The recommended type of polariser for autofocus cameras is circular.
Linear polarisers can interefere with the way AF Cameras focus, something to do with them blocking a certain type of light that the AF system uses.
I have a circular filter, and both my lenses are external focus, in that the barrel rotates the filter when it focuses.
All it requires is to set the shot up first, focus, then rotate the polariser to the desired position.