irqchip/sifive-plic: Skip contexts except supervisor in plic_init()

Modify plic_init() to skip .dts interrupt contexts other
than supervisor external interrupt.

The .dts entry for plic may specify multiple interrupt contexts.
For example, it may assign two entries IRQ_M_EXT and IRQ_S_EXT,
in that order, to the same interrupt controller. This patch
modifies plic_init() to skip the IRQ_M_EXT context since
IRQ_S_EXT is currently the only supported context.

If IRQ_M_EXT is not skipped, plic_init() will report "handler
already present for context" when it comes across the IRQ_S_EXT
context in the next iteration of its loop.

Without this patch, .dts would have to be edited to replace the
value of IRQ_M_EXT with -1 for it to be skipped.
Signed-off-by: Alan Mikhak <alan.mikhak@sifive.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Paul Walmsley <paul.walmsley@sifive.com> # arch/riscv
Link: https://lkml.kernel.org/r/1571933503-21504-1-git-send-email-alan.mikhak@sifive.com

drivers/irqchip/irq-sifive-plic.c

@@ -251,8 +251,8 @@ static int __init plic_init(struct device_node *node,
 			continue;
 		}
 
-		/* skip context holes */
-		if (parent.args[0] == -1)
+		/* skip contexts other than supervisor external interrupt */
+		if (parent.args[0] != IRQ_S_EXT)
 			continue;
 
 		hartid = plic_find_hart_id(parent.np);