Some PCI controllers may require more resources than the PCI bus can offer. In such cases, the PCI controller must not be visible to PCI drivers because resources were not allocated to the PCI controller. The PCI bus driver does not create a child handle for a PCI controller that does not have any allocated resources, and as a result, a PCI driver is never be passed a ControllerHandle for a PCI controller not having allocated resources.
The platform vendor controls the policy decisions that are made when this type of resource-constrained condition is encountered. The PCI driver writer never has to handle this case.