Re: Using a GPIO as an interrupt line

[Bjorn Andersson <bjorn.andersson@linaro.org>]

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

> On 19/11/2019 11:58, Russell King - ARM Linux admin wrote:
> 
> > 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:
> >>
> >>> On Tue, Nov 19, 2019 at 10:28:15AM +0100, Marc Gonzalez wrote:
> >>>
> >>>> 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).
> 
> 
> > 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