A system running inside a USB peripheral normally has at least three layers inside the kernel to handle USB protocol processing, and may have additional layers in user space code. The "gadget" API is used by the middle layer to interact with the lowest level (which directly handles hardware).
In Linux, from the bottom up, these layers are:
This is the lowest software level.
It is the only layer that talks to hardware,
through registers, fifos, dma, irqs, and the like.
The <linux/usb/gadget.h> API abstracts
the peripheral controller endpoint hardware.
That hardware is exposed through endpoint objects, which accept
streams of IN/OUT buffers, and through callbacks that interact
with gadget drivers.
Since normal USB devices only have one upstream
port, they only have one of these drivers.
The controller driver can support any number of different
gadget drivers, but only one of them can be used at a time.
Examples of such controller hardware include the PCI-based NetChip 2280 USB 2.0 high speed controller, the SA-11x0 or PXA-25x UDC (found within many PDAs), and a variety of other products.
The lower boundary of this driver implements hardware-neutral USB functions, using calls to the controller driver. Because such hardware varies widely in capabilities and restrictions, and is used in embedded environments where space is at a premium, the gadget driver is often configured at compile time to work with endpoints supported by one particular controller. Gadget drivers may be portable to several different controllers, using conditional compilation. (Recent kernels substantially simplify the work involved in supporting new hardware, by autoconfiguring endpoints automatically for many bulk-oriented drivers.) Gadget driver responsibilities include:
handling setup requests (ep0 protocol responses) possibly including class-specific functionality
returning configuration and string descriptors
(re)setting configurations and interface altsettings, including enabling and configuring endpoints
handling life cycle events, such as managing bindings to hardware, USB suspend/resume, remote wakeup, and disconnection from the USB host.
managing IN and OUT transfers on all currently enabled endpoints
Such drivers may be modules of proprietary code, although that approach is discouraged in the Linux community.
Most gadget drivers have an upper boundary that connects to some Linux driver or framework in Linux. Through that boundary flows the data which the gadget driver produces and/or consumes through protocol transfers over USB. Examples include:
user mode code, using generic (gadgetfs)
or application specific files in
/dev
networking subsystem (for network gadgets, like the CDC Ethernet Model gadget driver)
data capture drivers, perhaps video4Linux or a scanner driver; or test and measurement hardware.
input subsystem (for HID gadgets)
sound subsystem (for audio gadgets)
file system (for PTP gadgets)
block i/o subsystem (for usb-storage gadgets)
... and more
Other layers may exist. These could include kernel layers, such as network protocol stacks, as well as user mode applications building on standard POSIX system call APIs such as open(), close(), read() and write(). On newer systems, POSIX Async I/O calls may be an option. Such user mode code will not necessarily be subject to the GNU General Public License (GPL).
OTG-capable systems will also need to include a standard Linux-USB host side stack, with usbcore, one or more Host Controller Drivers (HCDs), USB Device Drivers to support the OTG "Targeted Peripheral List", and so forth. There will also be an OTG Controller Driver, which is visible to gadget and device driver developers only indirectly. That helps the host and device side USB controllers implement the two new OTG protocols (HNP and SRP). Roles switch (host to peripheral, or vice versa) using HNP during USB suspend processing, and SRP can be viewed as a more battery-friendly kind of device wakeup protocol.
Over time, reusable utilities are evolving to help make some gadget driver tasks simpler. For example, building configuration descriptors from vectors of descriptors for the configurations interfaces and endpoints is now automated, and many drivers now use autoconfiguration to choose hardware endpoints and initialize their descriptors. A potential example of particular interest is code implementing standard USB-IF protocols for HID, networking, storage, or audio classes. Some developers are interested in KDB or KGDB hooks, to let target hardware be remotely debugged. Most such USB protocol code doesn't need to be hardware-specific, any more than network protocols like X11, HTTP, or NFS are. Such gadget-side interface drivers should eventually be combined, to implement composite devices.