galois fields-conversion between polynomial and power represenation for extension

Hey All,

In an extended galois field of size p^m we can have basically two representations of the elements...one based on all polynomials over GF(p) of degree m-1 or less...and second powers of the primitive element. now given a power of primitive element u can find the polynomial representation by taking modulo by primitive polynomial ..but i cant figure out how to go back ..please kindly explain the other way round i.e given polynomial representation how to find which power of the primitive polynomial it corresponds too...