USB devices are represented as two levels of device tree nodes. A device node represents the entire USB device, and one or more child interface nodes represent the individual USB interfaces on the device. For special cases, the device and interface nodes are combined into a single combined node.
Driver binding is achieved by using the compatible name properties. Refer to 22.214.171.124 of the IEEE 1275 USB binding and Writing Device Drivers for more information. A driver can either bind to the entire device and control all the interfaces, or a driver can bind to just one interface, for example, a keyboard or mouse. If no vendor or class driver claims the entire device, a generic USB multi-interface driver is bound to the device-level node. This driver attempts to bind drivers to each interface by using compatible names properties, as defined in section 126.96.36.199 of the 1275 binding.
Figure 1–1 shows an example of a hub and printer as a compound device. Both the hub and the printer are enclosed in the same plastic case, but the hub and the printer have separate USB bus addresses. The same diagram shows an example of a composite device. The composite keyboard and controller are also enclosed in the same plastic case, but they have the same USB bus address. A cable connects the USB mouse to the composite keyboard/controller in this example.
The Solaris USB Architecture (USBA) adheres to the USB 1.0 and 1.1 specification plus Solaris driver requirements. The USBA model is similar to Sun Common SCSI Architecture (SCSA). The USBA is a thin layer that provides a generic USB transport-layer abstraction to the client driver.
The differences between SCSA and USBA are that the SCSA relies on .conf files to probe the bus, while USB hub drivers are self-probing nexus drivers.