linux-kernel - Re: [PATCH 0/6] twl4030: Various fixes for charing-from-USB

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Date:	Thu, 10 May 2012 10:45:50 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	NeilBrown <neilb@...e.de>
cc:	Peter Chen <peter.chen@...escale.com>,
	Samuel Ortiz <sameo@...ux.intel.com>,
	Felipe Balbi <balbi@...com>,
	Grazvydas Ignotas <notasas@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	<linux-kernel@...r.kernel.org>, <linux-usb@...r.kernel.org>,
	<linux-omap@...r.kernel.org>
Subject: Re: [PATCH 0/6] twl4030: Various fixes for charing-from-USB

On Thu, 10 May 2012, NeilBrown wrote:

>  Maybe we need to not suspend the USB OTG interface when the device as a
>  whole enters suspend-to-RAM.  Maybe we need to register a dummy gadget to
>  the bus active while in suspend?
>  Does the gadget have to be responsive while-ever the bus is not suspended?

Yes.

>  Maybe we can arrange for any USB activity to trigger a resume?

Probably not, at least, not fast enough.  The gadget is required to 
respond to USB packets within a few microseconds.  At a minimum, the 
USB device controller would have to remain fully powered.

>  Entering suspend while still charging really should be possible - at least
>  with a dump charger and hopefully from a host port as well.  I'll like to
>  find the best way to achieve that, and any help you can provide would be
>  greatly appreciated.

If the gadget is attached to a dumb charger rather than a host then 
there's no problem.  But otherwise it's not so simple.

Have you read the USB charger class specification?  (I haven't...)

Alan Stern

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