Re: [PATCHv2 02/10] ARM: vic: MULTI_IRQ_HANDLER handler

[Russell King - ARM Linux <linux-lFZ/pmaqli7XmaaqVzeoHQ@public.gmane.org>]

On Thu, Nov 03, 2011 at 01:29:08PM +0100, Linus Walleij wrote:
> No, if we receive another IRQ *after* the read of the register was the
> question, right?
> 
> Just replace
> 
> stat &= ~(1 << irq);
> 
> with a second
> 
> stat = readl_relaxed(vic->base + VIC_IRQ_STATUS);
> 
> It'll work just fine, the IRQ line should be low when you read
> it the second time, else it is probably fully proper to call
> the IRQ handler again anyway.

It depends on what kind of behaviour you want.  There are two solutions:

	stat = readl_relaxed(vic->base + VIC_IRQ_STATUS);
	while (stat) {
		irq = ffs(stat) - 1;
		handle_irq(irq);
		stat = readl_relaxed(vic->base + VIC_IRQ_STATUS);
	}

This gives priority to the lowest numbered interrupts; if these get stuck
then they can exclude higher numbered interrupts.  This is what we
implement in the assembly code versions, and as far as I know, no one has
ever complained about that behaviour.

	stat = readl_relaxed(vic->base + VIC_IRQ_STATUS);
	while (stat) {
		while (stat) {
			irq = ffs(stat) - 1;
			stat &= ~(1 << irq);
			handle_irq(irq);
		}
		stat = readl_relaxed(vic->base + VIC_IRQ_STATUS);
	}

This ensures that we process all interrupts found pending before we
re-check for any new interrupts pending.  Arguably this is a much
fairer implementation (and may mean if things get irrevokably stuck,
things like sysrq via the console uart may still work.)