The Avalon Memory-Mapped Interface has a property constantBurstBehavior which declares how the master/slave should handle the address and burstcount signals. When constantBurstBehavior is set to true, address and burstcount must be held stable throughout a burst. When constantBurstBehavior is set to false, address and burstcount are sampled on the first transaction of a burst so it does not matter if address and burstcount change during all subequent transactions of a burst.
The UniPHY based controllers have constantBurstBehavior set to false and only require that the address (avl_addr) and burstcount (avl_size) remain stable during the first transaction of a burst.