ARC: intc: Document arc_request_percpu_irq() better

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/kernel/irq.c

@@ -51,6 +51,18 @@ void arch_do_IRQ(unsigned int irq, struct pt_regs *regs)
 	set_irq_regs(old_regs);
 }
 
+/*
+ * API called for requesting percpu interrupts - called by each CPU
+ *  - For boot CPU, actually request the IRQ with genirq core + enables
+ *  - For subsequent callers only enable called locally
+ *
+ * Relies on being called by boot cpu first (i.e. request called ahead) of
+ * any enable as expected by genirq. Hence Suitable only for TIMER, IPI
+ * which are guaranteed to be setup on boot core first.
+ * Late probed peripherals such as perf can't use this as there no guarantee
+ * of being called on boot CPU first.
+ */
+
 void arc_request_percpu_irq(int irq, int cpu,
                             irqreturn_t (*isr)(int irq, void *dev),
                             const char *irq_nm,