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Date:	Wed, 18 Mar 2015 19:38:07 -0700
From:	Tony Lindgren <>
To:	Alan Stern <>
Cc:	Roger Quadros <>,,,,,,,,,
Subject: Re: [RFC][PATCH 1/9] usb: hcd: Introduce usb_start/stop_hcd()

* Alan Stern <> [150318 18:51]:
> On Wed, 18 Mar 2015, Tony Lindgren wrote:
> > > 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?
> This patch deals with the host side, not the device side.  The fact
> that the device is suspended is not relevant to the issues above.

OK, got it.
> Besides, the problems I outlined are more connected with the way 
> Linux's host-side USB stack is organized, and not so much with the 
> details of the OTG spec.

Yes thanks for the explanation.


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