mavlink_stream.cpp

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/**
 * @file mavlink_stream.cpp
 * Mavlink messages stream implementation.
 *
 * @author Anton Babushkin <anton.babushkin@me.com>
 * @author Lorenz Meier <lorenz@px4.io>
 */

#include <stdlib.h>

#include "mavlink_stream.h"
#include "mavlink_main.h"

MavlinkStream::MavlinkStream(Mavlink *mavlink) :
    _mavlink(mavlink)
{
    _last_sent = hrt_absolute_time();
}

/**
 * Update subscriptions and send message if necessary
 */
int
MavlinkStream::update(const hrt_abstime &t)
{
    update_data();

    // If the message has never been sent before we want
    // to send it immediately and can return right away
    if (_last_sent == 0) {
        // this will give different messages on the same run a different
        // initial timestamp which will help spacing them out
        // on the link scheduling
        if (send(t)) {
            _last_sent = hrt_absolute_time();

            if (!_first_message_sent) {
                _first_message_sent = true;
            }
        }

        return 0;
    }

    // One of the previous iterations sent the update
    // already before the deadline
    if (_last_sent > t) {
        return -1;
    }

    int64_t dt = t - _last_sent;
    int interval = (_interval > 0) ? _interval : 0;

    if (!const_rate()) {
        interval /= _mavlink->get_rate_mult();
    }

    // Send the message if it is due or
    // if it will overrun the next scheduled send interval
    // by 30% of the interval time. This helps to avoid
    // sending a scheduled message on average slower than
    // scheduled. Doing this at 50% would risk sending
    // the message too often as the loop runtime of the app
    // needs to be accounted for as well.
    // This method is not theoretically optimal but a suitable
    // stopgap as it hits its deadlines well (0.5 Hz, 50 Hz and 250 Hz)

    if (interval == 0 || (dt > (interval - (_mavlink->get_main_loop_delay() / 10) * 3))) {
        // interval expired, send message

        // If the interval is non-zero and dt is smaller than 1.5 times the interval
        // do not use the actual time but increment at a fixed rate, so that processing delays do not
        // distort the average rate. The check of the maximum interval is done to ensure that after a
        // long time not sending anything, sending multiple messages in a short time is avoided.
        if (send(t)) {
            _last_sent = ((interval > 0) && ((int64_t)(1.5f * interval) > dt)) ? _last_sent + interval : t;

            if (!_first_message_sent) {
                _first_message_sent = true;
            }

            return 0;

        } else {
            return -1;
        }
    }

    return -1;
}