CONFIG_USB_F_FS (not configurable)

modulename: usb_f_fs.ko

configname: CONFIG_USB_F_FS

Linux Kernel Configuration
└─>Device Drivers
└─>USB support
└─>USB Gadget Support
└─>USB_F_FS
In linux kernel since version 3.10 (release Date: 2013-06-30)  
source code:
drivers/usb/gadget/function//