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From: Viresh Kumar <viresh.kumar@linaro.org>
To: Arnd Bergmann <arnd@kernel.org>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
	Viresh Kumar <vireshk@kernel.org>,
	Linus Walleij <linus.walleij@linaro.org>,
	Cornelia Huck <cohuck@redhat.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"open list:DRM DRIVER FOR QEMU'S CIRRUS DEVICE" 
	<virtualization@lists.linux-foundation.org>,
	Bartosz Golaszewski <bgolaszewski@baylibre.com>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	"open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
	Marc Zyngier <maz@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Stratos Mailing List <stratos-dev@op-lists.linaro.org>,
	"Enrico Weigelt, metux IT consult" <info@metux.net>,
	Jason Wang <jasowang@redhat.com>
Subject: Re: [Stratos-dev] [PATCH V4 2/2] gpio: virtio: Add IRQ support
Date: Thu, 5 Aug 2021 18:19:22 +0530	[thread overview]
Message-ID: <20210805124922.j7lts7tfmm4t2kpf@vireshk-i7> (raw)
In-Reply-To: <CAK8P3a0DWkfQcZpmyfKcdNt1MHf8ha6a9L2LmLt1Tv-j0HDr3w@mail.gmail.com>

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.

> Also, I have no idea about whether using the handle_level_irq()
> function is actually correct here. I suppose if necessary, the driver
> could provide its own irq.handler callback in place of that.

After looking at internals of these, I felt handle_level_irq() suits
much better in our case as we need to queue the buffer only at
unmask(). With handle_fasteoi_irq(), we would be required to do the
same from multiple places, unmask(), eoi().

> > Also I have broken the rule from specs, maybe we should update spec
> > with that, where the specs said that the buffer must not be queued
> > before enabling the interrupt. I just queue the buffer unconditionally
> > now from unmask().
> >
> > I am not sure but there may be some race around the "queued" flag and
> > I wonder if we can land in a scenario where the buffer is left
> > un-queued somehow, while an interrupt is enabled.
> 
> Can that be integrated with the "masked" state now? It looks like
> the two flags are always opposites now.

Yeah, but then there can be races and keeping them separate is a
better thing IMO.

I was thinking of something on these lines, disable_irq() followed by
enable_irq():

CPU0                                                 CPU1

disable_irq()
 -> irq_bus_lock()
 -> irq_mask()
 -> irq_bus_sync_unlock()
   -> sends blocking VIRTIO_GPIO_MSG_IRQ_TYPE
      to disable interrupt
                                                     Backend (at host) disables irq and 
                                                     returns the unused buffer.

enable_irq()
 -> irq_bus_lock()
 -> irq_unmask()
   -> Tries to queues buffer again
      Finds it already queued and returns.
                                                     - virtio_gpio_event_vq() runs at guest
                                                     - Finds VIRTIO_GPIO_IRQ_STATUS_INVALID in status
                                                     - returns without doing anything
 -> irq_bus_sync_unlock()
   -> sends blocking VIRTIO_GPIO_MSG_IRQ_TYPE
      to enable interrupt



So the irq is still enabled and the buffer isn't queued. Yes, need
some locking here for sure, confirmed :)

-- 
viresh

  reply	other threads:[~2021-08-05 12:49 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-03 11:36 [PATCH V4 0/2] gpio: Add virtio based driver Viresh Kumar
2021-08-03 11:36 ` [PATCH V4 1/2] gpio: Add virtio-gpio driver Viresh Kumar
2021-08-03 11:36 ` [PATCH V4 2/2] gpio: virtio: Add IRQ support Viresh Kumar
2021-08-03 15:01   ` Arnd Bergmann
2021-08-04  7:05     ` Viresh Kumar
2021-08-04  8:27       ` Arnd Bergmann
2021-08-05  7:05         ` Viresh Kumar
2021-08-05 11:26     ` Viresh Kumar
     [not found]     ` <0100017b1610f711-c53c79f2-9e28-4c45-bb42-8db09688b18e-000000@email.amazonses.com>
2021-08-05 12:03       ` [Stratos-dev] " Arnd Bergmann
2021-08-05 12:49         ` Viresh Kumar [this message]
2021-08-05 13:10           ` Arnd Bergmann
2021-08-06  7:44             ` Viresh Kumar
     [not found]             ` <0100017b1a6c0a05-e41dc16c-b326-4017-a63d-a24a6c1fde70-000000@email.amazonses.com>
2021-08-06  8:00               ` Arnd Bergmann
2021-08-09  7:30                 ` Viresh Kumar
2021-08-09  7:55                   ` Arnd Bergmann
2021-08-09 10:46                     ` Viresh Kumar
     [not found]                     ` <0100017b2a85eaf8-08b905fc-89f7-43a4-857e-070ca9691ce1-000000@email.amazonses.com>
2021-08-09 11:19                       ` Arnd Bergmann
2021-08-10  7:35                         ` Viresh Kumar

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