Re: [PATCH] pinctrl: armada-37xx: Add edge both type gpio irq support

[Linus Walleij <linus.walleij@xxxxxxxxxx>]

On Tue, Mar 20, 2018 at 10:56 PM, Uwe Kleine-König
<u.kleine-koenig@xxxxxxxxxxxxxx> wrote:

> Maybe I'm wrong, but I wonder if there could be a set of helper
> functions provided by the gpio core that helps implementing this
> software simulation of IRQ_TYPE_EDGE_BOTH reliably (i.e. as good as
> possible in software) to prevent common mistakes.
>
> First draft:
>
>                 disable_irq_nosync(...);
>                 level = gpio_get(...);
>         retry:
>                 if (level)
>                         configure_for_falling_edge();
>                 else
>                         configure_for_raising_edge();
>                 postlevel = gpio_get(...);
>
>                 if (level != postlevel) {
>                         mark_irq_pending(); /* something like desc->istate |= IRQS_PENDING */
>                         level = postlevel;
>                         goto retry;
>                 }
>
>                 enable_irq(...); /* this resends the irq */
>
> I think this only looses an event if there is an edge between gpio_get
> and the configure_for_${some}_edge and another before postlevel = ...
> that make the two events invisible. But I think this is okish, as a
> short spike might also be missed by a hw-edge-detector. And compared to
> the current code there should be no way to end in a state where we
> configured for raising edge and the level is already high.

This is looking good compared to the solutions people have hacked up.

> When the gpio toggles quickly this might keep the cpu busy in an endless
> loop, but such a sequence would also block a controller that can trigger
> on both edges in hardware. Not sure if breaking the loop at some point
> is sensible anyhow. Also calling the irq handlers would be beneficial,
> but I don't know if/how this works without (more) racing.

What would make sense (if you want a perfect solution) is to enforce
some reasonable debouncing on double edges.

That may seem hard to do since not all HW has debounce.

In the past I had the idea to implement also generic debounce with a timer
in gpiolib, so that gpiod_set_debounce() would never fail, so in effect
to factor the code from drivers/input/keyboard/gpio_keys.c
over to gpiolib so they don't need a fallback at all, and then with
double edges, enforce some debouncing based on HZ.

At one point I tried to bring the debounce code over from the
input driver, but I hit some snag, I don't remember what though.
An optional per-gpiod timer can be created in struct gpio_desc
when needed.

> A similar approach would be great to have to "simulate" level sensitive
> irqs if the hardware only implements edge logic (which affects
> armada-37xx, too, which annoys me).

Yes that would be neat too...

Yours,
Linus Walleij
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