetGlobalInt::get_id_static),StreamListItem(&MavlinkStreamLocalPositionSetpoint::new_instance, &MavlinkStreamLocalPositionSetpoint::get_name_static, &MavlinkStreamLocalPositionSetpoint::get_id_static),StreamListItem(&MavlinkStreamAttitudeTarget::new_instance, &MavlinkStreamAttitudeTarget::get_name_static, &MavlinkStreamAttitudeTarget::get_id_static),StreamListItem(&MavlinkStreamRCChannels::new_instance, &MavlinkStreamRCChannels::get_name_static, &MavlinkStreamRCChannels::get_id_static),StreamListItem(&MavlinkStreamManualControl::new_instance, &MavlinkStreamManualControl::get_name_static, &MavlinkStreamManualControl::get_id_static),StreamListItem(&MavlinkStreamTrajectoryRepresentationWaypoints::new_instance, &MavlinkStreamTrajectoryRepresentationWaypoints::get_name_static, &MavlinkStreamTrajectoryRepresentationWaypoints::get_id_static),StreamListItem(&MavlinkStreamOpticalFlowRad::new_instance, &MavlinkStreamOpticalFlowRad::get_name_static, &MavlinkStreamOpticalFlowRad::get_id_static),StreamListItem(&MavlinkStreamActuatorControlTarget<0>::new_instance, &MavlinkStreamActuatorControlTarget<0>::get_name_static, &MavlinkStreamActuatorControlTarget<0>::get_id_static),StreamListItem(&MavlinkStreamActuatorControlTarget<1>::new_instance, &MavlinkStreamActuatorControlTarget<1>::get_name_static, &MavlinkStreamActuatorControlTarget<1>::get_id_static),StreamListItem(&MavlinkStreamActuatorControlTarget<2>::new_instance, &MavlinkStreamActuatorControlTarget<2>::get_name_static, &MavlinkStreamActuatorControlTarget<2>::get_id_static),StreamListItem(&MavlinkStreamActuatorControlTarget<3>::new_instance, &MavlinkStreamActuatorControlTarget<3>::get_name_static, &MavlinkStreamActuatorControlTarget<3>::get_id_static),StreamListItem(&MavlinkStreamNamedValueFloat::new_instance, &MavlinkStreamNamedValueFloat::get_name_static, &MavlinkStreamNamedValueFloat::get_id_static),StreamListItem(&MavlinkStreamDebug::new_instance, &MavlinkStreamDebug::get_name_static, &MavlinkStreamDebug::get_id_static),StreamListItem(&MavlinkStreamDebugVect::new_instance, &MavlinkStreamDebugVect::get_name_static, &MavlinkStreamDebugVect::get_id_static),StreamListItem(&MavlinkStreamNavControllerOutput::new_instance, &MavlinkStreamNavControllerOutput::get_name_static, &MavlinkStreamNavControllerOutput::get_id_static),StreamListItem(&MavlinkStreamCameraCapture::new_instance, &MavlinkStreamCameraCapture::get_name_static, &MavlinkStreamCameraCapture::get_id_static),StreamListItem(&MavlinkStreamCameraTrigger::new_instance, &MavlinkStreamCameraTrigger::get_name_static, &MavlinkStreamCameraTrigger::get_id_static),StreamListItem(&MavlinkStreamCameraImageCaptured::new_instance, &MavlinkStreamCameraImageCaptured::get_name_static, &MavlinkStreamCameraImageCaptured::get_id_static),StreamListItem(&MavlinkStreamDistanceSensor::new_instance, &MavlinkStreamDistanceSensor::get_name_static, &MavlinkStreamDistanceSensor::get_id_static),StreamListItem(&MavlinkStreamExtendedSysState::new_instance, &MavlinkStreamExtendedSysState::get_name_static, &MavlinkStreamExtendedSysState::get_id_static),StreamListItem(&MavlinkStreamAltitude::new_instance, &MavlinkStreamAltitude::get_name_static, &MavlinkStreamAltitude::get_id_static),StreamListItem(&MavlinkStreamADSBVehicle::new_instance, &MavlinkStreamADSBVehicle::get_name_static, &MavlinkStreamADSBVehicle::get_id_static),StreamListItem(&MavlinkStreamCollision::new_instance, &MavlinkStreamCollision::get_name_static, &MavlinkStreamCollision::get_id_static),StreamListItem(&MavlinkStreamWind::new_instance, &MavlinkStreamWind::get_name_static, &MavlinkStreamWind::get_id_static),StreamListItem(&MavlinkStreamMountOrientation::new_instance, &MavlinkStreamMountOrientation::get_name_static, &MavlinkStreamMountOrientation::get_id_static),StreamListItem(&MavlinkStreamHighLatency2::new_instance, &MavlinkStreamHighLatency2::get_name_static, &MavlinkStreamHighLatency2::get_id_static),StreamListItem(&MavlinkStreamGroundTruth::new_instance, &MavlinkStreamGroundTruth::get_name_static, &MavlinkStreamGroundTruth::get_id_static),StreamListItem(&MavlinkStreamPing::new_instance, &MavlinkStreamPing::get_name_static, &MavlinkStreamPing::get_id_static)
};

mavlink是怎么定时发送消息的？

添加消息、设置发送周期
在mavlink_main.cpp加入自定义的消息以及期望的更新频率，比如：

    configure_stream("CA_TRAJECTORY", 100.0f);

其实就是在_streams中查找stream_name，如果找到，则set_interval（新的interval）

数据结构
_streams是整个类维护的链表，用来记录目前活跃的stream消息。
发送消息
在mavlink_main.cpp的while循环中，通过下列代码遍历stream并发送：

/* update streams */
// 对_streams列表中的每个stream都更新.
// 这里的更新就包括了send stream message
MavlinkStream *stream;
LL_FOREACH(_streams, stream) {stream->update(t);   //t是时间if (!_first_heartbeat_sent) {if (_mode == MAVLINK_MODE_IRIDIUM) {if (stream->get_id() == MAVLINK_MSG_ID_HIGH_LATENCY2) {_first_heartbeat_sent = stream->first_message_sent();}} else {if (stream->get_id() == MAVLINK_MSG_ID_HEARTBEAT) {_first_heartbeat_sent = stream->first_message_sent();}}}
}

其中update会调用各个message对应的class（比如上面的MavlinkStreamHeartbeat）的send函数。

mavlink是怎么接收消息的？

mavlink_main.cpp的task_main中调用：

MavlinkReceiver::receive_start(&_receive_thread, this);

recieve_start创建了一个thread，在thread中，用MavlinkReceiver::start_helper创建MavlinkReceiver对象，并执行MavlinkReceiver::receive_thread(void *arg)，接收数据。

mavlink是怎么解析接收消息的？

在mavlink_receiver.h的class MavlinkReceiver中增加一个用来处理接收信息的函数，类似这样的：

 template<class T>void handle_message_command_both(mavlink_message_t *msg, const T &cmd_mavlink,const vehicle_command_s &vehicle_command);void handle_message_command_ack(mavlink_message_t *msg);void handle_message_optical_flow_rad(mavlink_message_t *msg);...

这个消息收到之后要转成uorb，再在内部传递，因此还要在这个类中增加一个uORB消息发布者，类似于：

 orb_advert_t _global_pos_pub;orb_advert_t _local_pos_pub;orb_advert_t _attitude_pub;orb_advert_t _gps_pub;...

然后再写一段上面声明的handle函数，该函数包括了对接收的消息进行解码：

void
MavlinkReceiver::handle_message_optical_flow_rad(mavlink_message_t *msg)
{/* optical flow */mavlink_optical_flow_rad_t flow;mavlink_msg_optical_flow_rad_decode(msg, &flow);int32_t flow_rot_int;param_get(param_find("SENS_FLOW_ROT"), &flow_rot_int);const enum Rotation flow_rot = (Rotation)flow_rot_int;struct optical_flow_s f = {};

并确定函数在MavlinkReceiver::handle_message()中被调用

 case MAVLINK_MSG_ID_OPTICAL_FLOW_RAD:handle_message_optical_flow_rad(msg);break;

这个handle_message是被MavlinkReceiver::receive_thread调用的。

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PX4中的mavlink

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