linux-kernel - Re: [PATCH 3/3] gpio: regmap: Support a custom ->to

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Date:   Tue, 5 Jul 2022 01:05:20 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Aidan MacDonald <aidanmacdonald.0x0@...il.com>
Cc:     michael@...le.cc, brgl@...ev.pl, linux-gpio@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] gpio: regmap: Support a custom ->to_irq() hook

On Sun, Jul 3, 2022 at 1:10 PM Aidan MacDonald
<aidanmacdonald.0x0@...il.com> wrote:

> Some GPIO chips require a custom to_irq() callback for mapping
> their IRQs, eg. because their interrupts come from a parent IRQ
> chip where the GPIO offset doesn't map 1-to-1 with hwirq number.
>
> Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@...il.com>

What is the usecase for this?

Since GPIO chips and IRQ chips are orthogonal there is absolutely
no guarantee that ->to_irq() is called before a driver start to use
an IRQ from the irqchip side:

(quoting Documentation/driver-api/gpio/driver.rst)

 It is legal for any IRQ consumer to request an IRQ from any irqchip even if it
 is a combined GPIO+IRQ driver. The basic premise is that gpio_chip and
 irq_chip are orthogonal, and offering their services independent of each
 other.

 gpiod_to_irq() is just a convenience function to figure out the IRQ for a
 certain GPIO line and should not be relied upon to have been called before
 the IRQ is used.

 Always prepare the hardware and make it ready for action in respective
 callbacks from the GPIO and irq_chip APIs. Do not rely on gpiod_to_irq() having
 been called first.

(end quote)

Using ->to_irq() makes sense in a few cases such as when
a GPIO key that can also poll for state want to get hold of an
IRQ to react to edges.

Now: if a consumer requests IRQ nr 3 from your driver say from ACPI or
from a device tree, and as you say GPIOs and IRQs are not 1-to-1 mapped,
so IRQ nr 3 may be coming from GPIO 314, isn't this going to be really
messy for users? One local numberspace for GPIO and another local
numberspace for IRQs?

To me it seems like the reasoning is something like

- I only use GPIO line numbers like <&gpio 3>;
- Then I call gpiod_to_irq() on that number so I do not need to
  deal with looking up the IRQ some other way
- request_irq();
- Profit.

There is no guarantee that the API will be used like that at all, actually
it is uncommon.

Yours,
Linus Walleij