Re: Using a GPIO as an interrupt line

Hello Marc,

On Tue, Nov 19, 2019 at 11:46:21AM +0100, Marc Gonzalez wrote:

On 19/11/2019 10:57, Russell King - ARM Linux admin wrote:

quoted

On Tue, Nov 19, 2019 at 10:28:15AM +0100, Marc Gonzalez wrote:

quoted

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>;

As pointed out by ukleinek on IRC, I might have (??) specified the wrong
trigger type. The data-sheet states:
"The TCA9539 open-drain interrupt (INTn) output is activated when any input state
differs from its corresponding Input Port register state, and is used to indicate
to the system master that an input state has changed."
(The data sheet speaks of "INT with a line on top"; what is the typical way to
write that in ASCII? I was told that adding a trailing 'n' or 'b' was common.)

Or a # as pre- or suffix ... (Didn't know about 'b')

According to that description, it looks like IRQ_TYPE_LEVEL_LOW?
(Weird, because it worked well with IRQ_TYPE_LEVEL_HIGH.)

quoted

This specifies an interrupt signal, number 42, on the tlmm interrupt
controller. It isn't a GPIO specification. Not every interrupt is a
GPIO, and some SoCs can have dedicated interrupt pins that are
exactly that.

Hence, needlessly limiting an external device to requiring a GPIO for
its interrupt is detrimental.

That makes complete sense.

IIUC, what is missing in my DT spec is defining pin 42 as a GPIO pin.
Looking more closely at top_exp_rst:

	top_exp_rst: top_exp_rst {
		mux {
			pins = "gpio96";
			function = "gpio";
		};

		config {
			pins = "gpio96";
			drive-strength = <2>;
			bias-pull-down;
		};
	};

IIUC, this defines pin 96 as a GPIO pin (as well as defining some low-level
properties of the pin). Likely I need something similar for pin 42?

Having a pin configured as GPIO is the boot default setting for many
SoCs/pins. So you might get away with not specifying a setting for pin
42, but that's not as robust as configuring that explicitly.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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