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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Message-ID: <CACRpkdbNAeDsi9B14kbkAeoqX7NE_Ua_yOX1iNF75oNK0ELefQ@mail.gmail.com>
Date:   Wed, 9 Dec 2020 13:53:15 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     "Enrico Weigelt, metux IT consult" <lkml@...ux.net>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "Enrico Weigelt, metux IT consult" <info@...ux.net>
Subject: Re: Howto listen to/handle gpio state changes ? Re: [PATCH v2 2/2]
 drivers: gpio: add virtio-gpio guest driver

On Wed, Dec 9, 2020 at 12:19 PM Arnd Bergmann <arnd@...nel.org> wrote:
> On Wed, Dec 9, 2020 at 9:51 AM Linus Walleij <linus.walleij@...aro.org> wrote:
> > On Tue, Dec 8, 2020 at 3:07 PM Enrico Weigelt, metux IT consult <lkml@...ux.net> wrote:
>
> > 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.
>
> I would expect the platform abstraction to actually be close enough
> to a chained irqchip that it actually works: the notification should
> come in via vring_interrupt(), which is a normal interrupt handler
> that calls vq->vq.callback(), calling generic_handle_irq() (and
> possibly chained_irq_enter()/chained_irq_exit() around it) like the
> other gpio drivers do should just work here I think, and if it did
> not, then I would expect this to be just a bug in the driver rather
> than something missing in the gpio framework.

Performance/latency-wise that would also be strongly encouraged.

Tglx isn't super-happy about the chained interrupts at times, as they
can create really nasty bugs, but a pure IRQ in fastpath of some
kinde is preferable and intuitive either way.

Yours,
Linus Walleij