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-rw-r--r--Documentation/usb/gadget-testing.txt23
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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