Re: [PATCH] IRQ: don't suspend nested_thread irqs over system suspend.

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

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On Saturday, January 31, 2015 02:37:47 PM NeilBrown wrote:
> On Sat, 31 Jan 2015 00:51:17 +0100 "Rafael J. Wysocki" <rjw@rjwysocki.net>
> wrote:
> 
> > On Saturday, January 31, 2015 12:06:37 AM Rafael J. Wysocki wrote:
> > > On Saturday, January 31, 2015 09:25:45 AM NeilBrown wrote:
> > > > 
> > > > Nested IRQs can only fire when the parent irq fires.
> > > > So when the parent is suspended, there is no need to suspend
> > > > the child irq.
> > > > 
> > > > Suspending nested irqs can cause a problem is they are suspended or
> > > > resumed in the wrong order.
> > > > If an interrupt fires while the parent is active but the child is
> > > > suspended, then the interrupt will not be acknowledged properly
> > > > and so an interrupt storm can result.
> > > > This is particularly likely if the parent is resumed before
> > > > the child, and the interrupt was raised during suspend.
> > > > 
> > > > Ensuring correct ordering would be possible, but it is simpler
> > > > to just never suspend nested interrupts.  This patch does that.
> > > 
> > > Clever. :-)
> > > 
> > > This is fine by me.  Thomas, what do you think?
> > 
> > It looks like I've overlooked a potential problem, though.
> > 
> > Can a nested interrupt be a wakeup one?  We won't set IRQD_WAKEUP_ARMED for it
> > then and may not handle wakeup correctly.
> > 
> 
> I only have a fairly narrow understanding of this stuff, but if you have
> nested interrupts, you would surely need the parent to be registered as a
> wakeup interrupt, else the device wouldn't wake and the nested interrupt
> would be ineffective until something else woke the device.
> 
> Very few files mention both '.irq_set_wake' and  'irq_set_nested'.
> 
> twl6040-irq.c has code to set irq_wake_enable  on the parent if any nested
> irqs have had irq_set_wake calls.
> tps6586x.c has something similar, but much simpler.
> arizona-irq.c and rc5t583-irq.c do the same as tps6586x.c
> 
> So I think that any nested interrupts which might want to be wakeup
> interrupts already deal with the issue, and I don't introduce a new problem
> here.

Fair enough.

I wonder if this means that it'll be useful to propagate IRQD_WAKEUP_STATE to
parents, then ...

Rafael

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