N3_sub/libraries/GCS_MAVLink/GCS.h

/// @file	GCS.h
/// @brief	Interface definition for the various Ground Control System
// protocols.
#pragma once

#include <AP_HAL/AP_HAL.h>
#include <AP_Common/AP_Common.h>
#include "GCS_MAVLink.h"
#include <AP_Mission/AP_Mission.h>
#include <stdint.h>
#include "MAVLink_routing.h"
#include <AP_Frsky_Telem/AP_Frsky_Telem.h>
#include <AP_AdvancedFailsafe/AP_AdvancedFailsafe.h>
#include <AP_RTC/JitterCorrection.h>
#include <AP_Common/Bitmask.h>
#include <AP_Devo_Telem/AP_Devo_Telem.h>
#include <RC_Channel/RC_Channel.h>

#include "MissionItemProtocol_Waypoints.h"
#include "MissionItemProtocol_Rally.h"
#include "ap_message.h"

#define GCS_DEBUG_SEND_MESSAGE_TIMINGS 0

// check if a message will fit in the payload space available
#define PAYLOAD_SIZE(chan, id) (GCS_MAVLINK::packet_overhead_chan(chan)+MAVLINK_MSG_ID_ ## id ## _LEN)
#define HAVE_PAYLOAD_SPACE(chan, id) (comm_get_txspace(chan) >= PAYLOAD_SIZE(chan, id))
#define CHECK_PAYLOAD_SIZE(id) if (comm_get_txspace(chan) < packet_overhead()+MAVLINK_MSG_ID_ ## id ## _LEN) return false
#define CHECK_PAYLOAD_SIZE2(id) if (!HAVE_PAYLOAD_SPACE(chan, id)) return false

// convenience macros for defining which ap_message ids are in which streams:
#define MAV_STREAM_ENTRY(stream_name)           \
    {                                           \
        GCS_MAVLINK::stream_name,               \
        stream_name ## _msgs,                   \
        ARRAY_SIZE(stream_name ## _msgs)        \
    }
#define MAV_STREAM_TERMINATOR { (streams)0, nullptr, 0 }

#define GCS_MAVLINK_NUM_STREAM_RATES 10
class GCS_MAVLINK_Parameters
{
public:

    GCS_MAVLINK_Parameters();

    static const struct AP_Param::GroupInfo        var_info[];

    // saveable rate of each stream
    AP_Int16        streamRates[GCS_MAVLINK_NUM_STREAM_RATES];
};

///
/// @class	GCS_MAVLINK
/// @brief	MAVLink transport control class
///
class GCS_MAVLINK
{
public:
    friend class GCS;

    GCS_MAVLINK(GCS_MAVLINK_Parameters &parameters, AP_HAL::UARTDriver &uart);
    virtual ~GCS_MAVLINK() {}

    void        update_receive(uint32_t max_time_us=1000);
    void        update_send();
    bool        init(uint8_t instance);
    void        send_message(enum ap_message id);
    void        send_text(MAV_SEVERITY severity, const char *fmt, ...) const FMT_PRINTF(3, 4);
    void        queued_param_send();
    void        queued_mission_request_send();

    bool sending_mavlink1() const;

    // returns true if we are requesting any items from the GCS:
    bool requesting_mission_items() const;

    void send_mission_ack(const mavlink_message_t &msg,
                          MAV_MISSION_TYPE mission_type,
                          MAV_MISSION_RESULT result) const {
        mavlink_msg_mission_ack_send(chan,
                                     msg.sysid,
                                     msg.compid,
                                     result,
                                     mission_type);
    }

    static const MAV_MISSION_TYPE supported_mission_types[2];

    // packetReceived is called on any successful decode of a mavlink message
    virtual void packetReceived(const mavlink_status_t &status,
                                const mavlink_message_t &msg);

    // send a mavlink_message_t out this GCS_MAVLINK connection.
    // Caller is responsible for ensuring space.
    void send_message(uint32_t msgid, const char *pkt) const {
        const mavlink_msg_entry_t *entry = mavlink_get_msg_entry(msgid);
        if (entry == nullptr) {
            return;
        }
        send_message(pkt, entry);
    }
    void send_message(const char *pkt, const mavlink_msg_entry_t *entry) const {
        _mav_finalize_message_chan_send(chan,
                                        entry->msgid,
                                        pkt,
                                        entry->min_msg_len,
                                        entry->max_msg_len,
                                        entry->crc_extra);
    }

    // accessor for uart
    AP_HAL::UARTDriver *get_uart() { return _port; }

    virtual uint8_t sysid_my_gcs() const = 0;
    virtual bool sysid_enforce() const { return false; }

    void send_parameter_value(const char *param_name,
                              ap_var_type param_type,
                              float param_value);

    // NOTE! The streams enum below and the
    // set of AP_Int16 stream rates _must_ be
    // kept in the same order
    enum streams : uint8_t {
        STREAM_RAW_SENSORS,
        STREAM_EXTENDED_STATUS,
        STREAM_RC_CHANNELS,
        STREAM_RAW_CONTROLLER,
        STREAM_POSITION,
        STREAM_EXTRA1,
        STREAM_EXTRA2,
        STREAM_EXTRA3,
        STREAM_PARAMS,
        STREAM_ADSB,
        NUM_STREAMS
    };

    // streams must be moved out into the top level for
    // GCS_MAVLINK_Parameters to be able to use it.  This is an
    // extensive change, so we 'll just keep them in sync with a
    // static assert for now:
    static_assert(NUM_STREAMS == GCS_MAVLINK_NUM_STREAM_RATES, "num streams must equal num stream rates");

    bool is_high_bandwidth() { return chan == MAVLINK_COMM_0; }
    // return true if this channel has hardware flow control
    bool have_flow_control();

    bool is_active() const {
        return GCS_MAVLINK::active_channel_mask() & (1 << (chan-MAVLINK_COMM_0));
    }
    bool is_streaming() const {
        return sending_bucket_id != no_bucket_to_send;
    }

    mavlink_channel_t get_chan() const { return chan; }
    uint32_t get_last_heartbeat_time() const { return last_heartbeat_time; };

    uint32_t        last_heartbeat_time; // milliseconds

    // last time we got a non-zero RSSI from RADIO_STATUS
    static uint32_t last_radio_status_remrssi_ms;

    // mission item index to be sent on queued msg, delayed or not
    uint16_t mission_item_reached_index = AP_MISSION_CMD_INDEX_NONE;

    // common send functions
    void send_heartbeat(void) const;
    void send_meminfo(void);
    void send_fence_status() const;
    void send_power_status(void);
    void send_battery_status(const uint8_t instance) const;
    bool send_battery_status() const;
    void send_distance_sensor() const;
    // send_rangefinder sends only if a downward-facing instance is
    // found.  Rover overrides this!
    virtual void send_rangefinder() const;
    void send_proximity() const;
    virtual void send_nav_controller_output() const = 0;
    virtual void send_pid_tuning() = 0;
    void send_ahrs2();
    void send_ahrs3();
    void send_system_time();
    void send_rc_channels() const;
    void send_rc_channels_raw() const;
    void send_raw_imu();

    void send_scaled_pressure_instance(uint8_t instance, void (*send_fn)(mavlink_channel_t chan, uint32_t time_boot_ms, float press_abs, float press_diff, int16_t temperature));
    void send_scaled_pressure();
    void send_scaled_pressure2();
    virtual void send_scaled_pressure3(); // allow sub to override this
    void send_sensor_offsets();
    virtual void send_simstate() const;
    void send_ahrs();
    void send_battery2();
    void send_opticalflow();
    virtual void send_attitude() const;
    void send_autopilot_version() const;
    void send_extended_sys_state() const;
    void send_local_position() const;
    void send_vfr_hud();
    void send_vibration() const;
    void send_mount_status() const;
    void send_named_float(const char *name, float value) const;
    void send_gimbal_report() const;
    void send_home_position() const;
    void send_gps_global_origin() const;
    virtual void send_position_target_global_int() { };
    virtual void send_position_target_local_ned() { };
    void send_servo_output_raw();
    void send_accelcal_vehicle_position(uint32_t position);
    void send_scaled_imu(uint8_t instance, void (*send_fn)(mavlink_channel_t chan, uint32_t time_ms, int16_t xacc, int16_t yacc, int16_t zacc, int16_t xgyro, int16_t ygyro, int16_t zgyro, int16_t xmag, int16_t ymag, int16_t zmag));
    void send_sys_status();
    void send_set_position_target_global_int(uint8_t target_system, uint8_t target_component, const Location& loc);
    void send_rpm() const;

    bool locked() const;

    // return a bitmap of active channels. Used by libraries to loop
    // over active channels to send to all active channels    
    static uint8_t active_channel_mask(void) { return mavlink_active; }

    // return a bitmap of streaming channels
    static uint8_t streaming_channel_mask(void) { return chan_is_streaming; }

    // set a channel as private. Private channels get sent heartbeats, but
    // don't get broadcast packets or forwarded packets
    static void set_channel_private(mavlink_channel_t chan);

    // return true if channel is private
    static bool is_private(mavlink_channel_t _chan) {
        return (mavlink_private & (1U<<(unsigned)_chan)) != 0;
    }
    
    // return true if channel is private
    bool is_private(void) const { return is_private(chan); }

    /*
      send a MAVLink message to all components with this vehicle's system id
      This is a no-op if no routes to components have been learned
    */
    static void send_to_components(const mavlink_message_t &msg) { routing.send_to_components(msg); }
    
    /*
      allow forwarding of packets / heartbeats to be blocked as required by some components to reduce traffic
    */
    static void disable_channel_routing(mavlink_channel_t chan) { routing.no_route_mask |= (1U<<(chan-MAVLINK_COMM_0)); }
    
    /*
      search for a component in the routing table with given mav_type and retrieve it's sysid, compid and channel
      returns if a matching component is found
     */
    static bool find_by_mavtype(uint8_t mav_type, uint8_t &sysid, uint8_t &compid, mavlink_channel_t &channel) { return routing.find_by_mavtype(mav_type, sysid, compid, channel); }

    // update signing timestamp on GPS lock
    static void update_signing_timestamp(uint64_t timestamp_usec);

    // return current packet overhead for a channel
    static uint8_t packet_overhead_chan(mavlink_channel_t chan);

    // alternative protocol function handler
    FUNCTOR_TYPEDEF(protocol_handler_fn_t, bool, uint8_t, AP_HAL::UARTDriver *);

    struct stream_entries {
        const streams stream_id;
        const ap_message *ap_message_ids;
        const uint8_t num_ap_message_ids;
    };
    // vehicle subclass cpp files should define this:
    static const struct stream_entries all_stream_entries[];

    virtual uint64_t capabilities() const;
    uint8_t get_stream_slowdown_ms() const { return stream_slowdown_ms; }

protected:

    virtual bool in_hil_mode() const { return false; }

    bool mavlink_coordinate_frame_to_location_alt_frame(MAV_FRAME coordinate_frame,
                                                        Location::AltFrame &frame);

    // overridable method to check for packet acceptance. Allows for
    // enforcement of GCS sysid
    bool accept_packet(const mavlink_status_t &status, const mavlink_message_t &msg);
    virtual AP_AdvancedFailsafe *get_advanced_failsafe() const { return nullptr; };
    virtual bool set_mode(uint8_t mode) = 0;
    void set_ekf_origin(const Location& loc);

    virtual MAV_MODE base_mode() const = 0;
    virtual MAV_STATE system_status() const = 0;

    virtual MAV_VTOL_STATE vtol_state() const { return MAV_VTOL_STATE_UNDEFINED; }
    virtual MAV_LANDED_STATE landed_state() const { return MAV_LANDED_STATE_UNDEFINED; }

    // return a MAVLink parameter type given a AP_Param type
    static MAV_PARAM_TYPE mav_param_type(enum ap_var_type t);

    AP_Param *                  _queued_parameter;      ///< next parameter to
                                                        // be sent in queue
    mavlink_channel_t           chan;
    uint8_t packet_overhead(void) const { return packet_overhead_chan(chan); }

    // saveable rate of each stream
    AP_Int16        *streamRates;

    virtual bool persist_streamrates() const { return false; }
    void handle_request_data_stream(const mavlink_message_t &msg);

    virtual void handle_command_ack(const mavlink_message_t &msg);
    void handle_set_mode(const mavlink_message_t &msg);
    void handle_command_int(const mavlink_message_t &msg);

    MAV_RESULT handle_command_int_do_set_home(const mavlink_command_int_t &packet);
    virtual MAV_RESULT handle_command_int_packet(const mavlink_command_int_t &packet);

    virtual bool set_home_to_current_location(bool lock) = 0;
    virtual bool set_home(const Location& loc, bool lock) = 0;

    MAV_RESULT handle_command_do_set_home(const mavlink_command_long_t &packet);

    void handle_mission_request_list(const mavlink_message_t &msg);
    void handle_mission_request(const mavlink_message_t &msg);
    void handle_mission_request_int(const mavlink_message_t &msg);
    void handle_mission_clear_all(const mavlink_message_t &msg);
    virtual void handle_mission_set_current(AP_Mission &mission, const mavlink_message_t &msg);
    void handle_mission_count(const mavlink_message_t &msg);
    void handle_mission_write_partial_list(const mavlink_message_t &msg);
    void handle_mission_item(const mavlink_message_t &msg);

    void handle_common_param_message(const mavlink_message_t &msg);
    void handle_param_set(const mavlink_message_t &msg);
    void handle_param_request_list(const mavlink_message_t &msg);
    void handle_param_request_read(const mavlink_message_t &msg);
    virtual bool params_ready() const { return true; }
    virtual void handle_rc_channels_override(const mavlink_message_t &msg);
    void handle_system_time_message(const mavlink_message_t &msg);
    void handle_common_rally_message(const mavlink_message_t &msg);
    void handle_rally_fetch_point(const mavlink_message_t &msg);
    void handle_rally_point(const mavlink_message_t &msg);
    virtual void handle_mount_message(const mavlink_message_t &msg);
    void handle_fence_message(const mavlink_message_t &msg);
    void handle_param_value(const mavlink_message_t &msg);
    void handle_radio_status(const mavlink_message_t &msg, bool log_radio);
    void handle_serial_control(const mavlink_message_t &msg);
    void handle_vision_position_delta(const mavlink_message_t &msg);

    void handle_common_message(const mavlink_message_t &msg);
    void handle_set_gps_global_origin(const mavlink_message_t &msg);
    void handle_setup_signing(const mavlink_message_t &msg);
    virtual bool should_zero_rc_outputs_on_reboot() const { return false; }
    MAV_RESULT handle_preflight_reboot(const mavlink_command_long_t &packet);

    // reset a message interval via mavlink:
    MAV_RESULT handle_command_set_message_interval(const mavlink_command_long_t &packet);
    MAV_RESULT handle_command_get_message_interval(const mavlink_command_long_t &packet);
    bool get_ap_message_interval(ap_message id, uint16_t &interval_ms) const;
    MAV_RESULT handle_command_request_message(const mavlink_command_long_t &packet);

    MAV_RESULT handle_rc_bind(const mavlink_command_long_t &packet);
    virtual MAV_RESULT handle_flight_termination(const mavlink_command_long_t &packet);

    void handle_send_autopilot_version(const mavlink_message_t &msg);
    MAV_RESULT handle_command_request_autopilot_capabilities(const mavlink_command_long_t &packet);

    virtual void send_banner();

    void handle_device_op_read(const mavlink_message_t &msg);
    void handle_device_op_write(const mavlink_message_t &msg);

    void send_timesync();
    // returns the time a timesync message was most likely received:
    uint64_t timesync_receive_timestamp_ns() const;
    // returns a timestamp suitable for packing into the ts1 field of TIMESYNC:
    uint64_t timesync_timestamp_ns() const;
    void handle_timesync(const mavlink_message_t &msg);
    struct {
        int64_t sent_ts1;
        uint32_t last_sent_ms;
        const uint16_t interval_ms = 10000;
    }  _timesync_request;

    void handle_statustext(const mavlink_message_t &msg);

    bool telemetry_delayed() const;
    virtual uint32_t telem_delay() const = 0;

    MAV_RESULT handle_command_preflight_set_sensor_offsets(const mavlink_command_long_t &packet);
    MAV_RESULT handle_command_flash_bootloader(const mavlink_command_long_t &packet);

    // generally this should not be overridden; Plane overrides it to ensure
    // failsafe isn't triggered during calibration
    virtual MAV_RESULT handle_command_preflight_calibration(const mavlink_command_long_t &packet);

    virtual MAV_RESULT _handle_command_preflight_calibration(const mavlink_command_long_t &packet);
    virtual MAV_RESULT _handle_command_preflight_calibration_baro();

    MAV_RESULT handle_command_preflight_can(const mavlink_command_long_t &packet);

    MAV_RESULT handle_command_battery_reset(const mavlink_command_long_t &packet);
    void handle_command_long(const mavlink_message_t &msg);
    MAV_RESULT handle_command_accelcal_vehicle_pos(const mavlink_command_long_t &packet);
    virtual MAV_RESULT handle_command_mount(const mavlink_command_long_t &packet);
    MAV_RESULT handle_command_mag_cal(const mavlink_command_long_t &packet);
    virtual MAV_RESULT handle_command_long_packet(const mavlink_command_long_t &packet);
    MAV_RESULT handle_command_camera(const mavlink_command_long_t &packet);
    MAV_RESULT handle_command_do_send_banner(const mavlink_command_long_t &packet);
    MAV_RESULT handle_command_do_set_roi(const mavlink_command_int_t &packet);
    MAV_RESULT handle_command_do_set_roi(const mavlink_command_long_t &packet);
    virtual MAV_RESULT handle_command_do_set_roi(const Location &roi_loc);
    MAV_RESULT handle_command_do_gripper(const mavlink_command_long_t &packet);
    MAV_RESULT handle_command_do_set_mode(const mavlink_command_long_t &packet);
    MAV_RESULT handle_command_get_home_position(const mavlink_command_long_t &packet);
    MAV_RESULT handle_command_do_fence_enable(const mavlink_command_long_t &packet);

    void handle_optical_flow(const mavlink_message_t &msg);

    // vehicle-overridable message send function
    virtual bool try_send_message(enum ap_message id);
    virtual void send_global_position_int();

    // message sending functions:
    bool try_send_mission_message(enum ap_message id);
    void send_hwstatus();
    void handle_data_packet(const mavlink_message_t &msg);

    // these two methods are called after current_loc is updated:
    virtual int32_t global_position_int_alt() const;
    virtual int32_t global_position_int_relative_alt() const;

    virtual float vfr_hud_climbrate() const;
    virtual float vfr_hud_airspeed() const;
    virtual int16_t vfr_hud_throttle() const { return 0; }
    virtual float vfr_hud_alt() const;
    virtual uint8_t get_battery_remaining_percentage(uint8_t instance=0) const;

    static constexpr const float magic_force_arm_value = 2989.0f;
    static constexpr const float magic_force_disarm_value = 21196.0f;

    virtual bool allow_disarm() const { return true; }

    void manual_override(RC_Channel *c, int16_t value_in, uint16_t offset, float scaler, const uint32_t tnow, bool reversed = false);

private:

    void log_mavlink_stats();

    MAV_RESULT _set_mode_common(const MAV_MODE base_mode, const uint32_t custom_mode);

    virtual void        handleMessage(const mavlink_message_t &msg) = 0;

    MAV_RESULT handle_servorelay_message(const mavlink_command_long_t &packet);

    static bool command_long_stores_location(const MAV_CMD command);
    static void convert_COMMAND_LONG_to_COMMAND_INT(const mavlink_command_long_t &in, mavlink_command_int_t &out);

    bool calibrate_gyros();

    /// The stream we are communicating over
    AP_HAL::UARTDriver *_port;

    /// Perform queued sending operations
    ///
    enum ap_var_type            _queued_parameter_type; ///< type of the next
                                                        // parameter
    AP_Param::ParamToken        _queued_parameter_token; ///AP_Param token for
                                                         // next() call
    uint16_t                    _queued_parameter_index; ///< next queued
                                                         // parameter's index
    uint16_t                    _queued_parameter_count; ///< saved count of
                                                         // parameters for
                                                         // queued send
    uint32_t                    _queued_parameter_send_time_ms;

    /// Count the number of reportable parameters.
    ///
    /// Not all parameters can be reported via MAVlink.  We count the number
    // that are
    /// so that we can report to a GCS the number of parameters it should
    // expect when it
    /// requests the full set.
    ///
    /// @return         The number of reportable parameters.
    ///
    uint16_t                    packet_drops;

    // number of extra ms to add to slow things down for the radio
    uint16_t         stream_slowdown_ms;

    // perf counters
    AP_HAL::Util::perf_counter_t _perf_packet;
    AP_HAL::Util::perf_counter_t _perf_update;
    char _perf_packet_name[16];
    char _perf_update_name[16];

    // outbound ("deferred message") queue.

    // "special" messages such as heartbeat, next_param etc are stored
    // separately to stream-rated messages like AHRS2 etc.  If these
    // were to be stored in buckets then they would be slowed down
    // based on stream_slowdown, which we have not traditionally done.
    struct deferred_message_t {
        const ap_message id;
        uint16_t interval_ms;
        uint16_t last_sent_ms; // from AP_HAL::millis16()
    } deferred_message[2] = {
        { MSG_HEARTBEAT, },
        { MSG_NEXT_PARAM, },
    };
    // returns index of id in deferred_message[] or -1 if not present
    int8_t get_deferred_message_index(const ap_message id) const;
    // returns index of a message in deferred_message[] which should
    // be sent (or -1 if none to send at the moment)
    int8_t deferred_message_to_send_index();
    // cache of which deferred message should be sent next:
    int8_t next_deferred_message_to_send_cache = -1;

    struct deferred_message_bucket_t {
        Bitmask<MSG_LAST> ap_message_ids;
        uint16_t interval_ms;
        uint16_t last_sent_ms; // from AP_HAL::millis16()
    };
    deferred_message_bucket_t deferred_message_bucket[10];
    static const uint8_t no_bucket_to_send = -1;
    static const ap_message no_message_to_send = (ap_message)-1;
    uint8_t sending_bucket_id = no_bucket_to_send;
    Bitmask<MSG_LAST> bucket_message_ids_to_send;

    ap_message next_deferred_bucket_message_to_send();
    void find_next_bucket_to_send();
    void remove_message_from_bucket(int8_t bucket, ap_message id);

    // bitmask of IDs the code has spontaneously decided it wants to
    // send out.  Examples include HEARTBEAT (gcs_send_heartbeat)
    Bitmask<MSG_LAST> pushed_ap_message_ids;

    // returns true if it is OK to send a message while we are in
    // delay callback.  In particular, when we are doing sensor init
    // we still send heartbeats.
    bool should_send_message_in_delay_callback(const ap_message id) const;

    // if true is returned, interval will contain the default interval for id
    bool get_default_interval_for_ap_message(const ap_message id, uint16_t &interval) const;
    //  if true is returned, interval will contain the default interval for id
    bool get_default_interval_for_mavlink_message_id(const uint32_t mavlink_message_id, uint16_t &interval) const;
    // returns an interval in milliseconds for any ap_message in stream id
    uint16_t get_interval_for_stream(GCS_MAVLINK::streams id) const;
    // set an inverval for a specific mavlink message.  Returns false
    // on failure (typically because there is no mapping from that
    // mavlink ID to an ap_message)
    bool set_mavlink_message_id_interval(const uint32_t mavlink_id,
                                         const uint16_t interval_ms);
    // map a mavlink ID to an ap_message which, if passed to
    // try_send_message, will cause a mavlink message with that id to
    // be emitted.  Returns MSG_LAST if no such mapping exists.
    ap_message mavlink_id_to_ap_message_id(const uint32_t mavlink_id) const;
    // set the interval at which an ap_message should be emitted (in ms)
    bool set_ap_message_interval(enum ap_message id, uint16_t interval_ms);
    // call set_ap_message_interval for each entry in a stream,
    // the interval being based on the stream's rate
    void initialise_message_intervals_for_stream(GCS_MAVLINK::streams id);
    // call initialise_message_intervals_for_stream on every stream:
    void initialise_message_intervals_from_streamrates();
    // boolean that indicated that message intervals have been set
    // from streamrates:
    bool deferred_messages_initialised;
    // return interval deferred message bucket should be sent after.
    // When sending parameters and waypoints this may be longer than
    // the interval specified in "deferred"
    uint16_t get_reschedule_interval_ms(const deferred_message_bucket_t &deferred) const;

    bool do_try_send_message(const ap_message id);

    // time when we missed sending a parameter for GCS
    static uint32_t reserve_param_space_start_ms;
    
    // bitmask of what mavlink channels are active
    static uint8_t mavlink_active;

    // bitmask of what mavlink channels are private
    static uint8_t mavlink_private;

    // bitmask of what mavlink channels are streaming
    static uint8_t chan_is_streaming;

    // mavlink routing object
    static MAVLink_routing routing;

    struct pending_param_request {
        mavlink_channel_t chan;
        int16_t param_index;
        char param_name[AP_MAX_NAME_SIZE+1];
    };

    struct pending_param_reply {
        mavlink_channel_t chan;        
        float value;
        enum ap_var_type p_type;
        int16_t param_index;
        uint16_t count;
        char param_name[AP_MAX_NAME_SIZE+1];
    };

    // queue of pending parameter requests and replies
    static ObjectBuffer<pending_param_request> param_requests;
    static ObjectBuffer<pending_param_reply> param_replies;

    // have we registered the IO timer callback?
    static bool param_timer_registered;

    // IO timer callback for parameters
    void param_io_timer(void);

    uint8_t send_parameter_async_replies();

    void send_distance_sensor(const class AP_RangeFinder_Backend *sensor, const uint8_t instance) const;

    virtual bool handle_guided_request(AP_Mission::Mission_Command &cmd) = 0;
    virtual void handle_change_alt_request(AP_Mission::Mission_Command &cmd) = 0;
    void handle_common_mission_message(const mavlink_message_t &msg);

    void handle_vicon_position_estimate(const mavlink_message_t &msg);
    void handle_vision_position_estimate(const mavlink_message_t &msg);
    void handle_global_vision_position_estimate(const mavlink_message_t &msg);
    void handle_att_pos_mocap(const mavlink_message_t &msg);
    void handle_common_vision_position_estimate_data(const uint64_t usec,
                                                     const float x,
                                                     const float y,
                                                     const float z,
                                                     const float roll,
                                                     const float pitch,
                                                     const float yaw,
                                                     const uint16_t payload_size);
    void log_vision_position_estimate_data(const uint64_t usec,
                                           const uint32_t corrected_msec,
                                           const float x,
                                           const float y,
                                           const float z,
                                           const float roll,
                                           const float pitch,
                                           const float yaw);

    void lock_channel(const mavlink_channel_t chan, bool lock);

    /*
      correct an offboard timestamp in microseconds to a local time
      since boot in milliseconds
     */
    uint32_t correct_offboard_timestamp_usec_to_ms(uint64_t offboard_usec, uint16_t payload_size);
    
    mavlink_signing_t signing;
    static mavlink_signing_streams_t signing_streams;
    static uint32_t last_signing_save_ms;
    
    static StorageAccess _signing_storage;
    static bool signing_key_save(const struct SigningKey &key);
    static bool signing_key_load(struct SigningKey &key);
    void load_signing_key(void);
    bool signing_enabled(void) const;
    static void save_signing_timestamp(bool force_save_now);

    // alternative protocol handler support
    struct {
        GCS_MAVLINK::protocol_handler_fn_t handler;
        uint32_t last_mavlink_ms;
        uint32_t last_alternate_ms;
        bool active;
    } alternative;

    JitterCorrection lag_correction;
    
    // we cache the current location and send it even if the AHRS has
    // no idea where we are:
    struct Location global_position_current_loc;

    void zero_rc_outputs();

    uint8_t last_tx_seq;
    uint16_t send_packet_count;

#if GCS_DEBUG_SEND_MESSAGE_TIMINGS
    struct {
        uint32_t longest_time_us;
        ap_message longest_id;
        uint32_t no_space_for_message;
        uint16_t statustext_last_sent_ms;
        uint32_t behind;
        uint32_t out_of_time;
        uint16_t fnbts_maxtime;
        uint32_t max_retry_deferred_body_us;
        uint8_t max_retry_deferred_body_type;
    } try_send_message_stats;
    uint16_t max_slowdown_ms;
#endif

    uint32_t last_mavlink_stats_logged;
};

/// @class GCS
/// @brief global GCS object
class GCS
{

public:

    GCS() {
        if (_singleton  == nullptr) {
            _singleton = this;
        } else {
#if CONFIG_HAL_BOARD == HAL_BOARD_SITL
            // this is a serious problem, but we don't need to kill a
            // real vehicle
            AP_HAL::panic("GCS must be singleton");
#endif
        }
    };

    static class GCS *get_singleton() {
        return _singleton;
    }

    virtual uint32_t custom_mode() const = 0;
    virtual MAV_TYPE frame_type() const = 0;
    virtual const char* frame_string() const { return nullptr; }

    void send_to_active_channels(uint32_t msgid, const char *pkt);

    void send_text(MAV_SEVERITY severity, const char *fmt, ...) FMT_PRINTF(3, 4);
    void send_textv(MAV_SEVERITY severity, const char *fmt, va_list arg_list);
    virtual void send_statustext(MAV_SEVERITY severity, uint8_t dest_bitmask, const char *text);
    void service_statustext(void);
    virtual GCS_MAVLINK *chan(const uint8_t ofs) = 0;
    virtual const GCS_MAVLINK *chan(const uint8_t ofs) const = 0;
    // return the number of valid GCS objects
    uint8_t num_gcs() const { return _num_gcs; };
    void send_message(enum ap_message id);
    void send_mission_item_reached_message(uint16_t mission_index);
    void send_named_float(const char *name, float value) const;

    void send_parameter_value(const char *param_name,
                              ap_var_type param_type,
                              float param_value);

    static MissionItemProtocol_Waypoints *_missionitemprotocol_waypoints;
    static MissionItemProtocol_Rally *_missionitemprotocol_rally;
    MissionItemProtocol *get_prot_for_mission_type(const MAV_MISSION_TYPE mission_type) const;
    void try_send_queued_message_for_type(MAV_MISSION_TYPE type);

    void update_send();
    void update_receive();

    // minimum amount of time (in microseconds) that must remain in
    // the main scheduler loop before we are allowed to send any
    // mavlink messages.  We want to prioritise the main flight
    // control loop over communications
    virtual uint16_t min_loop_time_remaining_for_message_send_us() const {
        return 200;
    }

    void setup_console();
    void setup_uarts();

    bool out_of_time() const;

    // frsky backend
    AP_Frsky_Telem *frsky;

#if !HAL_MINIMIZE_FEATURES
    // Devo backend
    AP_DEVO_Telem devo_telemetry;
#endif

    // install an alternative protocol handler
    bool install_alternative_protocol(mavlink_channel_t chan, GCS_MAVLINK::protocol_handler_fn_t handler);

    // get the VFR_HUD throttle
    int16_t get_hud_throttle(void) const { return num_gcs()>0?chan(0)->vfr_hud_throttle():0; }

    // update uart pass-thru
    void update_passthru();

    void get_sensor_status_flags(uint32_t &present, uint32_t &enabled, uint32_t &health);
    virtual bool vehicle_initialised() const { return true; }

    virtual bool simple_input_active() const { return false; }
    virtual bool supersimple_input_active() const { return false; }

protected:

    virtual GCS_MAVLINK *new_gcs_mavlink_backend(GCS_MAVLINK_Parameters &params,
                                                 AP_HAL::UARTDriver &uart) = 0;

    uint32_t control_sensors_present;
    uint32_t control_sensors_enabled;
    uint32_t control_sensors_health;
    virtual void update_vehicle_sensor_status_flags() {}

    GCS_MAVLINK_Parameters chan_parameters[MAVLINK_COMM_NUM_BUFFERS];
    uint8_t _num_gcs;
    GCS_MAVLINK *_chan[MAVLINK_COMM_NUM_BUFFERS];

private:

    static GCS *_singleton;

    void create_gcs_mavlink_backend(GCS_MAVLINK_Parameters &params,
                                    AP_HAL::UARTDriver &uart);

    struct statustext_t {
        uint8_t                 bitmask;
        mavlink_statustext_t    msg;
    };

    void update_sensor_status_flags();

#if HAL_MEM_CLASS <= HAL_MEM_CLASS_192 || CONFIG_HAL_BOARD == HAL_BOARD_SITL
    static const uint8_t _status_capacity = 5;
#else
    static const uint8_t _status_capacity = 30;
#endif

    // a lock for the statustext queue, to make it safe to use send_text()
    // from multiple threads
    HAL_Semaphore _statustext_sem;

    // queue of outgoing statustext messages
    ObjectArray<statustext_t> _statustext_queue{_status_capacity};

    // true if we have already allocated protocol objects:
    bool initialised_missionitemprotocol_objects;

    // handle passthru between two UARTs
    struct {
        bool enabled;
        bool timer_installed;
        AP_HAL::UARTDriver *port1;
        AP_HAL::UARTDriver *port2;
        uint32_t start_ms;
        uint32_t last_ms;
        uint32_t last_port1_data_ms;
        uint8_t timeout_s;
        HAL_Semaphore sem;
    } _passthru;

    // timer called to implement pass-thru
    void passthru_timer();
};

GCS &gcs();