Documentation: usb: LOOPBACK function testing

Summary of how to test LOOPBACK function of USB gadget.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

1 files changed, 23 insertions, 0 deletions

diff --git a/Documentation/usb/gadget-testing.txt b/Documentation/usb/gadget-testing.txt
index f117e5c84a1e..b13491d1e352 100644
--- a/Documentation/usb/gadget-testing.txt
+++ b/Documentation/usb/gadget-testing.txt
@@ -7,6 +7,7 @@ provided by gadgets.
 4. EEM function
 5. FFS function
 6. HID function
+7. LOOPBACK function
 
 
 1. ACM function
@@ -205,3 +206,25 @@ $ ./hid_gadget_test /dev/hidg0 keyboard
 
 Host:
 - observe the keystrokes from the gadget
+
+7. LOOPBACK function
+====================
+
+The function is provided by usb_f_ss_lb.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "Loopback".
+The LOOPBACK function provides these attributes in its function directory:
+
+        qlen            - depth of loopback queue
+        bulk_buflen     - buffer length
+
+Testing the LOOPBACK function
+-----------------------------
+
+device: run the gadget
+host: test-usb
+
+http://www.linux-usb.org/usbtest/testusb.c