USB (Universal Serial Bus) is a hardware and protocol specification for interconnecting various devices to a host controller. We supply a USB stack that implements the USB protocol and allows user-written class drivers to communicate with USB devices.
We also supply a USB driver library (usbd_*()) for class drivers to use in order to communicate with the USB stack. Note that a class driver can be considered a "client" of the USB stack.
The stack is implemented as a standalone process that registers the pathname of /dev/io-usb/io-usb (by default). Currently, the stack contains the hub class driver within it.