The MCA bus functions provide a generalised interface to find MCA bus cards, to claim them for a driver, and to read and manipulate POS registers without being aware of the motherboard internals or certain deep magic specific to onboard devices.
The basic interface to the MCA bus devices is the slot. Each slot is numbered and virtual slot numbers are assigned to the internal devices. Using a pci_dev as other busses do does not really make sense in the MCA context as the MCA bus resources require card specific interpretation.
Finally the MCA bus functions provide a parallel set of DMA functions mimicing the ISA bus DMA functions as closely as possible, although also supporting the additional DMA functionality on the MCA bus controllers.