linux-kernel - Re: [RFC PATCH] PM / Runtime: runtime: Add sysfs option for forcing runtime suspend

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Message-ID: <1645828.psusZpgl4n@vostro.rjw.lan>
Date:	Sat, 26 Sep 2015 01:04:29 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Oliver Neukum <oneukum@...e.de>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Irina Tirdea <irina.tirdea@...el.com>,
	Len Brown <len.brown@...el.com>,
	Octavian Purdila <octavian.purdila@...el.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Pavel Machek <pavel@....cz>,
	"linux-input@...r.kernel.org" <linux-input@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>
Subject: Re: [RFC PATCH] PM / Runtime: runtime: Add sysfs option for forcing runtime suspend

On Friday, September 25, 2015 11:52:23 PM Rafael J. Wysocki wrote:
> On Friday, September 25, 2015 05:13:04 PM Alan Stern wrote:
> > On Fri, 25 Sep 2015, Rafael J. Wysocki wrote:
> > 
> > > On Friday, September 25, 2015 10:29:55 AM Alan Stern wrote:
> > > > On Fri, 25 Sep 2015, Rafael J. Wysocki wrote:
> > > > 
> > > > > We are missing the "no remote wakeup" bit now (well, there is a PM QoS flag,
> > > > > but it isn't very useful, so I'd prefer to replace it with a "no remote wakeup"
> > > > > bit in struct dev_pm_info or something similar).
> > > > > 
> > > > > That is actually quite important, because (a) we can save energy but not
> > > > > configuring the device to do remote wakeup in the first place and (b) that
> > > > > may involve more than just the driver (for example, disabling PCI or ACPI
> > > > > remote wakeup involves the bus type or similar).
> > > > > 
> > > > > So it looks like we need to be able to distinguish between "runtime suspend
> > > > > with remote wakeup" and "runtime suspend without remote wakeup".
> > > > > 
> > > > > And if we do the latter, we may not even need the "inhibit" thing any more,
> > > > > because suspended devices without that are not configured to do remote wakeup
> > > > > cannot really signal anything in the majority of cases.
> > > > 
> > > > That works only for drivers that use autosuspend to go to low power in
> > > > between events.  It doesn't work for drivers that remain at full power 
> > > > as long as the device file is open.  That kind of driver does require 
> > > > an "inhibit" interface.
> > > 
> > > Or an interface allowing user space to trigger pm_request_idle() for them.
> > > 
> > > So user space would change the "no remote wakeup" setting and then do the
> > > "try to suspend now" thing.
> > 
> > So something like:
> > 
> > 	echo on >/sys/.../power/control  (in case the device was
> > 			already in runtime suspend with wakeups enabled)
> > 	echo off >/sys/.../power/wakeup
> > 	echo auto >/sys/.../power/control
> 
> That, or there may be an additional value, say "aggressive", to write to the
> control file in which case it becomes just
> 
> echo aggressive >/sys/.../power/control

That said I suppose that the "off" value for the "wakeup" file might also be
useful in some other cases, so it likely is a better approach.

Thanks,
Rafael

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