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Date: Mon, 5 Jun 2017 10:23:48 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Mason <slash.tmp@free.fr>
cc: Marc Zyngier <marc.zyngier@arm.com>,
        Jason Cooper <jason@lakedaemon.net>,
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        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: Design of interrupt controller driver
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On Mon, 5 Jun 2017, Mason wrote:
> On 04/06/2017 22:13, Thomas Gleixner wrote:
> > When you configure the interrupt as edge then you cannot share it. No
> > matter whether it stays high or not.
> 
> Could you explain why? (I must be missing something.)

Device A    Device B	 Combined Output  Edge detection
Low    	    Low	   	 0	  	  N

Low -> High Low		 1	    	  Y  -> Interrupt handled

High   	    Low -> High	 1	    	  N

When the A line stays high, which it does, then the edge detector will not
see a transition for B and you lose an interrupt.

> > The only way to share it is, to configure it as level interrupt. But that
> > requires that you can disable the interrupt at the DMA device level once it
> > triggered. Otherwise you get an interrupt storm.
> 
> I'm not sure what you mean with "disable the interrupt at the
> DMA device level". The interrupt can be masked at the system
> interrupt controller (i.e. before sharing the interrupt
> signal). The DMA engine just outputs 0 when busy, 1 when idle.

Sharing level interrupts requires a way to disable the device (in your case
the DMA engine) interrupt output in order to prevent irq storms.

Pseudo code (locking etc. omitted):

irq_handler_devA()
{
	if (!interrupt_active(devA))
		return IRQ_NONE;

	handle_device_irq();

	if (no_more_outstanding_requests(devA)) {
		reg = readl(devA->irq_control_reg);
		reg &= ~DEV_IRQ_ENABLE;
		writel(devA->irq_control_reg, reg);
	}
	return IRQ_HANDLED;
}

queue_reqeust_devA()
{
	if (no_more_outstanding_requests(devA)) {
		queue_request();
		
		start_engine();

		/* Reenable interrupt at device level */
		reg = readl(devA->irq_control_reg);
		reg |= DEV_IRQ_ENABLE;
		writel(devA->irq_control_reg, reg);
	} else {
	       queue_request();
	}
}

You get the idea.

Thanks,

	tglx