usbip: tools: add options and examples in man page related to device mode

The commit e0546fd8b748 ("usbip: tools: Start using VUDC backend in
usbip tools") implemented device mode for user space tools, however the
corresponding options are not documented in man page.

This commit documents the options and provides examples on device mode.
Also the command `usbip port` is documented.

Acked-by: Shuah Khan <[e-mail]>
Signed-off-by: Hongren Zheng <[e-mail]>
Link: https://lore.kernel.org/r/YFrdyKKx1nx8bktm@Sun
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

tools/usb/usbip/doc/usbip.8

···
 .PP
 
 .HP
+\fBattach\fR \-\-remote=<\fIhost\fR> \-\-device=<\fIdev_id\fR>
+.IP
+Attach a remote USB gadget.
+Only used when the remote usbipd is in device mode.
+.PP
+
+.HP
 \fBdetach\fR \-\-port=<\fIport\fR>
 .IP
-Detach an imported USB device.
+Detach an imported USB device/gadget.
 .PP
 
 .HP
···
 .PP
 
 .HP
+\fBlist\fR \-\-device
+.IP
+List USB gadgets of local usbip-vudc.
+Only used when the local usbipd is in device mode.
+Note that this can not list usbip-vudc USB gadgets of the remote device mode usbipd.
+.PP
+
+.HP
 \fBlist\fR \-\-local
 .IP
 List local USB devices.
+.PP
+
+.HP
+\fBport\fR
+.IP
+List imported devices/gadgets.
 .PP
 
 
···
     client:# usbip attach --remote=server --busid=1-2
         - Connect the remote USB device.
 
+    client:# usbip port
+        - List imported devices/gadgets.
+
     client:# usbip detach --port=0
         - Detach the usb device.
+
+The following example shows the usage of device mode
+
+    server:# usbip list --device
+        - List gadgets exported by local usbipd server.
+
+    client:# modprobe vhci-hcd
+
+    client:# usbip attach --remote=server --device=usbip-vudc.0
+        - Connect the remote USB gadget.
+
+    client:# usbip port
+        - List imported devices/gadgets.
+
+    client:# usbip detach --port=0
+        - Detach the usb gadget.
 
 .SH "SEE ALSO"
 \fBusbipd\fP\fB(8)\fB\fP

tools/usb/usbip/doc/usbipd.8

···
 .PP
 
 .HP
+\fB\-e\fR, \fB\-\-device\fR
+.IP
+Run in device mode. Rather than drive an attached device, create a virtual UDC to bind gadgets to.
+.PP
+
+.HP
 \fB\-D\fR, \fB\-\-daemon\fR
 .IP
 Run as a daemon process.
···
         - Bind usbip-host.ko to the device of busid 1-2.
         - A usb device 1-2 is now exportable to other hosts!
         - Use 'usbip unbind --busid=1-2' when you want to shutdown exporting and use the device locally.
+
+The following example shows the usage of device mode
+
+    server:# modprobe usbip-vudc
+        - Use /sys/class/udc/ interface.
+        - usbip-host is independent of this module.
+
+    server:# usbipd -e -D
+        - Start usbip daemon in device mode.
+
+    server:# modprobe g_mass_storage file=/tmp/tmp.img
+        - Bind a gadget to usbip-vudc.
+        - in this example, a mass storage gadget is bound.
+
+    server:# usbip list --device
+        - List gadgets exported by local usbipd server.
+
+    server:# modprobe -r g_mass_storage
+        - Unbind a gadget from usbip-vudc.
+        - in this example, the previous mass storage gadget is unbound.
 
 .SH "SEE ALSO"
 \fBusbip\fP\fB(8)\fB\fP