1. A data processing apparatus comprising: a central processing unit for executing a stream of instructions; a graphics processing unit for performing graphics processing operations on behalf of the central processing unit; shared memory accessible by the central processing unit and the graphics processing unit via which data structures are shareable between the central processing unit and the graphics processing unit; a bus via which the central processing unit, graphics processing unit and shared memory communicate, the central processing unit routing control signals via the bus as a first mechanism for controlling the graphics processing unit; and an interface between the central processing unit and the graphics processing unit, the central processing unit providing control signals over the interface as an additional mechanism for controlling the graphics processing unit.