Why does my Hard IP for PCI Express in Root Port mode not return a Vendor ID=0xFFFFFFFF for a device that's not present?

Description

This behavior is expected.  If a configuration read is attempted for a device that is not present on a downstream bus (or for busses not present), the Hard IP core will return a Completion without data, showing Unsupported Request (UR).

Resolution

The driver software communicating with the Hard IP core must be aware that a configuration read of the Vendor ID register was pending and that the Completion without data and UR indicate device/bus/function not present.  If this request was from higher layer software the driver should report a Vendor ID=0xFFFFFFFF to meet the expectations of system software components.

The Altera® Hard IP for PCI® Express is a root port device, it is the surrounding Root Complex that is responsible for reporting the 0xFFFFFFFF Vendor ID.