of/irq: Fix lookup to use 'interrupts-extended' property first

In case the Device Tree blob passed by the boot agent supplies both an
'interrupts-extended' and an 'interrupts' property in order to allow for
older kernels to be usable, prefer the new-style 'interrupts-extended'
property which conveys a lot more information.

This allows us to have bootloaders willingly maintaining backwards
compatibility with older kernels without entirely deprecating the
'interrupts' property.

Update the bindings documentation to describe a situation where both the
'interrupts-extended' and the 'interrupts' property are present, and
which one takes precedence over the other.

Cc: stable@vger.kernel.org # 3.13+
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>

drivers/of/irq.c

@@ -301,16 +301,17 @@ int of_irq_parse_one(struct device_node *device, int index, struct of_phandle_ar
 	/* Get the reg property (if any) */
 	addr = of_get_property(device, "reg", NULL);
 
+	/* Try the new-style interrupts-extended first */
+	res = of_parse_phandle_with_args(device, "interrupts-extended",
+					"#interrupt-cells", index, out_irq);
+	if (!res)
+		return of_irq_parse_raw(addr, out_irq);
+
 	/* Get the interrupts property */
 	intspec = of_get_property(device, "interrupts", &intlen);
-	if (intspec == NULL) {
-		/* Try the new-style interrupts-extended */
-		res = of_parse_phandle_with_args(device, "interrupts-extended",
-						"#interrupt-cells", index, out_irq);
-		if (res)
-			return -EINVAL;
-		return of_irq_parse_raw(addr, out_irq);
-	}
+	if (intspec == NULL)
+		return -EINVAL;
+
 	intlen /= sizeof(*intspec);
 
 	pr_debug(" intspec=%d intlen=%d\n", be32_to_cpup(intspec), intlen);