linux-kernel - Re: [PATCH 2/2] HID: i2c-hid: Add support for GPIO interrupts

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Date:	Tue, 27 Jan 2015 10:39:25 +0000
From:	Mark Rutland <mark.rutland@....com>
To:	Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:	Jiri Kosina <jkosina@...e.cz>,
	Benjamin Tissoires <benjamin.tissoires@...hat.com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <Pawel.Moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
	"linux-input@...r.kernel.org" <linux-input@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] HID: i2c-hid: Add support for GPIO interrupts

On Tue, Jan 27, 2015 at 10:16:10AM +0000, Mika Westerberg wrote:
> On Mon, Jan 26, 2015 at 04:39:30PM +0000, Mark Rutland wrote:
> > What I don't follow is why GpioInt seems to be translated as a GPIO
> > rather than as an interrupt which happens to be backed by a GPIO. Were
> > it not for that, the DT and ACPI cases would align better.
> 
> Because it *is* a GPIO.

I don't disagree on this point.

However, this is irrelevant from the PoV of the device in question. It
doesn't care what its interrupt line is wired up to so long as it
behaves like an interrupt. Hopefully the example below will clarify what
I'm getting at here.

> In my experience we have had two kinds of interrupts that the devices
> are connected to: pins connected directly to the interrupt controller
> (IO-APIC or whatever), and pins connected to the GPIO controller. This
> is the later.
> 
> Now, the device in question has following resources:
> 
> 	Name (_CRS, ResourceTemplate () {
> 		I2cSerialBus (0x004C, ControllerInitiated, 0x00061A80,
> 		    AddressingMode7Bit, "\\_SB.PCI0.I2C6",
> 		    0x00, ResourceConsumer,,)
> 		GpioInt (Level, ActiveLow, Shared, PullDefault, 0x0000,
> 		    "\\_SB.GPO0", 0x00, ResourceConsumer,,)
> 		    {
> 			0x004C
> 		    }
> 	})
> 
> The GpioInt() above refers to GPIO controller ("\_SB.GPO0") and its pin
> number 0x4c.
> 
> Normally I would do this in the driver regardless of where it is
> described (DT, ACPI, whatnot):
> 
> 	desc = gpiod_get(dev, NULL);
> 	gpiod_direction_input(desc);
> 	irq = gpiod_to_irq(desc);
> 
> Then the "irq" can be used for request_irq() and friends.
> 
> Note how both DT and ACPI cases align just fine.
> 
> If the above is not the right way to use GPIOs as interrupt, can you
> please tell me how it is done then?

So lets say we have a device which generates an interrupt:

	device@f00 {
		compatible = "some-interrupting-device";
		reg = <0xf00 0x100>;
		interrupts = < ... >;
	};

It's intended that this is connected to an interrupt controller:

	ic: interrupt-controller@b00 {
		compatible = "some-interrupt-controller";
		reg = <0xb00 0x100>;
		#interrupt-cells = <1>;
	};

	device@f00 {
		compatible = "some-interrupting-device";
		reg = <0xf00 0x100>;
		interrupt-parent = <&ic>;
		interrupts = <0x3>;
	};

But in some cases, this gets connected to a GPIO controller. In these
cases, the device is still logically generating an interrupt, and the
fact that the endpoint is an interrupt controller is irrelevant from the
PoV of the device. So we acknowledge that the GPIO controller is also
capable of acting as an interrupt controller, and mark it as such:

	gc: gpio-controller@000 {
		compatible = "some-gpio-controller";
		reg = <0x000 0x100>;
		#gpio-cells = <1>;
		#interrupt-cells = <1>;
	};

	device@f00 {
		compatible = "some-interrupting-device";
		reg = <0xf00 0x100>;
		interrupt-parent = <&gc>;
		interrupts = <0x1>;
	};

Thus the device binding only describes the logical interrupt, and the
driver only needs to handle interrupts.

In cases where the binding/driver actually care about the GPIO being a
GPIO (e.g. for card detect in an MMC controller), describing the GPIO as
a GPIO makes sense, and we can try gpio_to_irq as an optimisation over
polling the state of the GPIO.

> BTW, passing NULL to gpiod_get() implies property named "gpios" in DT
> (which is why I added it to the documentation).

Sure. My concern is that we should not need to deal with GPIOs in this
case were the GPIO is only there to function as an interrupt.

Given that GpioInt seems to describe an interrupt which happens to be
backed by a GPIO, I don't understand what it is necessary to translate
this as a GPIO rather than an interrupt. If it were going to be used as
a GPIO, then it would be a GpioIO object, no?

Thanks,
Mark.
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