linux-kernel - Re: [PATCH] PM / wakeirq: report wakeup events in dedicated wake-IRQs

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Date:   Thu, 10 Nov 2016 13:30:39 -0800
From:   Brian Norris <briannorris@...omium.org>
To:     Tony Lindgren <tony@...mide.com>
Cc:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Brian Norris <computersforpeace@...il.com>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>
Subject: Re: [PATCH] PM / wakeirq: report wakeup events in dedicated wake-IRQs

On Thu, Nov 10, 2016 at 01:49:11PM -0700, Tony Lindgren wrote:
> * Brian Norris <briannorris@...omium.org> [161110 11:49]:
> > On Thu, Nov 10, 2016 at 10:13:55AM -0800, Dmitry Torokhov wrote:
> > > On Thu, Nov 10, 2016 at 10:07 AM, Brian Norris <briannorris@...omium.org> wrote:
> > > > It's important that user space can figure out what device woke the
> > > > system from suspend -- e.g., for debugging, or for implementing
> > > > conditional wake behavior. Dedicated wakeup IRQs don't currently do
> > > > that.
> > > >
> > > > Let's report the event (pm_wakeup_event()) and also allow drivers to
> > > > synchronize with these events in their resume path (hence, disable_irq()
> > > > instead of disable_irq_nosync()).
> > > 
> > > Hmm, dev_pm_disable_wake_irq() is called from
> > > rpm_suspend()/rpm_resume() that take dev->power.lock spinlock and
> > > disable interrupts. Dropping _nosync() feels dangerous.
> > 
> > Indeed. So how do you suggest we get sane wakeup reports?
> 
> __pm_wakeup_event() ?

That's not the difficult part. (This patch already uses
pm_wakeup_event() correctly. It's in the ISR, and it doesn't get nested
within any other lock-holding code, so it should use the non-underscore
version, which grabs the lock.)

The difficult part is guaranteeing that the wake IRQ gets reported at
the appropriate time. It seems highly unlikely that a threaded IRQ like
this would take longer than the time for devices to resume, but it's not
guaranteed. So the question is where/when/how we call synchronize_irq().

Brian