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

[Viresh Kumar <viresh.kumar@linaro.org>]

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