Ok, as I promised here's what I found.
debug was a specific made for serial ports and, as USB is a serial port. it works on ANY usb 2.0 port.
it seems you cannot use this feature in 3.0 ports if your pc has at least one USB 2.0 port (it means not native usb 3.0 controller). It could be related to the need of speed increasing within the 3.0 standard. IIRC, it was difficult to make the usb 3.0 standard retro-compatible with 2.0.
I've also found that "Debugging over a USB 3.0 connection requires the following hardware:
A USB 3.0 debug cable. This is an A-A crossover cable that has only the USB 3.0 lines and no Vbus.
USB 3.0 host controllers on the host and target computers."
(as written here
http://social.msdn.microsoft.com/Forums/en-US/wdk/thread/74a03abb-ef5f-4e2a-8f27-e2282d56a986)
and that windows 7 CANNOT debug over usb 3.0 ports, that feature will be added in windows 8, as written here (
http://msdn.microsoft.com/en-us/library/windows/hardware/ff551063(v=vs.85).aspx):
"New for Windows 8
We have added support for debugging over a network connection or a USB 3.0 connection"
Anyway it depends ALSO on the controller (
www.ti.com/analog/docs/microsite.tsp?sectionId=590&tabId=2276µsiteId=11)
So, let's summarize:
1) you need debug support on your usb 3.0 controller (if your PC has at least one 2.0 port, connect it there. it won't work on 3.0 ones)
2) there are no PCs with only usb 3.0 ports (neither ultrabooks) so where's the problem?
3) if you like windows, you have to wait windows 8 as windows 7 doesn't support debug over usb 3.0 as above (again use 2.0 ports, it's not bad!)
3) if you like linux... don't know! I love Linux but let's talk about it when there'll be only 3.0 ports around
4) if you got the latest macbook air/pro/retina, it's another story (2012 released have only 3.0 ports)
Sorry if it is not a full-detailed explanation but I'm not an electronic engineering. Anyway, it gives us an idea why in described scenarios it does not work.
sources
http://stackoverflow.com/questions/9663719/usb-debugging-on-galaxy-nexus-causes-windows-7-to-freeze
http://pete.akeo.ie/2011/08/do-necrenesas-upd720200-based-usb-30.html
http://www.coreboot.org/EHCI_Debug_Port#Finding_the_USB_debug_port
http://www.google.it/url?sa=t&rct=j...sg=AFQjCNHFwNGEy8a58wmB5xAqhRnKPcB97A&cad=rja
http://channel9.msdn.com/Events/BUILD/BUILD2011/HW-258P