Re: [PATCH v1 1/8] dt-bindings: pinctrl: Add thead,th1520-pinctrl bindings

[Emil Renner Berthing <emil.renner.berthing@xxxxxxxxxxxxx>]

Linus Walleij wrote:
> On Sat, Dec 16, 2023 at 2:57 PM Emil Renner Berthing
> <emil.renner.berthing@xxxxxxxxxxxxx> wrote:
>
> > > > +          thead,strong-pull-up:
> > > > +            oneOf:
> > > > +              - type: boolean
> > > > +              - $ref: /schemas/types.yaml#/definitions/uint32
> > > > +                enum: [ 0, 2100 ]
> > > > +            description: Enable or disable strong 2.1kOhm pull-up.
> > >
> > > bias-pull-up can already specify the strength in Ohms.
> >
> > The strong pull up is a separate bit that can be enabled independently from the
> > regular pull-up/down, so in theory you could enable both the regular pull-up
> > and the strong pull-up at the same time, or even the regular poll-down and the
> > strong pull-up which is probably not advised.
>
> bias-pull-up; <- Just regular pulling up the ordinary
> bias-pull-up = <100>; <- Same thing if the ordinary is 100 Ohm (figure out what
>   resistance it actually is....)
> bias-pull-up = <21000000>; <- strong pull up
> bias-pull-up = <21000100>; <- both at the same time

Hmm.. the two pull-ups combined would be a stronger pull-up, eg. lower
resistance, right? So you'd need to calculate it using
https://en.wikipedia.org/wiki/Series_and_parallel_circuits#Resistance_units_2

The problem is that the documentation doesn't actually mention what will happen
if you combine the strong pull-up with the regular bias. My best guess for what
happens if you enable the strong pull-up and the regular pull-down is that you
create a sort of voltage divider. But how do you represent that as an Ohm value?

We would kind of have to, otherwise the pinconf_get callbacks have states that
it can't represent.

> > So the idea here was just to make sure that you can do eg.
> >
> >         thead,strong-pull-up = <0>;
> >
> > to make sure the bit is cleared.
>
> No use bias-disable; for this.
>
> Yours,
> Linus Walleij
>
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