genirq: Provide kstat_irqdesc_cpu()

Most users of kstat_irqs_cpu() have the irq descriptor already. No point in
calling into the core code and looking it up once more.

Use it in per_cpu_count_show() to start with.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201210194043.362094758@linutronix.de

include/linux/irqdesc.h

@@ -113,6 +113,12 @@ static inline void irq_unlock_sparse(void) { }
 extern struct irq_desc irq_desc[NR_IRQS];
 #endif
 
+static inline unsigned int irq_desc_kstat_cpu(struct irq_desc *desc,
+					      unsigned int cpu)
+{
+	return desc->kstat_irqs ? *per_cpu_ptr(desc->kstat_irqs, cpu) : 0;
+}
+
 static inline struct irq_desc *irq_data_to_desc(struct irq_data *data)
 {
 	return container_of(data->common, struct irq_desc, irq_common_data);

kernel/irq/irqdesc.c

@@ -147,12 +147,12 @@ static ssize_t per_cpu_count_show(struct kobject *kobj,
 				  struct kobj_attribute *attr, char *buf)
 {
 	struct irq_desc *desc = container_of(kobj, struct irq_desc, kobj);
-	int cpu, irq = desc->irq_data.irq;
 	ssize_t ret = 0;
 	char *p = "";
+	int cpu;
 
 	for_each_possible_cpu(cpu) {
-		unsigned int c = kstat_irqs_cpu(irq, cpu);
+		unsigned int c = irq_desc_kstat_cpu(desc, cpu);
 
 		ret += scnprintf(buf + ret, PAGE_SIZE - ret, "%s%u", p, c);
 		p = ",";