Re: Using a GPIO as an interrupt line

On Tue 19 Nov 03:46 PST 2019, Marc Gonzalez wrote:

On 19/11/2019 11:58, Russell King - ARM Linux admin wrote:

quoted

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

quoted

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:

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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.)

/INT or nINT are commonly used - I've never heard or seen 'b' (which is
commonly used as a suffix on binary numbers) or a trailing 'n'.

Perhaps the 'b' suffix is only used in French...
'b' might stand for "barre" (i.e. the line above the symbol).

quoted

Is pin 42 something that can be muxed?  If so, it seems sane to specify
configuration for it.  Whether it needs to be a GPIO or whether it has
a specific "interrupt" function mux state depends on the SoC.

According to drivers/pinctrl/qcom/pinctrl-msm8998.c
PINGROUP(42, EAST, blsp_spi6, blsp_uart3_b, blsp_uim3_b, _, qdss, _, _, _, _)

I don't think there is an explicit "interrupt" function in
this pinctrl driver... except FUNCTION(ssc_irq).

No there's no "interrupt" function, the function to be used is "gpio",
which will ensure that the irq logic is available. But in a modern
kernel we're implicitly selecting the "gpio" function if you're
requesting an interrupt. So you shouldn't need to specify this even.

static const char * const ssc_irq_groups[] = {
	"gpio58", "gpio59", "gpio60", "gpio61", "gpio62", "gpio63", "gpio78",
	"gpio79", "gpio80", "gpio117", "gpio118", "gpio119", "gpio120",
	"gpio121", "gpio122", "gpio123", "gpio124", "gpio125",
};

@Bjorn, do you know what these are used for?

The "ssc" would imply that it relates to the secure coprocessor somehow.

Regards,
Bjorn

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