How to use a global function in a recursive directive in angularjs

I'm trying to use a global function defined in a parent constructor within the directives on the page which will be called recursively. However not able to figure out how to pass it through all the layers cleanly. using the $parent I can pass it through one layer only.. What would be the best way to achieve this?

http://jsfiddle.net/J8tFS/

<div ng-app="myapp">
<div ng-controller="TreeCtrl">
    <tree family="treeFamily"></tree>
</div>

module.controller("TreeCtrl", function($scope) {
$scope.treeFamily = {
    name : "Parent",
    children: [{
        name : "Child1",
        children: [{
            name : "Grandchild1",
            children: []
        },{
            name : "Grandchild2",
            children: []
        },{
            name : "Grandchild3",
            children: []
        }]
    }, {
        name: "Child2",
        children: []
    }]
};
$scope.toUpper = function(strInput) {
     return strInput.toUpperCase();   
}
});

module.directive("tree", function($compile) {
return {
    restrict: "E",
    scope: {family: '='},
    template: 
        '<p>{{ family.name }}</p>'+
        '<p>{{ $parent.toUpper(family.name) }}</p>' +
        '<ul>' + 
            '<li ng-repeat="child in family.children">' + 
                '<tree family="child"></tree>' +
            '</li>' +
        '</ul>',
    compile: function(tElement, tAttr) {
        var contents = tElement.contents().remove();
        var compiledContents;
        return function(scope, iElement, iAttr) {
            if(!compiledContents) {
                compiledContents = $compile(contents);
            }
            compiledContents(scope, function(clone, scope) {
                     iElement.append(clone); 
            });
        };
    }
};

});