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: [e-mail] # 3.13+
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Brian Norris <[e-mail]>
Signed-off-by: Florian Fainelli <[e-mail]>
Signed-off-by: Grant Likely <[e-mail]>

Documentation/devicetree/bindings/interrupt-controller/interrupts.txt

···
 1) Interrupt client nodes
 -------------------------
 
-Nodes that describe devices which generate interrupts must contain an either an
-"interrupts" property or an "interrupts-extended" property. These properties
-contain a list of interrupt specifiers, one per output interrupt. The format of
-the interrupt specifier is determined by the interrupt controller to which the
-interrupts are routed; see section 2 below for details.
+Nodes that describe devices which generate interrupts must contain an
+"interrupts" property, an "interrupts-extended" property, or both. If both are
+present, the latter should take precedence; the former may be provided simply
+for compatibility with software that does not recognize the latter. These
+properties contain a list of interrupt specifiers, one per output interrupt. The
+format of the interrupt specifier is determined by the interrupt controller to
+which the interrupts are routed; see section 2 below for details.
 
   Example:
 	interrupt-parent = <&intc1>;

drivers/of/irq.c

···
 	/* 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);