On Wed, Nov 02, 2011 at 01:40:24PM +0000, Russell King - ARM Linux wrote:

On Thu, Sep 29, 2011 at 10:30:09AM +0100, Jamie Iles wrote:

quoted

+#ifdef CONFIG_MULTI_IRQ_HANDLER
+static void vic_single_handle_irq(struct vic_device *vic, struct pt_regs *regs)
+{
+	u32 stat, irq;
+
+	stat = readl_relaxed(vic->base + VIC_IRQ_STATUS);
+	while (stat) {
+		irq = ffs(stat) - 1;
+		handle_IRQ(irq_domain_to_irq(&vic->domain, irq), regs);
+		stat &= ~(1 << irq);
+	}
+}
+
+asmlinkage void __exception_irq_entry vic_handle_irq(struct pt_regs *regs)
+{
+	int i;
+
+	for (i = 0; i < vic_id; ++i)
+		vic_single_handle_irq(&vic_devices[i], regs);
+}

And if we receive another interrupt after the read of the register, we'll
have to exit all the way back (possibly to userspace) before re-entering
the IRQ handling paths back to this point to process it.

OK, so how about something like this instead:

static int vic_single_handle_irq(struct vic_device *vic,
				 struct pt_regs *regs)
{
	u32 stat, irq;
	int handled = 0;

	stat = readl_relaxed(vic->base + VIC_IRQ_STATUS);
	while (stat) {
		irq = ffs(stat) - 1;
		handle_IRQ(irq_domain_to_irq(&vic->domain, irq), regs);
		stat &= ~(1 << irq);
		handled = 1;
	}

	return handled;
}

asmlinkage void __exception_irq_entry vic_handle_irq(struct pt_regs *regs)
{
	int i, handled;

	do {
		handled = 0;
		for (i = 0; i < vic_id; ++i)
			if (vic_single_handle_irq(&vic_devices[i], regs))
				handled = 1;
	} while (handled);
}

which I think should keep handling IRQ's until no VIC has them pending 
(or as best can be determined).

Is there any particular reason folk are destroying the built-in efficiency
of the IRQ handling which is common-place in the existing assembly
approach?

Well this approach makes a single image kernel a bit easier.  The other 
thing is that it plays a lot nicer with dynamic irq_desc assignment.  
Grant's IRQ domain patches make this quite easy here, but I can't see an 
obvious way to do that with the assembly method.

Jamie