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

[Rafael J. Wysocki]

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

> 
> This should work.  But it would require that the driver doesn't
> increment the usage counter when the device file is opened.

Right.

> I can imagine this might lead to trouble if you're dealing with hardware that
> doesn't support remote wakeup very well.  The driver wouldn't be able
> to work around the hardware issue by incrementing the usage counter.

Or the "aggressive" mode wouldn't work for it.

> In real life this might not be a serious issue.  I don't know.

Me neither.

Thanks,
Rafael

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