On Tue, 2016-04-19 at 17:44 +0800, Peter Chen wrote:
> Hi all,
>
> When I review the patch [1] for adding companion controller support for
> OTG framework, I am puzzled several concepts, like shared hcd and
> companion controller, companion hcd, companion port, would someone
> please explain them? And why EHCI/OHCI do not use shared hcd, but
> xHCI uses it? Thanks.
>
> [1] http://www.spinics.net/lists/linux-usb/msg139344.html
Your question is a bit unspecific. A book on fundamentals
might be helpful.
In short, some HCDs are limited in the speeds they can operate at.
Obviously they cannot operate at the higher speeds of younger busses,
but EHCI can operate only at high speed.
If the port is required to operate at lower speed, it is switched
over to a companion controller.
XHCI can operate at all speeds, to allow for virtualization, but
in software we pretend that there is a second bus for slower than
SuperSpeed.
HTH
Oliver
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