PCIE clock rate stepping for graphics and platform processors

Circuits, methods, and apparatus for modifying the data rate of a data bus. In a circuit having two processors coupled by a data bus, the processors each learn that the other is capable of operating at a modified data rate. The data rate is then changed to the modified rate. Each processor may learn of the other's capability by reading a vendor identification, for example from a vendor defined message stored on the other processor. Alternately, each processor may provide an instruction to the other to operate at the modified rate, for example by writing to the other processor's extended capability registers. In another circuit having two processors communicating over a bus, it is determined that both are capable of transmitting and receiving data at a modified data rate. An instruction is provided to one or both of the processors to transmit at the modified rate.