usb: documentation update

Add some more informations to docs/usb2.txt about using usb2 (also usb1)
devices.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

docs/usb2.txt

@@ -31,6 +31,91 @@ a complete example:
 This attaches a usb tablet to the UHCI adapter and a usb mass storage
 device to the EHCI adapter.
 
+
+More USB tips & tricks
+======================
+
+Recently the usb pass through driver (also known as usb-host) and the
+qemu usb subsystem gained a few capabilities which are available only
+via qdev properties, i,e. when using '-device'.
+
+
+physical port addressing
+------------------------
+
+First you can (for all usb devices) specify the physical port where
+the device will show up in the guest.  This can be done using the
+"port" property.  UHCI has two root ports (1,2).  EHCI has four root
+ports (1-4), the emulated (1.1) USB hub has eight ports.
+
+Plugging a tablet into UHCI port 1 works like this:
+
+        -device usb-tablet,bus=usb.0,port=1
+
+Plugging a hub into UHCI port 2 works like this:
+
+        -device usb-hub,bus=usb.0,port=2
+
+Plugging a virtual usb stick into port 4 of the hub just plugged works
+this way:
+
+        -device usb-storage,bus=usb.0,port=2.4,drive=...
+
+You can do basically the same in the monitor using the device_add
+command.  If you want to unplug devices too you should specify some
+unique id which you can use to refer to the device ...
+
+        (qemu) device_add usb-tablet,bus=usb.0,port=1,id=my-tablet
+        (qemu) device_del my-tablet
+
+... when unplugging it with device_del.
+
+
+USB pass through hints
+----------------------
+
+The usb-host driver has a bunch of properties to specify the device
+which should be passed to the guest:
+
+  hostbus=<nr> -- Specifies the bus number the device must be attached
+  to.
+
+  hostaddr=<nr> -- Specifies the device address the device got
+  assigned by the guest os.
+
+  hostport=<str> -- Specifies the physical port the device is attached
+  to.
+
+  vendorid=<hexnr> -- Specifies the vendor ID of the device.
+  productid=<hexnr> -- Specifies the product ID of the device.
+
+In theory you can combine all these properties as you like.  In
+practice only a few combinations are useful:
+
+  (1) vendorid+productid -- match for a specific device, pass it to
+      the guest when it shows up somewhere in the host.
+
+  (2) hostbus+hostport -- match for a specific physical port in the
+      host, any device which is plugged in there gets passed to the
+      guest.
+
+  (3) hostbus+hostaddr -- most useful for ad-hoc pass through as the
+      hostaddr isn't stable, the next time you plug in the device it
+      gets a new one ...
+
+Note that USB 1.1 devices are handled by UHCI/OHCI and USB 2.0 by
+EHCI.  That means a device plugged into the very same physical port
+may show up on different busses depending on the speed.  The port I'm
+using for testing is bus 1 + port 1 for 2.0 devices and bus 3 + port 1
+for 1.1 devices.  Passing through any device plugged into that port
+and also assign them to the correct bus can be done this way:
+
+    qemu -M pc ${otheroptions}                           \
+        -usb                                             \
+        -device usb-ehci,id=ehci                         \
+        -device usb-host,bus=usb.0,hostbus=3,hostport=1  \
+        -device usb-host,bus=ehci.0,hostbus=1,hostport=1
+
 enjoy,
   Gerd