linux-kernel - Re: Non-enumerable devices on USB and other enumerable buses

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Message-ID: <20130814173028.GB2401@sirena.org.uk>
Date:	Wed, 14 Aug 2013 18:30:28 +0100
From:	Mark Brown <broonie@...nel.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Rob Herring <rob.herring@...xeda.com>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Stephen Warren <swarren@...dotorg.org>,
	Ian Campbell <ian.campbell@...rix.com>,
	Felipe Balbi <balbi@...com>,
	Grant Likely <grant.likely@...aro.org>,
	devicetree@...r.kernel.org, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Non-enumerable devices on USB and other enumerable buses

On Wed, Aug 14, 2013 at 12:14:06PM -0400, Alan Stern wrote:
> On Wed, 14 Aug 2013, Mark Brown wrote:

> > Yes, so you'd want callbacks when the device actually appears and
> > disappears.

> No, no -- this is exactly the point I was trying to make.  The on-board
> hub _won't_ appear on the USB bus until the GPIOs are set.  Therefore
> the callback to set the GPIOs needs to be at a different place, not
> when the device appears.

What I'm proposing is that we have a way of telling buses that devices
exist via a mechanism other than their actually being visible on the bus
at the current time.  If you're doing that the driver can be running
prior to the hardware being there, just like it does with all the
non-enumerable buses.

> > > Perhaps the platform-level code would need to hook into the places
> > > where the discoverable bus is registered and unregistered.

> > We'd need some way to get information to the drivers still, and to
> > handle drivers that want to stop and start things.

> Um...  What exactly does this mean?  It's so generic, it could apply to
> anything under drivers/.  After all, what driver doesn't want to stop
> and start things?

It means that there are situations where a driver wants to take the
hardware offline but still offer services to userspace which will
require the hardware to be brought on line again.  The fact that this is
a very generic thing is exactly why I'm raising this as a generic issue.

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