Loopback and sourcesink function (for testing)

configname: CONFIG_USB_CONFIGFS_F_LB_SS

Linux Kernel Configuration
└─>Device Drivers
└─>USB support
└─>USB Gadget Support
└─>Loopback and sourcesink function (for testing)
In linux kernel since version 3.14 (release Date: 2014-03-30)  
Loopback function loops back a configurable number of transfers.
Sourcesink function either sinks and sources bulk data.
It also implements control requests, for "chapter 9" conformance.
Make this be the first driver you try using on top of any new
USB peripheral controller driver. Then you can use host-side
test software, like the "usbtest" driver, to put your hardware
and its driver through a basic set of functional tests.