Date:	Tue, 27 Jan 2015 14:33:34 +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 11:30:41AM +0000, Mika Westerberg wrote:
> On Tue, Jan 27, 2015 at 11:14:58AM +0000, Mark Rutland wrote:
> > On Tue, Jan 27, 2015 at 10:59:31AM +0000, Mika Westerberg wrote:
> > > On Tue, Jan 27, 2015 at 10:39:25AM +0000, Mark Rutland wrote:
> > > > > 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.
> > > 
> > > OK.
> > > 
> > > > 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.
> > > 
> > > Well, I've seen touch panels where you actually need to switch the GPIO
> > > to be output and do some magic before you can use the same GPIO as an
> > > interrupt.
> > 
> > Ok. That's a nasty case, but surely in that case the relevant GPIO
> > shoiuld be a GpioIO object for output?
> 
> I can't remember the details anymore, possibly it was GpioIo().
> 
> Nothing prevents you from using GpioIo() as an interrupt.

Certainly. As I mention above, in the case of something like a card
detect pin, it makes sense to be able to acquire an interrupt for 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?
> > > 
> > > OK, so where do you propose we handle the translation if not in the
> > > driver? Also keep in mind that some of the devices may have multiple
> > > GpioInt()s.
> > 
> > To me it seems that GpioInt objects should be translated to interrupts
> > by some core code. How are interrupts described and handed in ACPI? Are
> > they resource along the lines of GpioInts, or are they a completely
> > separate class of device property?
> 
> They are similar resources in _CRS, like GpioIo/GpioInt etc. Below is
> from another touch panel:
> 
> 	Name (_CRS, ResourceTemplate () {
> 		I2cSerialBus (0x004C, ControllerInitiated, 0x00061A80,
> 			AddressingMode7Bit, "\\_SB.PCI0.I2C1", 0x00, ResourceConsumer,,)
> 		Interrupt (ResourceConsumer, Level, ActiveLow, Exclusive, ,, )
> 		{
> 		    0x00000022,
> 		}
> 	})
> 
> If we see one of the above we automatically add it to client->irq in
> case of I2C device.

Ok, that allays my fear w.r.t. ordering of the resources.

As I see it, the fact that we convert GpioInt entries to GPIOs rather
than irqs when parsing _CRS is the issue here, and to me it makes no
sense that we do so. Were we to treat them as interrupts, the binding is
fine as-is, and we'd do the same thing in DT and ACPI.

The reason GpioInt is separate from GpioIo is that a GpioInt _is_ an
interrupt (which happens to be backed by a GPIO), and is not something
that necessarily makes sense as a GPIO.

So why do we currently ignore the GpioInt/GpioIo distinction and treat
GpioInts as GPIOs rather than interrupts?

Thanks,
Mark.
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