linux-kernel - Re: [Stratos-dev] [PATCH V4 2/2] gpio: virtio: Add IRQ support

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Date:   Thu, 5 Aug 2021 15:10:21 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Viresh Kumar <vireshk@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Cornelia Huck <cohuck@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:DRM DRIVER FOR QEMU'S CIRRUS DEVICE" 
        <virtualization@...ts.linux-foundation.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Marc Zyngier <maz@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stratos Mailing List <stratos-dev@...lists.linaro.org>,
        "Enrico Weigelt, metux IT consult" <info@...ux.net>,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [Stratos-dev] [PATCH V4 2/2] gpio: virtio: Add IRQ support

On Thu, Aug 5, 2021 at 2:49 PM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> On 05-08-21, 14:03, Arnd Bergmann wrote:
> > On Thu, Aug 5, 2021 at 1:26 PM Viresh Kumar via Stratos-dev
> > > Based on discussion we had today (offline), I changed the design a bit
> > > and used handle_level_irq() instead, as it provides consistent calls
> > > to mask/unmask(), which simplified the whole thing a bit.
> >
> > The new flow looks much nicer to me, without the workqueue, and
> > doing the requeue directly in the unmask() operation.
> >
> > I don't quite understand the purpose of the type_pending and
> > mask_pending flags yet, can you explain what they actually
> > do?
>
> They are required to make sure we don't send unnecessary
> VIRTIO_GPIO_MSG_IRQ_TYPE events to the device, every time bus_unlock()
> is called.
>
> mask_pending tracks if the masked state has changed since the time
> last bus_unlock() was called. So on an interrupt, both mask() and
> unmask() will get called by the irq-core now and mask_pending will
> change to true (in mask()} and then false (in unmask()). And
> eventually in bus_unlock() we won't send an extra
> VIRTIO_GPIO_MSG_IRQ_TYPE message.

I hope this can still be simplified by working out better which state
transitions are needed exactly. In particular, I would expect that we
can get away with not sending a VIRTIO_GPIO_MSG_IRQ_TYPE
for 'mask' state changes at all, but use that only for forcing 'enabled'
state changes.

One part that I think is missing though is remembering the case
when an eventq message came in after an interrupt got masked
when the message was already armed. In this case, the
virtio_gpio_event_vq() function would not call the irq handler,
but the subsequent "unmask" callback would need to arrange
having it called.

        Arnd