Re: [Qemu-devel] RFC: ehci -> uhci handoff suggestions

["David S. Ahern" <daahern@cisco.com>]

On 05/26/2010 06:48 AM, Gerd Hoffmann wrote:
> 
>   Hi,
> 
>>> USB devices can support both 1.1 and 2.0, right?  Who decides which
>>> protocol is used then?  I think the OS can speak 1.1 to the device even
>>> in case a ehci controller is present (but unused by the OS), right?
>>
>> AFAIK the OS must tell the EHCI that it should hand the device off to
>> the UHCI/OHCI companion before it can use it there.
> 
> Huh?  Compatibility-wise it makes sense to do it the other way around
> (i.e. have it @ UHCI/OHCI by default and move to EHCI on request), so a
> OS which knows nothing about EHCI can cope.
> 
>> If they should be accessed via the EHCI or a companion controller
>> depends on what the OS requests. And USB 2.0 says that any device that
>> supports High Speed must also support Full Speed and therefore be
>> accessible using the companion (at least that's what I understand).
> 
> Hmm, ok, so no shortcut even for emulated devices.  Not that it would
> have helped much as we have to cover host devices anyway.
> 
> Also I think one ehci controller can have multiple uhci companion
> controllers.  At least lspci on my T60 suggests that:
> 
> 00:1d.0 USB Controller: Intel Corporation 82801G (ICH7 Family) USB UHCI
> Controller #1 (rev 02)
> 00:1d.1 USB Controller: Intel Corporation 82801G (ICH7 Family) USB UHCI
> Controller #2 (rev 02)
> 00:1d.2 USB Controller: Intel Corporation 82801G (ICH7 Family) USB UHCI
> Controller #3 (rev 02)
> 00:1d.3 USB Controller: Intel Corporation 82801G (ICH7 Family) USB UHCI
> Controller #4 (rev 02)
> 00:1d.7 USB Controller: Intel Corporation 82801G (ICH7 Family) USB2 EHCI
> Controller (rev 02)
> 
> cheers,
>   Gerd
> 

Yes, that is the ehci feature to be implemented.

My understanding is that the port routing happens internally to the host
controller based on device speed - section 4.2 (pag 64) of:
http://www.intel.com/technology/usb/download/ehci-r10.pdf

ehci does have more overhead from an emulation perspective, so it would
be best to keep mice, keyboard, serial ports, etc on the uhci/ohci bus
and have storage devices and webcams and such on ehci. And any
transition should happen automagically within the device model.

David