genirq: Add ipi_offset to irq_common_data

IPIs are always assumed to be consecutively allocated, hence virqs and hwirqs
can be inferred by using CPU id as an offset. But the first cpu doesn't always
have to start at offset 0. ipi_offset stores the position of the first cpu so
that we can easily calculate the virq or hwirq of an IPI associated with a
specific cpu.

Signed-off-by: Qais Yousef <qais.yousef@imgtec.com>
Cc: <jason@lakedaemon.net>
Cc: <marc.zyngier@arm.com>
Cc: <jiang.liu@linux.intel.com>
Cc: <ralf@linux-mips.org>
Cc: <linux-mips@linux-mips.org>
Cc: <lisa.parratt@imgtec.com>
Cc: Qais Yousef <qsyousef@gmail.com>
Link: http://lkml.kernel.org/r/1449580830-23652-6-git-send-email-qais.yousef@imgtec.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/irq.h

@@ -137,6 +137,7 @@ struct irq_domain;
  *			related irq, then this is the mask of the
  *			CPUs to which an IPI can be sent.
  * @msi_desc:		MSI descriptor
+ * @ipi_offset:		Offset of first IPI target cpu in @affinity. Optional.
  */
 struct irq_common_data {
 	unsigned int		state_use_accessors;
@@ -146,6 +147,9 @@ struct irq_common_data {
 	void			*handler_data;
 	struct msi_desc		*msi_desc;
 	cpumask_var_t		affinity;
+#ifdef CONFIG_GENERIC_IRQ_IPI
+	unsigned int		ipi_offset;
+#endif
 };
 
 /**