What is the direction of friction when a cylinder rolls up an inclined plane?

Question

anonymous · Answer

In this case the role of frictional force is to prevent slipping down. So the direction of friction is opposite to the direction in which the object would slip, that is, friction is in the direction of motion of the cylinder. It is about Newton action-reaction law. The cylinder wants to slip down and pushes down but the friction wouldn't let to and pushes it up.

vincent-lyon.fr · Answer

I confirm the answer given by seemanstreue.
If the cylinder is rolling down, the direction of friction is the same (up); it's always the one that opposes the direction in which slipping would occur.