Gadget Drivers

In addition to Gadget Zero (used primarily for testing and development with drivers for usb controller hardware), other gadget drivers exist.

There's an ethernet gadget driver, which implements one of the most useful Communications Device Class (CDC) models. One of the standards for cable modem interoperability even specifies the use of this ethernet model as one of two mandatory options. Gadgets using this code look to a USB host as if they're an Ethernet adapter. It provides access to a network where the gadget's CPU is one host, which could easily be bridging, routing, or firewalling access to other networks. Since some hardware can't fully implement the CDC Ethernet requirements, this driver also implements a "good parts only" subset of CDC Ethernet. (That subset doesn't advertise itself as CDC Ethernet, to avoid creating problems.)

There is also support for user mode gadget drivers, using gadgetfs. This provides a User Mode API that presents each endpoint as a single file descriptor. I/O is done using normal read() and read() calls. Familiar tools like GDB and pthreads can be used to develop and debug user mode drivers, so that once a robust controller driver is available many applications for it won't require new kernel mode software.

There's a USB Mass Storage class driver, which provides a different solution for interoperability with systems such as MS-Windows and MacOS. That File-backed Storage driver uses a file or block device as backing store for a drive, like the loop driver. The USB host uses the BBB, CB, or CBI versions of the mass storage class specification, using transparent SCSI commands to access the data from the backing store.

Support for other kinds of gadget is expected to be developed and contributed over time, as this driver framework evolves.