Using a GPIO as an interrupt line

[Marc Gonzalez <marc.w.gonzalez@xxxxxxx>]

Hello,

The board I'm working on provides a TCA9539 I/O expander.
Or, as the datasheet(*) calls it, a "Low Voltage 16-Bit I2C and
SMBus Low-Power I/O Expander with Interrupt Output, Reset Pin,
and Configuration Registers"

(*) http://www.ti.com/lit/ds/symlink/tca9539.pdf

The binding is documented in Documentation/devicetree/bindings/gpio/gpio-pca953x.txt

I have some doubts about the interrupt output, described as:

Optional properties:
 - interrupts: interrupt specifier for the device's interrupt output.

In my board's DT, the I/O expander is described as:

	exp1: gpio@74 {
		compatible = "ti,tca9539";
		reg = <0x74>;
		gpio-controller;
		#gpio-cells = <2>;
		reset-gpios = <&tlmm 96 GPIO_ACTIVE_LOW>;
		pinctrl-names = "default";
		pinctrl-0 = <&top_exp_rst>;
		interrupt-parent = <&tlmm>;
		interrupts = <42 IRQ_TYPE_LEVEL_HIGH>;
	};


("tlmm" is a pinctrl for qualcomm SoCs.)

The problem with this DT node is that nowhere have I specified that
the interrupt output is itself a GPIO line... I see other drivers
calling gpiod_to_irq() to handle this kind of setup. I assume this
function marks the GPIO as "used for interrupt, no longer available
as software-controlled GPIO"?

The tlmm node has the following properties:

	tlmm: pinctrl@03400000 {
		compatible = "qcom,msm8998-pinctrl";
		reg = <0x03400000 0xc00000>;
		interrupts = <0 208 0>;
		gpio-controller;
		#gpio-cells = <2>;
		interrupt-controller;
		#interrupt-cells = <2>;

Does gpio-controller and/or interrupt-controller mean something
is automagically happening when exp1's interrupts = <42 IRQ_TYPE_LEVEL_HIGH>
prop is being processed?

Or maybe I just have no idea what gpiod_to_irq() is used for?

I'd appreciate any help clearing my confusion.

Regards.