genirq: Provide an IRQ affinity mask in non-SMP configs

IRQ affinity masks are not allocated in uniprocessor configurations.
This requires special case non-SMP code in drivers for irqchips which
have per-CPU enable or mask registers.

Since IRQ affinity is always the same in a uniprocessor configuration,
we can provide a correct affinity mask without allocating one per IRQ.

By returning a real cpumask from irq_data_get_affinity_mask even when
SMP is disabled, irqchip drivers which iterate over that mask will
automatically do the right thing.
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220701200056.46555-9-samuel@sholland.org

include/linux/irq.h

@@ -151,7 +151,9 @@ struct irq_common_data {
 #endif
 	void			*handler_data;
 	struct msi_desc		*msi_desc;
+#ifdef CONFIG_SMP
 	cpumask_var_t		affinity;
+#endif
 #ifdef CONFIG_GENERIC_IRQ_EFFECTIVE_AFF_MASK
 	cpumask_var_t		effective_affinity;
 #endif
@@ -882,13 +884,19 @@ static inline int irq_data_get_node(struct irq_data *d)
 static inline
 const struct cpumask *irq_data_get_affinity_mask(struct irq_data *d)
 {
+#ifdef CONFIG_SMP
 	return d->common->affinity;
+#else
+	return cpumask_of(0);
+#endif
 }
 
 static inline void irq_data_update_affinity(struct irq_data *d,
 					    const struct cpumask *m)
 {
+#ifdef CONFIG_SMP
 	cpumask_copy(d->common->affinity, m);
+#endif
 }
 
 static inline const struct cpumask *irq_get_affinity_mask(int irq)