The gic_all_vpes_local_irq_controller chip currently attempts to operate
on all CPUs/VPs in the system when masking or unmasking an interrupt.
This has a few drawbacks:

 - In multi-cluster systems we may not always have access to all CPUs in
   the system. When all CPUs in a cluster are powered down that
   cluster's GIC may also power down, in which case we cannot configure
   its state.

 - Relatedly, if we power down a cluster after having configured
   interrupts for CPUs within it then the cluster's GIC may lose state &
   we need to reconfigure it. The current approach doesn't take this
   into account.

 - It's wasteful if we run Linux on fewer VPs than are present in the
   system. For example if we run a uniprocessor kernel on CPU0 of a
   system with 16 CPUs then there's no point in us configuring CPUs
   1-15.

 - The implementation is also lacking in that it expects the range
   0..gic_vpes-1 to represent valid Linux CPU numbers which may not
   always be the case - for example if we run on a system with more VPs
   than the kernel is configured to support.

Fix all of these issues by only configuring the affected interrupts for
CPUs which are online at the time, and recording the configuration in a
new struct gic_all_vpes_chip_data for later use by CPUs being brought
online. We register a CPU hotplug state (reusing
CPUHP_AP_IRQ_GIC_STARTING which the ARM GIC driver uses, and which seems
suitably generic for reuse with the MIPS GIC) and execute
irq_cpu_online() in order to configure the interrupts on the newly
onlined CPU.

Signed-off-by: Paul Burton <redacted>
Cc: Jason Cooper <redacted>
Cc: Marc Zyngier <redacted>
Cc: Thomas Gleixner <redacted>
Cc: linux-mips@linux-mips.org
---

 drivers/irqchip/irq-mips-gic.c | 72 ++++++++++++++++++++++++++++++++----------
 1 file changed, 56 insertions(+), 16 deletions(-)

diff --git a/drivers/irqchip/irq-mips-gic.c b/drivers/irqchip/irq-mips-gic.c
index 6fdcc1552fab..dd9da773db90 100644
--- a/drivers/irqchip/irq-mips-gic.c
+++ b/drivers/irqchip/irq-mips-gic.c

@@ -622,6 +653,13 @@ static const struct irq_domain_ops gic_ipi_domain_ops = {
 	.match = gic_ipi_domain_match,
 };
 
+static int gic_cpu_startup(unsigned int cpu)
+{
+	/* Invoke irq_cpu_online callbacks to enable desired interrupts */
+	irq_cpu_online();
+
+	return 0;
+}
 
 static int __init gic_of_init(struct device_node *node,
 			      struct device_node *parent)

@@ -768,6 +806,8 @@ static int __init gic_of_init(struct device_node *node,
 		}
 	}
 
-	return 0;
+	return cpuhp_setup_state(CPUHP_AP_IRQ_GIC_STARTING,
+				 "irqchip/mips/gic:starting",
+				 gic_cpu_startup, NULL);