Documentation: usb: NCM function testing

Summary of how to test NCM 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

@@ -10,6 +10,7 @@ provided by gadgets.
 7. LOOPBACK function
 8. MASS STORAGE function
 9. MIDI function
+10. NCM function
 
 
 1. ACM function
@@ -366,3 +367,36 @@ $ aconnect 24:0 128:0 # try it on the host
 After the gadget's MIDI port is connected to timidity's MIDI port,
 whatever is played at the gadget side with aplaymidi -l is audible
 in host's speakers/headphones.
+
+10. NCM function
+================
+
+The function is provided by usb_f_ncm.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "ncm".
+The NCM 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/ncm.<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.
+
+Testing the NCM 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>