linux-kernel - Re: [linux-pm] [RFC][PATCH] PM: Introduce new top level suspend and hibernation callbacks (rev. 2)

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Date:	Tue, 25 Mar 2008 10:19:45 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Oliver Neukum <oliver@...kum.org>
cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	<linux-pm@...ts.linux-foundation.org>,
	Alexey Starikovskiy <astarikovskiy@...e.de>,
	Johannes Berg <johannes@...solutions.net>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [linux-pm] [RFC][PATCH] PM: Introduce new top level suspend	and hibernation callbacks (rev. 2)

On Tue, 25 Mar 2008, Oliver Neukum wrote:

> Am Dienstag, 25. MÃ¤rz 2008 14:06:15 schrieb Rafael J. Wysocki:
> > On Tuesday, 25 of March 2008, Oliver Neukum wrote:
> 
> > > A device that cannot wake up is unusable. Shouldn't the pm core disconnect()
> > > such a device?

It's not safe for the PM core to do such things unilaterally.  The 
decision to unregister a device should be made by the driver or the 
subsystem.

(The only problem is that it's impossible to unregister a device from 
within its suspend or resume methods.  Perhaps there should be a way 
for the driver to tell the PM core that the core should unregister the 
device as soon as the method returns.  I don't know if such a facility 
would get used...)

> > Well, if ->resume() returns an error, the driver already knows there's a
> > problem and it can act upon that, at least in principle.
> 
> Then why return an error? If a driver returns an error I would assume that
> to indicate an irrecoverable error.

The PM core prints a warning in the system log whenever an error is 
returned.  There isn't much more it can do.

> > However, the PM core probably shouldn't try to resume the children of a failing
> > device.  Also, if ->resume_noirq() fails, it probably is not a good idea to
> > call ->resume() and ->complete() for the same device and for it's children.
> 
> Exactly. But we need to define what happens in these cases. If we simply
> ignore the errors, the drivers must be able to deal with IO to half suspended
> devices.

Just so -- it's up to the drivers to deal with this sort of thing.  The 
PM core can't know the details of what should be done in each case.

For example, if a USB hub can't be resumed then usbcore marks all of
its descendants with USB_STATE_NOTATTACHED.  When the children's resume
methods are called, they return without trying to do anything.  Later
on khubd takes care of unregistering everything.

Alan Stern

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