A small roadmap of the USB files:

FILES			this file
Makefile		to install .h files
Makefile.usbdevs	to run devlist2h.awk
TODO			just a list of things to do
devlist2h.awk		script to generate usbdevs*.h
ezload.c		EZ-USB firmware download subroutines
ezload.h		API for ezload.c
files.usb		config include file
hid.c			subroutines to parse and access HID data
hid.h			API for hid.c
if_aue.c		USB Pegasus Ethernet driver
if_auereg.h		  and definitions for it
if_axe.c                USB ASIX Electronics Ethernet driver
if_axereg.h               and definitions for it
if_cue.c		USB CATC Ethernet driver
if_cuereg.h		  and definitions for it
if_kue.c		USB Kawasaki Ethernet driver
if_kuereg.h		  and definitions for it
if_udav.c		USB Davicom Ethernet driver
if_udavreg.h		  and definitions for it
if_upl.c		USB Prolofic host-to-host driver
kue_fw.h		Kawasaki Ethernet firmware
ohci.c			Host controller driver for OHCI
ohcireg.h		Hardware definitions for OHCI
ohcivar.h		API for ohci.c
uaudio.c		USB audio class driver
uaudioreg.h		  and definitions for it
ucom.c			TTY emulation for two bulk pipes
ucomvar.h		TTY emulation interface
uftdi.c			USB FTDI serial driver
uftdireg.h		  and definitions for it
ugen.c			generic driver that can handle access to any USB device
uhci.c			Host controller driver for UHCI
uhcireg.h		Hardware definitions for UHCI
uhcivar.h		API for uhci.c
uhid.c			USB generic HID driver
uhidev.c		USB HID class driver
uhidev.h		  and definitions for it
uhub.c			USB hub driver
ukbd.c			USB keyboard driver
ukbdmap.c		wscons key mapping for ukbd
ukbdvar.h		API for ukbd.c
ulpt.c			USB printer class driver
umass.c			USB mass storage wire protocol driver
umass_isdata.c		In-System Design ATA over bulk-only driver
umass_isdata.h		  and definitions for it
umass_quirks.c		Table of strange umass devices
umass_quirks.h		  and definitions for it
umass_scsi.c		umass command protocol driver
umass_scsi.h		  and definitions for it
umassvar.h		definitions for umass.c
umidi.c			USB MIDI driver
umidi_quirks.c		Strange MIDI devices
umidi_quirks.h		  and definitions for it
umidireg.h		Protocol definitions for umidi.c
umidivar.h		definitions for umidi.c
umodem.c		USB modem (CDC ACM) driver
ums.c			USB mouse driver
urio.c			USB Diamond Rio500 driver
urio.h			USB Diamond Rio500 defines
usb.c			usb (bus) device driver
usb.h			general USB defines
usb_mem.c		memory allocation for DMAable memory
usb_mem.h		API for usb_mem.c
usb_port.h		compatibility defines for different OSs
usb_quirks.c		table of non-conforming USB devices and their problems
usb_quirks.h		API for usb_quirks.c
usb_subr.c		various subroutines used by USB code
usbcdc.h		USB CDC class definitions
usbdevs			data base of known device
usbdevs.h		generated from usbdevs
usbdevs_data.h		generated from usbdevs
usbdi.c			implementation of the USBDI API, which all drivers use
usbdi.h			API for usbdi.c
usbdi_util.c		utilities built on top of usbdi.h
usbdi_util.h		API for usbdi_util.c
usbdivar.h		internal defines and structures for usbdi.c
uscanner.c		minimal USB scanner driver
usscanner.c		driver for some SCSI-over-USB scanners
usbhid.h		USB HID class definitions
uvisor.c		USB Handsping Visor driver
uyap.c			Initial firmware downloader for Y@P phones
uyap_firmware.h		Firmware for the Y@P phone