linux-kernel - Re: [RFC][PATCH 1/9] usb: hcd: Introduce usb_start/stop

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Date:	Thu, 19 Mar 2015 10:17:46 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Roger Quadros <rogerq@...com>
cc:	gregkh@...uxfoundation.org, <balbi@...com>,
	<dan.j.williams@...el.com>, <peter.chen@...escale.com>,
	<jun.li@...escale.com>, <mathias.nyman@...ux.intel.com>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-omap@...r.kernel.org>
Subject: Re: [RFC][PATCH 1/9] usb: hcd: Introduce usb_start/stop_hcd()

On Thu, 19 Mar 2015, Roger Quadros wrote:

> > 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.
> 
> So you mean directly calling usb_add/remove_hcd() from the OTG core?

Well, actually I meant calling device_release_driver() and
driver_probe_device().  But usb_add/remove_hcd() might work just about
as well, although I think some of the HCDs may still require a little
rewriting.  Peter Chen says it works for Chipidea.

> I don't see any issues with that other than it being a heavy-duty operation
> like you said and hope that it doesn't violate the OTG spec timing.

The timing is a tough issue.  I'm not sure what we can do about it, 
though.

> Looking at Figure 5-3: "HNP Sequence of Events (FS)" of the OTG 2.0 spec
> we have about 150ms (X10) to switch from B-Device detected A connect (b_wait_acon state)
> to driving bus reset (b_host state). I don't think this should be an issue in modern SoCs
> but I'm not very sure.
> 
> In any case I can migrate to the add/remove hcd approach to simplify things.

Good luck.

Alan Stern

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