The RISC OS USB stack is derived from the NetBSD USB stack, customised to work within a module, and the RISC OS execution model. USBDriver contains the higher level aspects of the USB stack and talks down to host controller (HC) drivers which sit between it and the HC hardware registers.
Central to this arrangement are 5 header files exported by USBDriver for inclusion in HC drivers :
Header Contents Required by HC?
usb.h Structure definitions relating to the USB standard Required by usbdivar.h
usbdi.h Enumerated error numbers Required by usbdivar.h
usbdivar.h Structures to communicate with USBDriver Essential
usb_port.h Macros for porting from NetBSD to RISC OS No
usbdevs.h Vendor/device ID database No
The most important is usbdivar.h which contains definitions of a usbd_bus, usbd_device, usbd_interface, usbd_pipe, and usbd_xfer all of which must agree in their layout to the matching definitions that USBDriver uses. The value of RISCOS_USBDRIVER_API_VERSION is defined to denote the current usbdivar.h, and this value is changed whenever incompatible changes are made to the header. The HC can check the value it assumed against the one USBDriver assumes.
The usb_port.h header contains commonly used macros which may be helpful in porting source code written with the NetBSD stack in mind; your HC driver may choose to do this differently. The usbdevs.h database is a big table of all the known vendors and their devices; it is only likely to be useful to a HC driver if it needs to check for non compliant devices somehow or performs a dual host & device role.
For speed the interface between USBDriver and HC drivers use direct function pointers (referred to as "methods" in the above header files). However, because the two drivers reside in separate modules the function pointers will need to point to a small CMHG-like veneer to swap the value of R12 before continuing to the actual function.
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