genirq: irqchip: Add mask to block out invalid irqs

Some controllers have irqs that aren't wired up and must never be used.
For the generic chip attached to an irq_domain this provides a mask that
can be used to block out particular irqs so that they never get mapped.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
Link: http://lkml.kernel.org/r/1369793454-19197-2-git-send-email-grant.likely@linaro.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/irq.h

@@ -679,6 +679,7 @@ struct irq_chip_type {
  * @num_ct:		Number of available irq_chip_type instances (usually 1)
  * @private:		Private data for non generic chip callbacks
  * @installed:		bitfield to denote installed interrupts
+ * @unused:		bitfield to denote unused interrupts
  * @domain:		irq domain pointer
  * @list:		List head for keeping track of instances
  * @chip_types:		Array of interrupt irq_chip_types
@@ -702,6 +703,7 @@ struct irq_chip_generic {
 	unsigned int		num_ct;
 	void			*private;
 	unsigned long		installed;
+	unsigned long		unused;
 	struct irq_domain	*domain;
 	struct list_head	list;
 	struct irq_chip_type	chip_types[0];

kernel/irq/generic-chip.c

@@ -359,6 +359,9 @@ static int irq_map_generic_chip(struct irq_domain *d, unsigned int virq,
 
 	idx = hw_irq % dgc->irqs_per_chip;
 
+	if (test_bit(idx, &gc->unused))
+		return -ENOTSUPP;
+
 	if (test_bit(idx, &gc->installed))
 		return -EBUSY;