APM: much faster parameter fetching

allow up to 30% of bandwidth to be used for parameter send

ArduPlane/GCS.h

@@ -153,6 +153,7 @@ private:
     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.
     ///

ArduPlane/GCS_Mavlink.pde

@@ -1989,9 +1989,23 @@ GCS_MAVLINK::_count_parameters()
 void
 GCS_MAVLINK::queued_param_send()
 {
-    // Check to see if we are sending parameters
-    if (NULL == _queued_parameter) return;
+    if (_queued_parameter == NULL) {
+        return;
+    }
+
+    uint16_t bytes_allowed;
+    uint8_t count;
+    uint32_t tnow = millis();
 
+    // use at most 30% of bandwidth on parameters. The constant 26 is
+    // 1/(1000 * 1/8 * 0.001 * 0.3)
+    bytes_allowed = g.serial3_baud * (tnow - _queued_parameter_send_time_ms) * 26;
+    if (bytes_allowed > comm_get_txspace(chan)) {
+        bytes_allowed = comm_get_txspace(chan);
+    }
+    count = bytes_allowed / (MAVLINK_MSG_ID_PARAM_VALUE_LEN + MAVLINK_NUM_NON_PAYLOAD_BYTES);
+
+    while (_queued_parameter != NULL && count--) {
         AP_Param      *vp;
         float value;
 
@@ -2015,6 +2029,8 @@ GCS_MAVLINK::queued_param_send()
         _queued_parameter = AP_Param::next_scalar(&_queued_parameter_token, &_queued_parameter_type);
         _queued_parameter_index++;
     }
+    _queued_parameter_send_time_ms = tnow;
+}
 
 /**
  * @brief Send the next pending waypoint, called from deferred message