Documentation: usb: RNDIS function testing

Summary of how to test RNDIS function of USB gadget.
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

Documentation/usb/gadget-testing.txt

@@ -13,6 +13,7 @@ provided by gadgets.
 10. NCM function
 11. OBEX function
 12. PHONET function
+13. RNDIS function
 
 
 1. ACM function
@@ -493,3 +494,38 @@ $ ./pnxmit -a 0x10 -r
 On the device:
 
 $ ./pnxmit -a 0x6c -s 0x10
+
+13. RNDIS function
+==================
+
+The function is provided by usb_f_rndis.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "rndis".
+The RNDIS function provides these attributes in its function directory:
+
+	ifname		- network device interface name associated with this
+			function instance
+	qmult		- queue length multiplier for high and super speed
+	host_addr	- MAC address of host's end of this
+			Ethernet over USB link
+	dev_addr	- MAC address of device's end of this
+			Ethernet over USB link
+
+and after creating the functions/rndis.<instance name> they contain default
+values: qmult is 5, dev_addr and host_addr are randomly selected.
+Except for ifname they can be written to until the function is linked to a
+configuration. The ifname is read-only and contains the name of the interface
+which was assigned by the net core, e. g. usb0.
+
+By default there can be only 1 RNDIS interface in the system.
+
+Testing the RNDIS function
+--------------------------
+
+Configure IP addresses of the device and the host. Then:
+
+On the device: ping <host's IP>
+On the host: ping <device's IP>