AMD Answer 73152
A processor can use the wait for event (WFE) or wait for interrupt (WFI) mechanism to enter low-power state. A processor can enter low-power state only after the Embedded Trace Macrocell (ETM) drains all trace bytes on the AMBA® ATB interface. Under certain conditions, anAMBA ATB stall can cause the processor to hang until the AMBA ATB stall condition is cleared. Some trace subsystems might require instructions to be executed on the processor to clear the AMBA ATB stall condition. An example of such trace subsystems involves draining the trace to the memory subsystem via an SMMU. A processor deadlock might occur when such trace subsystems are used.
This is a third-party errata (Arm, Inc. 850321); this issue will not be fixed.