This layer of interfaces is implemented by the lowest level layer of drivers, using DKI services. This set of drivers can itself be composed of multiple sub-layers to reflect the bus hierarchy of a given platform.
Typically, only the primary (host) bus driver is built solely using DKI services. Subsequent drivers, those occupying a "downstream" position in the hierarchy, interface with the primary (host) bus. As all different I/O buses share a subset of features, and then have their particular specificities, the bus driver interfaces layer offers a subset of services called "Common bus driver interface" (CBDI) , which is independent of the bus type, offering a set of services common for all bus classes.
In addition to the CBDI, there is of course a collection of bus specific interfaces (such as PCI, VME, ISA) to implement bus-specific driver services.