The Function Filesystem (FunctioFS) lets one create USB
composite functions in user space in the same way as GadgetFS
lets one create USB gadgets in user space. This allows creation
of composite gadgets such that some of the functions are
implemented in kernel space (for instance Ethernet, serial or
mass storage) and other are implemented in user space.

Say "y" to link the driver statically, or "m" to build
a dynamically linked module called "g_ffs".

source code:

drivers/usb/gadget/g_ffs.c