Re: how to request gpiochip line which is only valid as an interrupt?

["Marek Behún" <kabel@kernel.org>]

On Mon, 10 Jun 2024 16:01:55 +0200
Marek Behún <kabel@kernel.org> wrote:

> Hello Bartosz,
> 
> I would like to ask you if you could find some time to look at
> 
>   [PATCH v11 6/8] platform: cznic: turris-omnia-mcu: Add support for
>                   MCU provided TRNG
> 
>   https://lore.kernel.org/soc/20240605161851.13911-7-kabel@kernel.org/
> 
> Andy Shevchenko added you to that conversation asking you about how to
> correctly do the following part:
> 
>   irq = gpiod_to_irq(gpiochip_get_desc(&mcu->gc, irq_idx));
> 
> I am writing this to give some more light into the problem. What is
> going on:
> - the turris-omnia-mcu driver provides a gpio chip with interrupts
> - some lines are gpio + irq, but some lines are interrupt only
> - later, after the gpiochip is registered, another part of the
>   turris-omnia-mcu driver wants to use one interrupt only line
> 
> To use the gpiod_to_irq() function, I need gpio descriptor for that
> line. I can get that with gpiochip_get_desc(), since this is within the
> driver, I have access to the gpiochip. But this is semantically a
> little weird, because
> 
>   1. gpiochip_get_desc() is supposed to be used by gpio driver, not
>      consumer (and the trng part of the turris-omnia-mcu code is a
>      consumer of the gpio)
> 
>   2. reference counting?
> 
> Looking at gpiolib, maybe the better function to use would be
> gpiochip_request_own_desc(). This also is defined in
> include/gpio/driver.c instead of include/gpio/consumer.c, but at least
> it's name suggests that it is used by code that also owns the
> gpiochip...
> 
> One problem is that gpiochip_request_own_desc() won't work, because the
> gpiochip initializes valid masks for both gpios and irqs, and the 
> gpiochip_request_own_desc() function calls gpiod_request_commit(),
> which executes the following code
> 
>   if (guard.gc->request) {
>     offset = gpio_chip_hwgpio(desc);
>     if (gpiochip_line_is_valid(guard.gc, offset))
>       ret = guard.gc->request(guard.gc, offset);
>     else
>       ret = -EINVAL;
>     ...
>   }
> 
> So if a gpiochip line is not valid GPIO, only valid IRQchip line, then
> the GPIO cannot be requested, even for interrupts.
> 
> What is the proper solution here?
> 
> Thank you
> 
> Marek

Bart, Andy,

it seems that if I write the mcu DT node interrupt property which
refers to self, i.e.:

  mcu: system-controller@2a {
    ...

    interrupts-extended = <&gpio1 11 IRQ_TYPE_NONE>,
                          <&mcu 13 IRQ_TYPE_NONE>;
    interrupt-names = "irq", "trng";

    ...
  };

it seems to work and I can use

  irq = fwnode_irq_get_byname(dev_fwnode(dev), "trng");

even if this is called from the mcu probe method.

Do you think this is a proper solution?

I find it a little bit weird that the mcu DT node refers to itself in
it's interrupt properties.

Marek