Re: [update] Re: [RFC][PATCH] PM: Avoid losing wakeup events during suspend

From: Rafael J. Wysocki
Date: Thu Jun 24 2010 - 10:50:45 EST

On Thursday, June 24, 2010, Andy Lutomirski wrote:
> Rafael J. Wysocki wrote:
> > On Tuesday, June 22, 2010, Rafael J. Wysocki wrote:
> >> On Tuesday, June 22, 2010, Alan Stern wrote:
> >>> On Tue, 22 Jun 2010, Rafael J. Wysocki wrote:
> > ...
> >>>> So, even if we can say when the kernel has finished processing the event
> >>>> (although that would be complicated in the PCIe case above), I don't think
> >>>> it's generally possible to ensure that the entire processing of a wakeup event
> >>>> has been completed. This leads to the question whether or not it is worth
> >>>> trying to detect the ending of the processing of a wakeup event.
> >>> As Arve pointed out, in some cases it definitely is worthwhile (the
> >>> gpio keypad matrix example). In other cases there may be no reasonable
> >>> way to tell. That doesn't mean we have to give up entirely.
> >> Well, I'm not sure, because that really depends on the hardware and bus in
> >> question. The necessary condition seems to be that the event be detected
> >> and handled entirely by the same functional unit (eg. a device driver) within
> >> the kernel and such that it is able to detect whether or not user space has
> >> acquired the event information. That doesn't seem to be a common case to me.
> >
> > Anyway, below's an update that addresses this particular case.
> >
> > It adds two more functions, pm_wakeup_begin() and pm_wakeup_end()
> > that play similar roles to suspend_block() and suspend_unblock(), but they
> > don't operate on suspend blocker objects. Instead, the first of them increases
> > a counter of events in progress and the other one decreases this counter.
> > Together they have the same effect as pm_wakeup_event(), but the counter
> > of wakeup events in progress they operate on is also checked by
> > pm_check_wakeup_events().
> >
> > Thus there are two ways kernel subsystems can signal wakeup events. First,
> > if the event is not explicitly handed over to user space and "instantaneous",
> > they can simply call pm_wakeup_event() and be done with it. Second, if the
> > event is going to be delivered to user space, the subsystem that processes
> > the event can call pm_wakeup_begin() right when the event is detected and
> > pm_wakeup_end() when it's been handed over to user space.
> How does userspace handle this without races? (I don't see an example
> in a driver that talks to userspace in your code...)
> For example, if I push a button on my keyboard, the driver calls
> pm_wakeup_begin(). Then userspace reads the key from the evdev device
> and tells the userspace suspend manager not to go to sleep.
> But there's a race: the keyboard driver (or input subsystem) could call
> pm_wakeup_end() before the userspace program has a chance to tell the
> suspend manager not to sleep.

That basically is what /sys/power/wakeup_count is for. The power manager is
supposed to first read from it (that will block if there are any events in
progress in the last, recently posted, version of the patch), obtain ACKs from
user space event consumers known to it, then write to
/sys/power/wakeup_count (that will fail if there were any wakeup events in the
meantime) and write to /sys/power/state only if that was successful.

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