1To understand all the Linux-USB framework, you'll use these resources:
   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.
   9    * The USB 2.0 specification (from, 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".
  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.
  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 team.
  23Here is a list of what each subdirectory here is, and what is contained in
  26core/           - This is for the core USB host code, including the
  27                  usbfs files and the hub class driver ("khubd").
  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.
  33gadget/         - This is for USB peripheral controller drivers and
  34                  the various gadget drivers which talk to them.
  37Individual USB driver directories.  A new driver should be added to the
  38first subdirectory in the list below that it fits into.
  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.