RE: [RFC PATCH 00/15] rework port power control

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> > I have a USB 3.0 hub (not with me right now), which has either three or
> > four USB 3.0 ports exposed, and seven USB 2.0 ports.  The OS sees it as
> > one USB 3.0 hub and two USB 2.0 hubs chained together.  My guess is that
> > the manufacturer wanted to say they had a "seven port USB 3.0 hub", but
> > only four-port USB 3.0 hub chips were available, and they didn't have
> > room in the hardware design or didn't want to pay for the second USB 3.0
> > hub chip.
> >
> > There's no real way to tell how port power is routed across the ports
> > without doing some physical layout tests.  I could see how they could
> > meet the requirement in section 10.3.3 by pairing the USB 3.0 ports with
> > either USB 2.0 hub.
> >
> > If they wanted to attach to the parent hub, they could physically expose
> > three USB 3.0 ports, leave one port unexposed and not connected to any
> > physical wires, and say the USB 3.0 hub has four ports.  That ensures
> > compatibility with section 10.3.3 while still allowing the USB 2.0 port
> > to be used by the second-tier hub without exposing a USB 3.0-only port.
> >
> > However, that's technically violating the spirit of section, 10.3.3, and
> > it's probably easier to pair the USB 3.0 ports with the child USB 2.0
> > hub.  That would also allow them to expose all four USB 3.0 ports.
> 
> In fact, it seems like this hub design is in actual violation of
> Section 3.2.6.2 (not just the spirit):
> 
> 	A USB 3.0 hub connects upstream as two devices; a SuperSpeed
> 	hub on the SuperSpeed bus and a USB 2.0 hub on the USB 2.0 bus.
> 
> Your hub connects as _two_ USB 2.0 hubs on the USB 2.0 bus, thus
> appearing upstream as _three_ devices.

The diagram in section D.1 of the xhci spec seems to imply that there
can be additional USB2 hubs between the root hub and a USB3 hub.
So if the USB3 ports are connected to the second tier USB2 port
it might be conformant.

	David



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