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Date: Wed, 26 May 2010 14:48:21 +0200
From: Gerd Hoffmann <kraxel@redhat.com>
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Subject: Re: [Qemu-devel] RFC: ehci -> uhci handoff suggestions
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To: Kevin Wolf <kwolf@redhat.com>
Cc: qemu-devel <qemu-devel@nongnu.org>, "David S. Ahern" <daahern@cisco.com>

   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