[PATCH v3 6/6] irqchip: sifive-plic: Implement irq_set_affinity() for SMP host

[Anup Patel]

Currently on SMP host, all CPUs take external interrupts routed via
PLIC. All CPUs will try to claim a given external interrupt but only
one of them will succeed while other CPUs would simply resume whatever
they were doing before. This means if we have N CPUs then for every
external interrupt N-1 CPUs will always fail to claim it and waste
their CPU time.

Instead of above, external interrupts should be taken by only one CPU
and we should have provision to explicitly specify IRQ affinity from
kernel-space or user-space.

This patch provides irq_set_affinity() implementation for PLIC driver.
It also updates irq_enable() such that PLIC interrupts are only enabled
for one of CPUs specified in IRQ affinity mask.

With this patch in-place, we can change IRQ affinity at any-time from
user-space using procfs.

Example:

/ # cat /proc/interrupts
           CPU0       CPU1       CPU2       CPU3
  8:         44          0          0          0  SiFive PLIC   8  virtio0
 10:         48          0          0          0  SiFive PLIC  10  ttyS0
IPI0:        55        663         58        363  Rescheduling interrupts
IPI1:         0          1          3         16  Function call interrupts
/ #
/ #
/ # echo 4 > /proc/irq/10/smp_affinity
/ #
/ # cat /proc/interrupts
           CPU0       CPU1       CPU2       CPU3
  8:         45          0          0          0  SiFive PLIC   8  virtio0
 10:        160          0         17          0  SiFive PLIC  10  ttyS0
IPI0:        68        693         77        410  Rescheduling interrupts
IPI1:         0          2          3         16  Function call interrupts

Signed-off-by: Anup Patel <anup@xxxxxxxxxxxxxx>
---
 drivers/irqchip/irq-sifive-plic.c | 35 +++++++++++++++++++++++++++++--
 1 file changed, 33 insertions(+), 2 deletions(-)

diff --git a/drivers/irqchip/irq-sifive-plic.c b/drivers/irqchip/irq-sifive-plic.c
index 17269622be21..c5bbb12e74e0 100644
--- a/drivers/irqchip/irq-sifive-plic.c
+++ b/drivers/irqchip/irq-sifive-plic.c
@@ -106,14 +106,42 @@ static void plic_irq_toggle(const struct cpumask *mask, int hwirq, int enable)
 
 static void plic_irq_enable(struct irq_data *d)
 {
-	plic_irq_toggle(irq_data_get_affinity_mask(d), d->hwirq, 1);
+	unsigned int cpu = cpumask_any_and(irq_data_get_affinity_mask(d),
+					   cpu_online_mask);
+	WARN_ON(cpu >= nr_cpu_ids);
+	plic_irq_toggle(cpumask_of(cpu), d->hwirq, 1);
 }
 
 static void plic_irq_disable(struct irq_data *d)
 {
-	plic_irq_toggle(irq_data_get_affinity_mask(d), d->hwirq, 0);
+	plic_irq_toggle(cpu_possible_mask, d->hwirq, 0);
 }
 
+#ifdef CONFIG_SMP
+static int plic_set_affinity(struct irq_data *d, const struct cpumask *mask_val,
+			    bool force)
+{
+	unsigned int cpu;
+
+	if (!force)
+		cpu = cpumask_any_and(mask_val, cpu_online_mask);
+	else
+		cpu = cpumask_first(mask_val);
+
+	if (cpu >= nr_cpu_ids)
+		return -EINVAL;
+
+	if (!irqd_irq_disabled(d)) {
+		plic_irq_toggle(cpu_possible_mask, d->hwirq, 0);
+		plic_irq_toggle(cpumask_of(cpu), d->hwirq, 1);
+	}
+
+	irq_data_update_effective_affinity(d, cpumask_of(cpu));
+
+	return IRQ_SET_MASK_OK_DONE;
+}
+#endif
+
 static struct irq_chip plic_chip = {
 	.name		= "SiFive PLIC",
 	/*
@@ -122,6 +150,9 @@ static struct irq_chip plic_chip = {
 	 */
 	.irq_enable	= plic_irq_enable,
 	.irq_disable	= plic_irq_disable,
+#ifdef CONFIG_SMP
+	.irq_set_affinity = plic_set_affinity,
+#endif
 };
 
 static int plic_irqdomain_map(struct irq_domain *d, unsigned int irq,
-- 
2.17.1