Asynchronous call inside a Knockout computed function

Right now I have a Knockout computed function that takes certain variables and make an ajax call to google directionsService to get distance between two locations. And I have some calculations done on the callback function of the ajax request. What I would like to do is to make the ajax call only when location variables change so that it does not make api calls every time a ko observable change.

This was my initial code:

ko.computed(function () {

    if (checkStatus() !== "invalid-data") {
      // Encode address for google API
      var start = self.start().split(' ').join('+');
      var end = self.end().split(' ').join('+');

        var request = {
          origin: start,
          destination: end,
          travelMode: google.maps.TravelMode.DRIVING
        };

        directionsServices.route(request, function (response, status) {

          if (status === "OK") {
            // calculations
          } else {
            console.error("Error: " + status);
          }
        });
      }
    }

  }, self);

And this was my attempt to refactor the API call:

function getTripData() {

    var request = {
      origin: start,
      destination: start,
      waypoints: [{
        location: end
    }],
      travelMode: google.maps.TravelMode.DRIVING
    };

    return $.Deferred(function(dfd) {
      directionsServices.route(request, dfd.resolve());
    }).promise();
}

if (locationsChanged) {
  trip = getTripData();
  trip.done (function() {
    console.log(trip.status);
    console.log(trip.response);
  });
}

I know this isn't right and the code doesnt work either. I could not really understand the JQuery docs on deferred and promise objects. If anybody can help me figure this out that would be great.