hi peter.
yes, you are right. when the filter is added, filter function is fired once.
but when i add filter by dropdownlist, filter function is fired twice. that is problem.
(See the attached video)
so The below of the method can not be used.
http://www.jqwidgets.com/jquery-widgets-documentation/documentation/jqxgrid/jquery-grid-php-server-side-processing.htm