A root bridge driver does not produce any instances of the
EFI_DRIVER_BINDING_PROTOCOL. It is responsible for initializing and
immediately creating physical controller handles for the root bridge
controllers or root devices in a platform. The driver must install the Device
Path Protocol onto a physical controller handle because the root bridge
controllers or root devices represent physical devices. An example root bridge
driver, PcAtChipsetPkg/PciHostBridgeDxe, is shown in the EDK II.
This driver also installs the PCI Root Bridge I/O Protocol―the protocol abstraction for a PCI Bus. This protocol is used by a bus driver for the PCI Bus to enumerate the PCI controllers attached to the PCI root bridge.
A driver for a root device may produce a protocol that is more directly usable as a console or boot device. For example, a Serial I/O Protocol for a serial device that is not attached to an industry standard bus type supported by the UEFI Specification, or a Block I/O Protocol for a block-oriented media device that is not attached to an industry standard bus type supported by the UEFI Specification.