linux-kernel - Re: [RFC PATCH v3] drivercore: Add driver probe deferral mechanism

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Date:	Mon, 26 Sep 2011 16:12:10 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
Cc:	Grant Likely <grant.likely@...retlab.ca>,
	Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <greg@...ah.com>,
	linux-kernel@...r.kernel.org,
	Manjunath GKondaiah <manjunath.gkondaiah@...aro.org>,
	Dilan Lee <dilee@...dia.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH v3] drivercore: Add driver probe deferral mechanism

On Mon, Sep 26, 2011 at 03:16:43PM +0100, Mark Brown wrote:
> On Thu, Sep 22, 2011 at 12:51:23PM -0600, Grant Likely wrote:
> > Allow drivers to report at probe time that they cannot get all the resources
> > required by the device, and should be retried at a later time.
> 
> > This should completely solve the problem of getting devices
> > initialized in the right order.  Right now this is mostly handled by
> > mucking about with initcall ordering which is a complete hack, and
> > doesn't even remotely handle the case where device drivers are in
> > modules.  This approach completely sidesteps the issues by allowing
> > driver registration to occur in any order, and any driver can request
> > to be retried after a few more other drivers get probed.
> 
> So, one issue I did think of the other day while putting some support in
> the regulator core for using this: what happens with devices which can
> optionally use a resource but don't rely on it?  One example here is
> that a lot of the MMC drivers have an optional regulator to control some
> of the supplies for the cards.  If the reglator isn't there it won't be
> used but it's not a blocker for anything.  Devices doing this would need
> some way to figure out if they should -EBUSY or fail otherwise.

Just to avoid confusion - ITYM -EAGAIN there.  -EBUSY is already used
by drivers to mean "someone else claimed a resource I need" be it the
IO region or an IRQ resource...
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