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

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Mar 06, 2015 at 11:06:18AM +0000, Mark Rutland 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.

I don't think we should want to make such distinction; we should treat
all suspend states the same.

Drivers should not want to rely on the fact that one state
(suspend-to-idle) might maybe deal with interrupts while other states do
not.

> > > 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.

Note that COND_SUSPEND must have SHARED set.

> 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.

While I appreciate the use-case; we should be careful not to make of
mess of things either.