genirq: Add a new function to get IPI reverse mapping

When dealing with coprocessors we need to find out the actual hwirqs values to
pass on to the firmware so that it knows what it needs to use to receive IPIs
from and send IPIs to Linux cpus.

[ tglx: Fixed the single hwirq IPI case. The hardware irq number does not
  	change due to the cpu number ]
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/1449580830-23652-10-git-send-email-qais.yousef@imgtec.comSigned-off-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/irq.h

@@ -942,5 +942,6 @@ static inline u32 irq_reg_readl(struct irq_chip_generic *gc,
 
 /* Contrary to Linux irqs, for hardware irqs the irq number 0 is valid */
 #define INVALID_HWIRQ	(~0UL)
+irq_hw_number_t ipi_get_hwirq(unsigned int irq, unsigned int cpu);
 
 #endif /* _LINUX_IRQ_H */

kernel/irq/ipi.c

@@ -135,3 +135,37 @@ void irq_destroy_ipi(unsigned int irq)
 
 	irq_domain_free_irqs(irq, nr_irqs);
 }
+
+/**
+ * ipi_get_hwirq - Get the hwirq associated with an IPI to a cpu
+ * @irq:	linux irq number
+ * @cpu:	the target cpu
+ *
+ * When dealing with coprocessors IPI, we need to inform the coprocessor of
+ * the hwirq it needs to use to receive and send IPIs.
+ *
+ * Returns hwirq value on success and INVALID_HWIRQ on failure.
+ */
+irq_hw_number_t ipi_get_hwirq(unsigned int irq, unsigned int cpu)
+{
+	struct irq_data *data = irq_get_irq_data(irq);
+	struct cpumask *ipimask = data ? irq_data_get_affinity_mask(data) : NULL;
+
+	if (!data || !ipimask || cpu > nr_cpu_ids)
+		return INVALID_HWIRQ;
+
+	if (!cpumask_test_cpu(cpu, ipimask))
+		return INVALID_HWIRQ;
+
+	/*
+	 * Get the real hardware irq number if the underlying implementation
+	 * uses a seperate irq per cpu. If the underlying implementation uses
+	 * a single hardware irq for all cpus then the IPI send mechanism
+	 * needs to take care of this.
+	 */
+	if (irq_domain_is_ipi_per_cpu(data->domain))
+		data = irq_get_irq_data(irq + cpu - data->common->ipi_offset);
+
+	return data ? irqd_to_hwirq(data) : INVALID_HWIRQ;
+}
+EXPORT_SYMBOL_GPL(ipi_get_hwirq);