irqchip/gic-v3: Add __gic_get_ppi_index() to find the PPI number from hwirq

gic_get_ppi_index() is a useful concept for ppi partitions, as the GIC
has two PPI ranges but needs mapping to a single range when used as an
index in the gic_data.ppi_descs[] array.

Add a double-underscore version which takes just the intid. This will
be used in the partition domain select and translate helpers to enable
partition support for the EPPI range.

Signed-off-by: James Morse <james.morse@arm.com>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210729172748.28841-2-james.morse@arm.com

drivers/irqchip/irq-gic-v3.c

@@ -446,18 +446,23 @@ static void gic_irq_set_prio(struct irq_data *d, u8 prio)
 	writeb_relaxed(prio, base + offset + index);
 }
 
-static u32 gic_get_ppi_index(struct irq_data *d)
+static u32 __gic_get_ppi_index(irq_hw_number_t hwirq)
 {
-	switch (get_intid_range(d)) {
+	switch (__get_intid_range(hwirq)) {
 	case PPI_RANGE:
-		return d->hwirq - 16;
+		return hwirq - 16;
 	case EPPI_RANGE:
-		return d->hwirq - EPPI_BASE_INTID + 16;
+		return hwirq - EPPI_BASE_INTID + 16;
 	default:
 		unreachable();
 	}
 }
 
+static u32 gic_get_ppi_index(struct irq_data *d)
+{
+	return __gic_get_ppi_index(d->hwirq);
+}
+
 static int gic_irq_nmi_setup(struct irq_data *d)
 {
 	struct irq_desc *desc = irq_to_desc(d->irq);