1To understand all the Linux-USB framework, you'll use these resources:
2
3    * This source code.  This is necessarily an evolving work, and
4      includes kerneldoc that should help you get a current overview.
5      ("make pdfdocs", and then look at "usb.pdf" for host side and
6      "gadget.pdf" for peripheral side.)  Also, Documentation/usb has
7      more information.
8
9    * The USB 2.0 specification (from www.usb.org), with supplements
10      such as those for USB OTG and the various device classes.
11      The USB specification has a good overview chapter, and USB
12      peripherals conform to the widely known "Chapter 9".
13
14    * Chip specifications for USB controllers.  Examples include
15      host controllers (on PCs, servers, and more); peripheral
16      controllers (in devices with Linux firmware, like printers or
17      cell phones); and hard-wired peripherals like Ethernet adapters.
18
19    * Specifications for other protocols implemented by USB peripheral
20      functions.  Some are vendor-specific; others are vendor-neutral
21      but just standardized outside of the www.usb.org team.
22
23Here is a list of what each subdirectory here is, and what is contained in
24them.
25
26core/		- This is for the core USB host code, including the
27		  usbfs files and the hub class driver ("hub_wq").
28
29host/		- This is for USB host controller drivers.  This
30		  includes UHCI, OHCI, EHCI, and others that might
31		  be used with more specialized "embedded" systems.
32
33gadget/		- This is for USB peripheral controller drivers and
34		  the various gadget drivers which talk to them.
35
36
37Individual USB driver directories.  A new driver should be added to the
38first subdirectory in the list below that it fits into.
39
40image/		- This is for still image drivers, like scanners or
41		  digital cameras.
42../input/	- This is for any driver that uses the input subsystem,
43		  like keyboard, mice, touchscreens, tablets, etc.
44../media/	- This is for multimedia drivers, like video cameras,
45		  radios, and any other drivers that talk to the v4l
46		  subsystem.
47../net/		- This is for network drivers.
48serial/		- This is for USB to serial drivers.
49storage/	- This is for USB mass-storage drivers.
50class/		- This is for all USB device drivers that do not fit
51		  into any of the above categories, and work for a range
52		  of USB Class specified devices. 
53misc/		- This is for all USB device drivers that do not fit
54		  into any of the above categories.
55