The Virtex-4 and Virtex-II Pro do not support Receiver Detect as defined in section 18.104.22.168. How is this handled by the core?
Although Receiver Detect is not supported by the device, it does not affect the operation much of an actual PCI Express link or the behavior of Xilinx core.
Receiver Detect would allow Xilinx devices to determine if there is another device attached. This would serve two purposes; it would allow the physical layer to save power in the event no other device is attached, and it would also allow Xilinx devices to enter what is called the compliance state (a test mode that is not used in normal operation).
Without Receiver Detect, Xilinx core must make the only assumption possible, that a receiver is present. Because of this, the core cannot save power when no other device is attached. Further, since the core assumes another device is present and cannot detect electrical idle, it must not enter the compliance state.
In summary, the PCI Express Core assumes another device is present, and proceeds with polling on exit from the detect state. For more information on the LTSSM, refer to section 4.2.6 of the PCI Express Base Specification.