genirq: Restrict effective affinity to interrupts actually using it

Just because CONFIG_GENERIC_IRQ_EFFECTIVE_AFF_MASK is selected
doesn't mean that all the interrupts are using the effective
affinity mask. For a number of them, this mask is likely to
be empty.

In order to deal with this, let's restrict the use of the
effective affinity mask to these interrupts that have a non empty
effective affinity.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: Chris Zankel <chris@zankel.net>
Cc: Kevin Cernekee <cernekee@gmail.com>
Cc: Wei Xu <xuwei5@hisilicon.com>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Gregory Clement <gregory.clement@free-electrons.com>
Cc: Matt Redfearn <matt.redfearn@imgtec.com>
Cc: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Link: http://lkml.kernel.org/r/20170818083925.10108-2-marc.zyngier@arm.com

include/linux/irq.h

@@ -781,7 +781,10 @@ static inline struct cpumask *irq_data_get_affinity_mask(struct irq_data *d)
 static inline
 struct cpumask *irq_data_get_effective_affinity_mask(struct irq_data *d)
 {
-	return d->common->effective_affinity;
+	if (!cpumask_empty(d->common->effective_affinity))
+		return d->common->effective_affinity;
+
+	return d->common->affinity;
 }
 static inline void irq_data_update_effective_affinity(struct irq_data *d,
 						      const struct cpumask *m)