Re: [PATCH] PM / runtime: Add support for wake-up reason for wakeirqs

[Tony Lindgren]

* Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> [191009 18:51]:
> On Wed, 9 Oct 2019, Tony Lindgren wrote:
> 
> > With generic wakeirqs we can wake a device, but do not know if the
> > device woke to a wakeirq. Let's add pm_runtime_wakeup_is_wakeirq() so
> > a device can check the wake-up reason.
> 
> People have tried many times over the years to do something like this.  
> It's never right.
> 
> The problem is simple: It's impossible to know for certain why the
> system woke up from suspend.  In fact, there may be many wakeup sources
> all active at the same time, and any of them could be the one
> responsible for actually waking the system.

Hmm yeah good point. Even with dedicated wakeirq it could race
against a timer for the wake-up event.

> All you can do is check to see whether a particular wakeup source is
> active at the present moment.  You can't tell whether it was active in
> the past (while the system was suspended) or whether it caused the
> system to resume.

We can actually do more than that now though :)

With handle_threaded_wake_irq() we could optionally call a handler
before we call pm_runtime_resume() and let the consumer device
driver figure out what the state is.

Regards,

Tony