linux-kernel - Re: Howto listen to/handle gpio state changes ? Re: [PATCH v2 2/2] drivers: gpio: add virtio-gpio guest driver

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Date:   Wed, 9 Dec 2020 09:51:30 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     "Enrico Weigelt, metux IT consult" <lkml@...ux.net>
Cc:     Bartosz Golaszewski <bgolaszewski@...libre.com>,
        "Enrico Weigelt, metux IT consult" <info@...ux.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        linux-riscv@...ts.infradead.org
Subject: Re: Howto listen to/handle gpio state changes ? Re: [PATCH v2 2/2]
 drivers: gpio: add virtio-gpio guest driver

On Tue, Dec 8, 2020 at 3:07 PM Enrico Weigelt, metux IT consult
<lkml@...ux.net> wrote:

> I've been looking for some more direct notification callback for gpio
> consumers: here the consumer would register itself as a listener on
> some gpio_desc and called back when something changes (with data what
> exactly changed, eg. "gpio #3 input switched to high").
>
> Seems we currently just have the indirect path via interrupts.

I don't know how indirect it is, it seems pretty direct to me. The subsystem
was designed in response to how the hardware in front of the developers
worked.

So far we have had:
- Cascaded interrupts
- Dedicated (hieararchical) interrupts
- Message Signalled Interrupts

And if you now bring something else to the show then it's not like the
subsystem was designed for some abstract quality such as
generic notification of events that occurred, all practical instances
have been around actual IRQs and that is why it is using
struct irq_chip.

What we need to understand is if your new usecase is an outlier
so it is simplest modeled by a "mock" irq_chip or we have to design
something new altogether like notifications on changes. I suspect
irq_chip would be best because all drivers using GPIOs for interrupts
are expecting interrupts, and it would be an enormous task to
change them all and really annoying to create a new mechanism
on the side.

Yours,
Linus Walleij