Re: Using a GPIO as an interrupt line

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

On Tue 19 Nov 11:23 PST 2019, Jeffrey Hugo wrote:

> On Tue, Nov 19, 2019 at 12:20 PM Bjorn Andersson
> <bjorn.andersson@linaro.org> wrote:
> >
> > 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.
> 
> Are you sure?  "SSC" is the short hand for the sensor subsystem in the
> documentation I see.

No, you're right. I guess the last two abbreviations where getting
old...

Regards,
Bjorn