Device Drivers Interface (DDI)

The DDI defines several layers of interface between different layers of device drivers in the driver's hierarchy. Typically an API is defined for each class of bus or device, as a part of the DDI.

Note that a driver's client application may itself be a driver component (as a device driver is a client of the bus driver API). In this way, it can be seen that all DDI services are implemented by a driver component, and are in turn called by upper-layer drivers (or directly by the driver's client applications).

As illustrated earlier, in Figure 1-3, the DDI set of APIs is further divided along hierarchical lines into two principle interface layers -- Bus Driver Interfaces and Device Driver Interfaces.

Bus Driver Interface APIs

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.

Device Driver Interface APIs

This layer of interfaces is implemented by the device drivers, and is built upon the lower layer of services (bus driver interfaces). This set of device drivers provides different interfaces for each different class/type of device. Typically, there are different interfaces for timer devices, UART devices, Ethernet devices and so on.

Each of these APIs may be used by the driver's client application to manage the associated devices. Note that the ddi(9) manpage gives an entry point to a detailed description of all DDI APIs.