irqchip: vic: Support cascaded VIC in device tree

When transitioning some elder platforms to device tree it
becomes necessary to cascade VIC IRQ chips off another
interrupt controller.

Tested with the cascaded VIC on the Integrator/AP attached
logic module IM-PD1.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200219153543.137153-1-linus.walleij@linaro.org

drivers/irqchip/irq-vic.c

@@ -509,21 +509,22 @@ static int __init vic_of_init(struct device_node *node,
 	void __iomem *regs;
 	u32 interrupt_mask = ~0;
 	u32 wakeup_mask = ~0;
-
-	if (WARN(parent, "non-root VICs are not supported"))
-		return -EINVAL;
+	int parent_irq;
 
 	regs = of_iomap(node, 0);
 	if (WARN_ON(!regs))
 		return -EIO;
 
 	of_property_read_u32(node, "valid-mask", &interrupt_mask);
 	of_property_read_u32(node, "valid-wakeup-mask", &wakeup_mask);
+	parent_irq = of_irq_get(node, 0);
+	if (parent_irq < 0)
+		parent_irq = 0;
 
 	/*
 	 * Passing 0 as first IRQ makes the simple domain allocate descriptors
 	 */
-	__vic_init(regs, 0, 0, interrupt_mask, wakeup_mask, node);
+	__vic_init(regs, parent_irq, 0, interrupt_mask, wakeup_mask, node);
 
 	return 0;
 }