linux-kernel - Re: [PATCH] PM: suspend_device_irqs(): don't disable wakeup IRQs

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Message-Id: <200905071354.08319.rjw@sisk.pl>
Date:	Thu, 7 May 2009 13:54:07 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Kevin Hilman <khilman@...prootsystems.com>
Cc:	Arve Hjønnevåg <arve@...roid.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] PM: suspend_device_irqs(): don't disable wakeup IRQs

On Thursday 07 May 2009, Kevin Hilman wrote:
> "Rafael J. Wysocki" <rjw@...k.pl> writes:
> 
> > On Wednesday 06 May 2009, Kevin Hilman wrote:
> >> Kevin Hilman <khilman@...prootsystems.com> writes:
> >> 
> >> > "Rafael J. Wysocki" <rjw@...k.pl> writes:
> >> >
> >> >> On Wednesday 06 May 2009, Kevin Hilman wrote:
> >> 
> >> [...]
> >> 
> >> >>> 
> >> >>> [...]
> >> >>> 
> >> >>> >>>
> >> >>> >>> If this fixes some bug then please provide a description of that bug?
> >> >>> >>
> >> >>> >> The bug is that on TI OMAP, interrupts that are used for wakeup events
> >> >>> >> are disabled by this code causing the system to no longer wake up.
> >> >>> >
> >> >>> > What do you do if the interrupt triggers right after your driver has
> >> >>> > returned from its late suspend hook?  
> >> >>> 
> >> >>> If it's a wakeup IRQ, I assume you want it to prevent suspend.
> >> >>> 
> >> >>> But I don't see how that can happen in the current code. IIUC, by the
> >> >>> time your late suspend hook is run, your device IRQ is already
> >> >>> disabled, so it won't trigger an interrupt that will be caught by
> >> >>> check_wakeup_irqs() anyways.
> >> >>
> >> >> My understanding of __disable_irq() was that it didn't actually disable the
> >> >> IRQ at the hardware level, allowing the CPU to actually receive the interrupt
> >> >> and acknowledge it, but preventing the device driver for receiving it.  
> >> >
> >> >> Does it work differently on the affected systems?
> >> >
> >> > Yes.
> >> >
> >> > __disable_irq() calls the irq_chip's disable method which is platform
> >> > specific.  On OMAP, this masks the IRQ at the hardware level
> >> > preventing the CPU from seeing the interrupt.
> >> 
> >> Looking at x86, the i8259 disable hook also seems to mask the IRQ at
> >> the PIC level.
> >> 
> >> The various IO-APIC irq_chips do not have a disable hook so the
> >> __disable_irq() here is a NOP.
> >
> > Except that it sets IRQ_DISABLED.
> >
> > All right there.
> 
> OK, right. I missed the setting of IRQ_DISABLED there.
> 
> > We can either avoid disabling wake-up interrupts, in which case we
> > should drop check_wakeup_irqs() IMO, or rework things so that
> > check_wakeup_irqs() will catch them.  Doing both doesn't seem to
> > make sense to me.
> >
> > Which one would be the right approach, then?
> 
> Not sure if there is right one as they are solving two different
> problems.
> 
> check_wakeup_irqs() seems meant to address handling interrupts that
> happen during the suspend path.  My fix for not disabling wakups is
> meant to allow those interrupts after suspend.

Still, with your fix in place, check_wakeup_irqs() is useless, because
the wake-up interrupts are not going to have IRQ_PENDING set.

> An alternative to my original approach is the one taken on x86
> IO-APICs where the irq_chip's disable hook is empty, thus not masking
> in HW but still preventing the ISR from running.

That sounds quite right.

> I need to explore whether just dropping the irq_chip's disable hook
> would work for us on OMAP.
> 
> There is at least one problem with that which is why Kyuwon Kim added
> the ->disable hook to OMAP's irq_chip.  The problem is with drivers
> that call disable_irq() in their suspend hook, usually done to prevent
> the device from waking the system since on OMAP, any IRQ can be
> configured to wake the system.
> 
> If a driver's suspend hook calls disable_irq() and the system is
> suspended before the lazy disable happens in the next handler, then
> the system will be suspended with that device's IRQ still enabled.
> Without an irq_chip->disable hook, that will result in that device IRQ
> waking up the system if it fires.
> 
> I now have a patch for this which ensures that the lazy-disable
> happens in the suspend path using the ->mask hook just like
> it does in the handler.  Will send to LKML and here shortly.

Great, thanks!

Best,
Rafael
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