Earlier quoted context omitted.
It's backwards compatible on the device side (you can plug a 3.0 device into a 2.0 port and work at lower speed). But I really doubt that you can use an xHCI controller through EHCI drivers.
> But I really doubt that you can use an xHCI controller through EHCI drivers. Indeed, you can't. xHCI subsumes all USB operation and doesn't need companion controllers, but presents a _very_ different interface. That's different from the USB1/2 transition where USB2 was a separate controller that could take over the ports from a USB1 controller, giving them back if a USB1 device is attached. Some boards/chipsets do…
I'm simultaneously disappointed that this is not (as far as I can tell) widely known information. It sounds like the thing that may prevent you from going on goose chase if you ever need to debug it.