linux-kernel - Re: [PATCH 3/3] irqchip: orion: clear stale interrupts in irq

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Date:	Thu, 23 Jan 2014 15:52:08 -0700
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	Andrew Lunn <andrew@...n.ch>,
	Gregory Clement <gregory.clement@...e-electrons.com>,
	Ezequiel Garcia <ezequiel.garcia@...e-electrons.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] irqchip: orion: clear stale interrupts in irq_enable

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
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