The first hardware supporting this API was the NetChip 2280
controller, which supports USB 2.0 high speed and is based on PCI.
This is the net2280 driver module.
The driver supports Linux kernel versions 2.4 and 2.6;
contact NetChip Technologies for development boards and product
information.
Other hardware working in the "gadget" framework includes:
Intel's PXA 25x and IXP42x series processors
(pxa2xx_udc),
Toshiba TC86c001 "Goku-S" (goku_udc),
Renesas SH7705/7727 (sh_udc),
MediaQ 11xx (mq11xx_udc),
Hynix HMS30C7202 (h7202_udc),
National 9303/4 (n9604_udc),
Texas Instruments OMAP (omap_udc),
Sharp LH7A40x (lh7a40x_udc),
and more.
Most of those are full speed controllers.
At this writing, there are people at work on drivers in this framework for several other USB device controllers, with plans to make many of them be widely available.
A partial USB simulator,
the dummy_hcd driver, is available.
It can act like a net2280, a pxa25x, or an sa11x0 in terms
of available endpoints and device speeds; and it simulates
control, bulk, and to some extent interrupt transfers.
That lets you develop some parts of a gadget driver on a normal PC,
without any special hardware, and perhaps with the assistance
of tools such as GDB running with User Mode Linux.
At least one person has expressed interest in adapting that
approach, hooking it up to a simulator for a microcontroller.
Such simulators can help debug subsystems where the runtime hardware
is unfriendly to software development, or is not yet available.
Support for other controllers is expected to be developed and contributed over time, as this driver framework evolves.