linux-kernel - Re: [PATCH v5 2/3] gpio: loongson: add gpio driver support

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Date:   Thu, 24 Nov 2022 09:54:43 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Yinbo Zhu <zhuyinbo@...ngson.cn>
Cc:     Bartosz Golaszewski <brgl@...ev.pl>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        WANG Xuerui <kernel@...0n.name>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Juxin Gao <gaojuxin@...ngson.cn>,
        Bibo Mao <maobibo@...ngson.cn>,
        Yanteng Si <siyanteng@...ngson.cn>, linux-gpio@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        loongarch@...ts.linux.dev, linux-mips@...r.kernel.org,
        Arnaud Patard <apatard@...driva.com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Jianmin Lv <lvjianmin@...ngson.cn>,
        Hongchen Zhang <zhanghongchen@...ngson.cn>,
        Liu Peibao <liupeibao@...ngson.cn>
Subject: Re: [PATCH v5 2/3] gpio: loongson: add gpio driver support

On Thu, Nov 24, 2022 at 3:22 AM Yinbo Zhu <zhuyinbo@...ngson.cn> wrote:
> 在 2022/11/24 上午6:05, Linus Walleij 写道:

> > But these drivers can not rely on the .gpio_to_irq() callback
> > to be called before an IRQ is requested and used.
>
> I may not have made it clear before that the gpio irq chip for other
> platforms may need to be implemented, but the loongson platform may be
> special.
>
> I mean that the loongson platform use gpio irq does not need to rely on
> gpio_to_irq, because loongson interrupt controller driver has covered
> gpio irq.  The specific reason is my above explanation.
>
> so, Can I not realize gpio irq chip?

Isn't this a hierarchical irqchip then?

Please consult the following from
Documentation/driver-api/gpio/driver.rst:

---------------------------------

GPIO drivers providing IRQs
===========================

It is custom that GPIO drivers (GPIO chips) are also providing interrupts,
most often cascaded off a parent interrupt controller, and in some special
cases the GPIO logic is melded with a SoC's primary interrupt controller.

The IRQ portions of the GPIO block are implemented using an irq_chip, using
the header <linux/irq.h>. So this combined driver is utilizing two sub-
systems simultaneously: gpio and irq.

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.

We can divide GPIO irqchips in two broad categories:

- CASCADED INTERRUPT CHIPS: this means that the GPIO chip has one common
  interrupt output line, which is triggered by any enabled GPIO line on that
  chip. The interrupt output line will then be routed to an parent interrupt
  controller one level up, in the most simple case the systems primary
  interrupt controller. This is modeled by an irqchip that will inspect bits
  inside the GPIO controller to figure out which line fired it. The irqchip
  part of the driver needs to inspect registers to figure this out and it
  will likely also need to acknowledge that it is handling the interrupt
  by clearing some bit (sometime implicitly, by just reading a status
  register) and it will often need to set up the configuration such as
  edge sensitivity (rising or falling edge, or high/low level interrupt for
  example).

- HIERARCHICAL INTERRUPT CHIPS: this means that each GPIO line has a dedicated
  irq line to a parent interrupt controller one level up. There is no need
  to inquire the GPIO hardware to figure out which line has fired, but it
  may still be necessary to acknowledge the interrupt and set up configuration
  such as edge sensitivity.

---------------------------------

You find an example of a hierarchical GPIO irqchip using the
GPIOLIB_IRQCHIP in drivers/gpio/gpio-ixp4xx.c.

Yours,
Linus Walleij