linux-kernel - Re: Attempted summary of suspend-blockers LKML thread

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Message-Id: <201008070004.43923.rjw@sisk.pl>
Date:	Sat, 7 Aug 2010 00:04:43 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Arve Hjønnevåg <arve@...roid.com>,
	Matthew Garrett <mjg59@...f.ucam.org>, david@...g.hm,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	pavel@....cz, florian@...kler.org, swetland@...gle.com,
	peterz@...radead.org, tglx@...utronix.de, alan@...rguk.ukuu.org.uk
Subject: Re: Attempted summary of suspend-blockers LKML thread

On Friday, August 06, 2010, Alan Stern wrote:
> On Thu, 5 Aug 2010, Arve Hjønnevåg wrote:
...
> But what sorts of things qualify as wakeup events?  Right now, the code
> handles only events coming by way of the PME# signal (or its platform
> equivalent).  But that signal usually gets activated only when a PCI
> device is in a low-power mode; if the device is at full power then it
> simply generates an IRQ.  It's the same event, but reported to the
> kernel in a different way.  So consider...
> 
> Case 1: The system is suspending and the PCI device has already been
> 	placed in D3hot when an event occurs.  PME# is activated,
> 	the wakeup event is reported, the suspend is aborted, and the 
> 	system won't try to suspend again for at least 100 ms.  Good.
> 
> Case 2: The system is running normally and the PCI device is at full
> 	power when an event occurs.  PME# isn't activated and 
> 	pm_wakeup_event doesn't get called.  Then when the system
> 	tries to suspend 25 ms later, there's nothing to prevent it
> 	even though the event is still being processed.  Bad.
> 
> In case 2 the race has not been resolved.  It seems to me that the 
> only proper solution is to call pm_wakeup_event for _every_ PCI 
> interrupt.  This may be too much to add to a hot path, but what's the 
> alternative?

Arguably not every PCI interrupt should be regarded as a wakeup event, so
I think we can simply say in the cases when that's necessary the driver should
be responsible for using pm_wakeup_event() or pm_stay_awake() / pm_relax() as
appropriate.

My patch only added it to the bus-level code which covered the PME-based
wakeup events that _cannot_ be handled by device drivers.

Thanks,
Rafael
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