Re: [PATCH V3 2/3] gpio: virtio: Add IRQ support

[Linus Walleij <linus.walleij@linaro.org>]

Hi Viresh!

thanks for this interesting patch!

On Thu, Jun 10, 2021 at 2:16 PM Viresh Kumar <viresh.kumar@linaro.org> wrote:

> This patch adds IRQ support for the virtio GPIO driver. Note that this
> uses the irq_bus_lock/unlock() callbacks since the operations over
> virtio can sleep.
>
> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

>  drivers/gpio/gpio-virtio.c       | 256 ++++++++++++++++++++++++++++++-
>  include/uapi/linux/virtio_gpio.h |  15 ++

You also need to
select GPIOLIB_IRQCHIP
in the Kconfig entry for the gpio-virtio driver, because you make
use of it.

> +static void virtio_gpio_irq_mask(struct irq_data *d)
> +{
> +       struct gpio_chip *gc = irq_data_to_gpio_chip(d);
> +       struct virtio_gpio *vgpio = gpio_chip_to_vgpio(gc);
> +       struct vgpio_line *line = &vgpio->lines[d->hwirq];
> +
> +       line->masked = true;
> +       line->masked_pending = true;
> +}
> +
> +static void virtio_gpio_irq_unmask(struct irq_data *d)
> +{
> +       struct gpio_chip *gc = irq_data_to_gpio_chip(d);
> +       struct virtio_gpio *vgpio = gpio_chip_to_vgpio(gc);
> +       struct vgpio_line *line = &vgpio->lines[d->hwirq];
> +
> +       line->masked = false;
> +       line->masked_pending = true;
> +}

This looks dangerous in combination with this:

> +static void virtio_gpio_interrupt(struct virtqueue *vq)
> +{
(...)
> +       local_irq_disable();
> +       ret = generic_handle_irq(irq);
> +       local_irq_enable();

Nominally slow IRQs like those being marshalled over
virtio should be nested, handle_nested_irq(irq);
but are they? Or are they just quite slow not super slow?

If a threaded IRQF_ONESHOT was requested the
IRQ core will kick the thread and *MASK* this IRQ,
which means it will call back to your .irq_mask() function
and expect it to be masked from this
point.

But the IRQ will not actually be masked until you issue
your callbacks in the .irq_bus_sync_unlock() callback
right?

So from this point until .irq_bus_sync_unlock()
get called and actually mask the IRQ, it could be
fired again? I suppose the IRQ handler is reentrant?
This would violate the API.

I would say that from this point and until you sync
you need a spinlock or other locking primitive to
stop this IRQ from fireing again, and a spinlock will
imply local_irq_disable() so this gets really complex.

I would say only using nesting IRQs or guarantee this
some other way, one way would be to specify that
whatever is at the other side of virtio cannot send another
GPIO IRQ message before the last one is handled,
so you would need to send a specific (new)
VIRTIO_GPIO_REQ_IRQ_ACK after all other messages
have been sent in .irq_bus_sync_unlock()
so that the next GPIO IRQ can be dispatched after that.

(Is this how messaged signalled interrupts work? No idea.
When in doubt ask the IRQ maintainers.)

Thanks,
Linus Walleij