Port manipulation PINx commands with Arduino Due ?

Hello, again.
Did any of you know how to start the clock that makes input mode posible in arduino DUE.

I'm doing direct port manipulation, but to be able to read direct from a pin, i'm currently using

pinMode(pin,input), where pin, is a non used pin of the port.

acording to the SAMx documentation, you do need to start this clock for each port. but i couldn't find how to do this, any other way