Re: [PATCH v2 5/6] watchdog: at91sam9: request the irq with IRQF_NO_SUSPEND

Rafael J. Wysocki
2015-03-06T12:39:15Z

On Fri, Mar 6, 2015 at 12:06 PM, Mark Rutland <mark.rutland@arm.com> wrote:
> [...]
>
>> > The request_irq path never results in a call to chip->irq_set_wake(),
>> > even with the IRQF_NO_SUSPEND flag. So requesting an irq with
>> > IRQF_NO_SUSPEND does not guarantee wakeup; it only guarantees that the
>> > CPU can take the interrupt _around_ the suspended state, not necessarily
>> > while _in_ the suspended state.
>>
>> Right. "Suspended state" meaning full suspend here I suppose?
>
> Yes; any state deeper than suspend-to-idle.
>
> [...]
>
>> > We seem to be conflating some related properties:
>> >
>> > [a] The IRQ will be left unmasked.
>> > [b] The IRQ will be handled immediately when taken.
>> > [c] The IRQ will wake the system from suspend.
>> >
>> > Requesting an IRQ with IRQF_NO_SUSPEND guarantees [a,b], but does not
>> > guarantee [c].
>>
>> That's correct. IRQF_NO_SUSPEND does not guarantee that interrupts from
>> that IRQ will have any effect after arch_suspend_disable_irqs() in
>> suspend_enter().
>
> [...]
>
>> > It sounds like for this kind of watchdog device we want [a,b,c], even if
>> > the IRQ is not shared with an IRQF_NO_SUSPEND user.
>>
>> We can't guarantee that, though. arch_suspend_disable_irqs() disables
>> interrupts on the last working CPU and it won't get any. It may be
>> brought out of a low-power state by a pending interrupt, but it won't act
>> upon that interrupt immediately anyway, only after the arch_suspend_enable_irqs()
>> in suspend_enter().
>
> Ok, so [b] needs the caveat that it's only handled "immediately" outside
> of the arch_suspend_disable_irqs() ... arch_suspend_enable_irqs()
> section.
>
>> But then it might as well be deferred until after
>> resume_device_irqs().
>
> That was my original line of thinking, in which case the watchdog driver
> should use IRQF_COND_SUSPEND rather than IRQF_NO_SUSPEND, with
> enable_irq_wake() if we care about the watchdog during suspend. I'm
> happy with this.
>
> Considering that the use-case of a watchdog is to alert us to something
> going hideously wrong in the kernel, we want to handle the IRQ after
> executing the smallest amount of kernel code possible. For that, they
> need to have their handlers to be called "immediately" outside of the
> arch_suspend_disable_irqs() ... arch_suspend_enable_irqs() window, and
> need to be enabled during suspend to attempt to catch bad wakeup device
> configuration.
>
> I think it's possible (assuming the caveats on [b] above) to provide
> [a,b,c] for this case.

OK

But in this case the request_irq() passing IRQF_NO_SUSPEND *and* requiring
enable_irq_wake() in addition to that needs a big fat comment explaining the
whole thing or we'll forget about the gory details at one point and no one will
know what's going on in there.

Rafael