Dynamic security in SSAS tabular model

Hi All,
 
I am using SSAS 2012 to create a tabular model. While creating tabular model am enabling Direct query mode as ON ,to get Real time data from database.
 
I need to restrict the users accessing that tabular model and they should get only data which are specific to his role or his country. when we enable the Direct query mode we are not allowed to use DAX functions to use USERNAME Function (DAX) and LOOKUPVALUE Function (DAX). 

It would be helpful if I get any approach to solve this.

Thanks,

Balaji