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

From: Alan Stern
Date: Sun Sep 27 2015 - 10:27:33 EST

On Sun, 27 Sep 2015, Rafael J. Wysocki wrote:

> On Saturday, September 26, 2015 11:20:50 AM Alan Stern wrote:
> > On Sat, 26 Sep 2015, Rafael J. Wysocki wrote:
> >
> > > > > 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

> > We still need some sort of "inhibit" callback for cases where the
> > driver doesn't want to go into runtime suspend but does want to turn
> > off all I/O. Should this callback be triggered when the user writes
> > "off" to power/wakeup, or when the user writes "inhibit" to
> > power/control, or should there be a separate sysfs attribute?
> My first thought is that if there is a separate attribute, then it only actually
> makes sense for devices that generate input events, while the "off" thing may
> be generally useful in principle (eg. it may indicate to disable PME for the
> device to the PCI layer etc).

I'm not sure how much sense that distinction makes. It seems to me the
only time you want to ignore potential wakeup events is if you want to
ignore _all_ input. Which is basically what "inhibit" means.

This suggests we forget about power/wakeup == "off" and introduce an
"inhibit" attribute instead.

> OTOH, the additional "inhibit" attribute may only be exposed if the corresponding
> callback is present, so I'm not really sure.

It could be a separate attribute, or it could be a new entry for
power/control. Come to think of it, a separate attribute might be
better. Otherwise we would lose track of whether runtime suspend was
permitted (the "on" vs. "auto" distinction) when the device was
inhibited. I can imagine someone might want to forbid runtime suspend
but still inhibit a device.

However, I agree that there's no point registering a separate attribute
or accepting a write of "inhibit" to power/control if there's no
corresponding callback.

> Question is, though, what's the use case for turning off I/O when we don't
> go into runtime suspend. After all, runtime suspend need not mean putting
> the device into any kind of low-power state and the "off" thing may very
> well be defined to mean that all input is discarded if the device is
> runtime-suspended and the device is not configured to do remote wakeup
> then.

Well, I suppose there might be a driver that supports inhibit but
doesn't support runtime PM, unlikely as that seems. Or the driver
might support both but the user might leave power/control == "on" while
inhibiting the device.

Alan Stern

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