* Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> [150318 12:50]:
> On Wed, 18 Mar 2015, Roger Quadros wrote:
>
> > To support OTG we want a mechanism to start and stop
> > the HCD from the OTG state machine. Add usb_start_hcd()
> > and usb_stop_hcd().
> >
> > Signed-off-by: Roger Quadros <rogerq@xxxxxx>
>
> There are a few problems in this proposed patch.
>
> > +int usb_start_hcd(struct usb_hcd *hcd)
> > +{
> > + int retval;
> > + struct usb_device *rhdev = hcd->self.root_hub;
> > +
> > + if (hcd->state != HC_STATE_HALT) {
> > + dev_err(hcd->self.controller, "not starting a running HCD\n");
> > + return -EINVAL;
> > + }
> > +
> > + hcd->state = HC_STATE_RUNNING;
> > + retval = hcd->driver->start(hcd);
> > + if (retval < 0) {
> > + dev_err(hcd->self.controller, "startup error %d\n", retval);
> > + hcd->state = HC_STATE_HALT;
> > + return retval;
> > + }
> > +
> > + /* starting here, usbcore will pay attention to this root hub */
> > + if ((retval = register_root_hub(hcd)) != 0)
> > + goto err_register_root_hub;
>
> If the host controller is started more than once, you will end up
> unregistering and re-registering the root hub. The device core does
> not allow this. Once a device has been unregistered, you must not try
> to register it again -- you have to allocate a new device and register
> it instead.
>
> Also, although you call the driver's ->start method multiple times, the
> ->reset method is called only once, when the controller is first
> probed. It's not clear that this will work in a situation where the HC
> and the UDC share hardware state; after the UDC is stopped it may be
> necessary to reset the HC before it can run again.
>
> It might be possible to make this work, but I suspect quite a few
> drivers would need rewriting first. As another example of the problems
> you face, consider how stopping a host controller will interact with
> the driver's PM support (both system suspend and runtime suspend).
>
> It would be a lot simpler to unbind the host controller driver
> completely when switching to device mode and rebind it when switching
> back. I guess that is the sort of heavy-duty approach you want to
> avoid, but it may be the only practical way forward.
Hmm from memory I think the OTG spec assumes the USB devices are
suspended when attempting the role change? I could be totally wrong,
it's been a really long time since I've looked at the OTG spec, but
maybe that would make it easier to deal with thing?
Regards,
Tony
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