Re: [PATCH] ARM: dts: allwinner: minimize irq debounce filter per default

[pelzi]

Am 13.02.23 um 09:43 schrieb Maxime Ripard:
> On Fri, Feb 10, 2023 at 10:18:14AM +0000, Andre Przywara wrote:
> > > > Not sure if you were actually arguing this, but the change I sketched
> > > > above (interpreting 0 as 24MHz/1) is separate though, as the current
> > > > default is "no DT property", and not 0. There is no input-debounce
> > > > property user in the kernel tree at the moment, so we wouldn't break
> > > > anyone. The only thing that would change is if a downstream user was
> > > > relying on "0" being interpreted as "skip the setup", which isn't
> > > > really documented and could be argued to be an implementation detail.
> > > >
> > > > So I'd suggest to implement 0 as "lowest possible", and documenting that
> > > > and the 32KHz/1 default if no property is given.
> > > Ah, my bad.
> > >
> > > There's another thing to consider: there's already a generic per-pin
> > > input-debounce property in pinctrl.
> > >
> > > Since we can't control it per pin but per bank, we moved it to the
> > > controller back then, but there's always been this (implicit)
> > > expectation that it was behaving the same way.
> > >
> > > And the generic, per-pin, input-debounce documentation says:
> > >
> > > > Takes the debounce time in usec as argument or 0 to disable debouncing
> > > I agree that silently ignoring it is not great, but interpreting 0 as
> > > the lowest possible is breaking that behaviour which, I believe, is a
> > > worse outcome.
> > Is it really? If I understand the hardware manuals correctly, we cannot
> > really turn that feature off, so isn't the lowest possible time period (24
> > MHz/1 at the moment) the closest we can get to "turn it off"? So
> > implementing this would bring us actually closer to the documented
> > behaviour? Or did I get the meaning of this time period wrong?
> > At least that's my understanding of how it fixed Andreas' problem: 1µs
> > is still not "off", but much better than the 31µs of the default. The new
> > 0 would then be 0.041µs.
> My point was that the property we share the name (and should share the
> semantics with) documents 0 as disabled. We would have a behavior that
> doesn't disable it. It's inconsistent.
>
> The reason doesn't really matter, we would share the same name but have
> a completely different behavior, this is super confusing to me.

I got the point. As far as I can tell from the datasheet, it is not possible
to actually switch off input-debounce. But as a debounce filter is actually
a low-pass filter, setting the cut-off frequency as high as possible,
appears to be the equivalent to switching it off.
To me it does not appear inconsistent, as any hardware will have an
implicit cut-off frequency given by its physical capabilites.

Cheers,

Andreas.