The policy is that the device tree should match hardware bus topology.
(Or at least the control bus, for devices which use multiple busses.)
+In particular, this means that a device registration may fail if the parent of
+the device is suspending (ie. has been chosen by the PM core as the next
+device to suspend) or has already suspended, as well as after all of the other
+devices have been suspended. Device drivers must be prepared to cope with such
+situations.
Suspending Devices