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

[Johannes Stezenbach <js@sig21.net>]

On Wed, May 26, 2010 at 08:00:33AM -0600, David S. Ahern wrote:
> On 05/26/2010 07:23 AM, Kevin Wolf wrote:
> > Am 26.05.2010 15:06, schrieb David S. Ahern:
> >>
> >> 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
> > 
> > The routing may happen internally, but the OHCI/UHCI appears just like a
> > normal controller to the OS. You can't access the devices on a companion
> > with your EHCI driver.
...
> > Any transition between High Speed (directly handled by EHCI) and
> > Low/Full Speed (OHCI/UHCI companion controller) must not happen
> > automagically, but be requested by the guest OS. And you probably don't
> > want to re-implement UHCI or OHCI inside the EHCI emulation, so you
> > can't keep things inside the EHCI device model.
> 
> I'm still confused by the guest OS interaction -- more code/spec reading
> I guess.
> 
> Key points are that lspci in the VM shows both buses, and the qemu
> monitor would still scan both buses and show devices. And definitely no
> code duplication - some kind of movement to current uhci/ohci ports is
> what I am after.

My understanding of the EHCI spec: It's best to see the
port router as a seperate device.  USB devices are not
connected directly to EHCI or [UO]HCI, they are connected
to the port router.  If the port routing is changed
one of the HCs will see a connect event, the other one
a disconnect event, i.e. the handling is as if you
had unplugged the cable from one HC and plugged in into
the other HC.

By default the bus is connected to [OU]HCI, but when the
OS loads an EHCI driver it will set the Configured Flag to change
the default routing to EHCI.  If the EHCI driver determines
a device is not high speed, it will change the routing for
that port back to [UO]HCI.  The port router will see a physical
unplug event so the EHCI driver can change the routing back to
EHCI on unplug.

I think a typical EHCI controller has more ports than a UHCI
or OHCI controller, this is why you see more companion controllers
in lspci.  I think this doesn't matter, the key point is that
there is a fixed connection in hw for each port to the port router.


Johannes