linux-kernel - Re: [RFC][PATCH 1/3] PM: Introduce new top level suspend and hibernation callbacks

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Date:	Thu, 20 Mar 2008 10:45:38 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
cc:	"Rafael J. Wysocki" <rjw@...k.pl>, Pavel Machek <pavel@....cz>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Greg KH <greg@...ah.com>, Len Brown <lenb@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexey Starikovskiy <astarikovskiy@...e.de>,
	David Brownell <david-b@...bell.net>
Subject: Re: [RFC][PATCH 1/3] PM: Introduce new top level suspend and
 hibernation callbacks

On Thu, 20 Mar 2008, Benjamin Herrenschmidt wrote:

> > An important point I tried to make in the earlier email is that drivers
> > will want a simple way to know when it is illegal for them to register
> > new children.  For example, suppose the registration is done by a
> > workqueue routine.  The most reliable way for the driver to insure that
> > the routine won't try to register new children improperly is to have
> > the routine check a flag which gets set _before_ prepare() is called.
> 
> I don't totally agree here. Drivers could have their own flag set
> internally with appropriate locking. The problem with your approach is
> locking. Just setting a flag is mostly useless, -unless- there
> appropriate locking between setter and testers. 

I didn't mention locking because it seemed obvious that locking is
needed.  Of course a flag alone is mostly useless.

But we already _do_ have a lock (dev->sem) and we _don't_ have a flag.  
I was pointing out that some drivers will want to have a flag added.
Maybe so many drivers will want it that the flag should be added to the
PM structure, where it will be available for every device.  If only a
few drivers want this new flag, then yes, they can put it in their
private data structures.

> However, I think this is mostly a non-issue, because the core _does_
> provide something here that is useful for drivers who don't want to
> bother with the above: The failure return from device_add. If drivers
> don't want to do something akin to what I described, they can just be
> made to deal gracefully with the failure from device_add that would
> happen due to the core internal flag being set after the return from
> prepare().

Relying on a registration failure to handle this sort of thing is _not_
a good idea.  For example, how would the driver distinguish failure
caused by impending suspend from other sorts of failure?

Alan Stern

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