USB is a host/device protocol, organized with one host (such as a
PC) controlling up to 127 peripheral devices.
The USB hardware is asymmetric, which makes it easier to set up:
you can't connect a "to-the-host" connector to a peripheral.

Linux can run in the host, or in the peripheral. In both cases
you need a low level bus controller driver, and some software
talking to it. Peripheral controllers are often discrete silicon,
or are integrated with the CPU in a microcontroller. The more
familiar host side controllers have names like "EHCI", "OHCI",
or "UHCI", and are usually integrated into southbridges on PC
motherboards.

Enable this configuration option if you want to run Linux inside
a USB peripheral device. Configure one hardware driver for your
peripheral/device side bus controller, and a "gadget driver" for
your peripheral protocol. (If you use modular gadget drivers,
you may configure more than one.)

If in doubt, say "N" and don't enable these drivers; most people
don't have this kind of hardware (except maybe inside Linux PDAs).

For more information, see <http://www.linux-usb.org/gadget> and
the kernel documentation for this API.

source code:

drivers/usb/gadget/udc//