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

From: [email-masked] (Jason Gunthorpe)
Date: Thu, 23 Jan 2014 15:52:08 -0700

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?

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.

Regards,
Jason