Date:   Tue, 14 Feb 2023 19:49:36 +0100
From:   pelzi@...ing-snail.de
To:     Andre Przywara <andre.przywara@....com>,
        Maxime Ripard <maxime@...no.tech>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Chen-Yu Tsai <wens@...e.org>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Samuel Holland <samuel@...lland.org>,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-sunxi@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: dts: allwinner: minimize irq debounce filter per
 default

Am 13.02.23 um 12:56 schrieb Andre Przywara:
> On Mon, 13 Feb 2023 10:18:03 +0100
> Maxime Ripard <maxime@...no.tech> wrote:
>
> Hi,
>
>> On Mon, Feb 13, 2023 at 09:49:55AM +0100, pelzi@...ing-snail.de wrote:
>>> 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.
>> It's not really a matter of hardware here, it's a matter of driver
>> behavior vs generic behavior from the framework. The hardware obviously
>> influences the former, but it's marginal in that discussion.
>>
>> As that whole discussion shows, whether the frequency would be high
>> enough is application dependent, and thus we cannot just claim that it's
>> equivalent in all circumstances.
>>
>> Making such an assumption will just bite someone else down the road,
>> except this time we will have users (you, I'd assume) relying on that
>> behavior so we wouldn't be able to address it.
>>
>> But I also agree with the fact that doing nothing with 0 is bad UX and
>> confusing as well.
>>
>> I still think that we can address both by just erroring out on 0 /
>> printing an error message so that it's obvious that we can't support it,
>> and we wouldn't change the semantics of the property either.
>>
>> And then you can set the actual debouncing time you need instead of
>> "whatever" in the device tree.
> I am on the same page with regards to discouraging 0 as a proper value, and
> that we should warn if this is being used.
> However I think we should at the same time try to still get as low as
> possible when 0 is specified. The debounce property uses microseconds as
> the unit, but even the AW hardware allows us to go lower than this. So we
> would leave that on the table, somewhat needlessly: input-debounce = <1>
> would give us 1333 ns, when the lowest possible is about 42 ns (1/24MHz).
>
> So what about the following:
> We document that 0 does not mean off, but tries to get as low as possible.
> If the driver sees 0, it issues a warning, but still tries to lower the
> debounce period as much as possible, and reports that, like:
> [1.2345678] 1c20800.pinctrl: cannot turn off debouncing, setting to 41.7 ns
That would be trivial to implement. My only concern: this
leaves no way to configure the minimum setting without getting a
warning.

I'd like to show the acutally selected timing for values >= 1 as well (level
info, though), as it hardly ever exactly matches the value given.
> Alternatively we use a different property name, if that is a concern. We
> could then use nanoseconds as a unit value, and then can error out on 0.
> Re-using input-debounce is somewhat dodgy anyway, since the generic
> property is for a single value only, per pin (in the pinmux DT node, not
> in the controller node), whereas we use an array of some non-obvious
> subset of ports.

How to avoid breaking existing devicetrees? Wouldn't it be required to
handle input-debounce as well, but somehow obsolete it?

Cheers,

Andreas

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