Not sure whether that's accurate semantics, but I like to think of plaform_drv and pci_decode as a H.A.L. (i.e. a hardware abstraction layer, as the term was used many eons ago by Dave Haynie et alia). Write your OS to 'talk' to the HAL, and then you can port your OS to many platforms with more ease. Well the state of the art probably moved by leaps and bounds since I heard the term, but I'm the nostalgic type ^^.
Cedric