Re: [PATCH 3/3] irqchip: orion: clear stale interrupts in irq_enable

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Thu, Jan 23, 2014 at 03:52:08PM -0700, Jason Gunthorpe wrote:
> On Thu, Jan 23, 2014 at 11:38:06PM +0100, Sebastian Hesselbarth wrote:
> > Bridge IRQ_CAUSE bits are asserted regardless of the corresponding bit in
> > IRQ_MASK register. To avoid interrupt events on stale irqs, we have to clear
> > them before unmask. This installs an .irq_enable callback to ensure stale
> > irqs are cleared before initial unmask.
> 
> I'm not sure if putting this in irq_enable is correct. I think this
> should only happen at irq_startup.
> 
> The question boils down to what is supposed to happen with this code
> sequence:
> 
> disable_irq(..);
> write(.. something to cause an interrupt edge ..);
> .. synchronize ..
> enable_irq(..);
> 
> Do we get the interrupt or not?

The answer is... yes, the interrupt should be delivered after the
interrupt is re-enabled.

> I found this message from Linus long ago:
>  http://yarchive.net/comp/linux/edge_triggered_interrupts.html
> > Btw, the "disable_irq()/enable_irq()" subsystem has been written so that
> > when you disable an edge-triggered interrupt, and the edge happens while
> > the interrupt is disabled, we will re-play the interrupt at enable time.
> > Exactly so that drivers can have an easier time and don't have to
> > normally worry about whether something is edge or level-triggered.
> 
> And found this note in Documentation/DocBook/genericirq.tmpl:
> 
> > This prevents losing edge interrupts on hardware which does
> > not store an edge interrupt event while the interrupt is disabled at
> > the hardware level. 
> 
> So I think it is very clear that the chip driver should not discard
> edges that happened while the interrupt was disabled.

Correct.

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