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From: Emil Renner Berthing <emil.renner.berthing@canonical.com>
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Date: Thu, 21 Dec 2023 04:28:46 -0800
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Subject: Re: [PATCH v1 1/8] dt-bindings: pinctrl: Add thead,th1520-pinctrl bindings
To: Linus Walleij <linus.walleij@linaro.org>, 
	Emil Renner Berthing <emil.renner.berthing@canonical.com>
Cc: Rob Herring <robh@kernel.org>, linux-gpio@vger.kernel.org, devicetree@vger.kernel.org, 
	linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org, 
	Hoan Tran <hoan@os.amperecomputing.com>, Serge Semin <fancer.lancer@gmail.com>, 
	Bartosz Golaszewski <brgl@bgdev.pl>, Andy Shevchenko <andy@kernel.org>, 
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>, Conor Dooley <conor+dt@kernel.org>, 
	Jisheng Zhang <jszhang@kernel.org>, Guo Ren <guoren@kernel.org>, Fu Wei <wefu@redhat.com>, 
	Paul Walmsley <paul.walmsley@sifive.com>, Palmer Dabbelt <palmer@dabbelt.com>
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Linus Walleij wrote:
> On Sat, Dec 16, 2023 at 2:57=E2=80=AFPM Emil Renner Berthing
> <emil.renner.berthing@canonical.com> 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 pu=
ll-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 =3D <100>; <- Same thing if the ordinary is 100 Ohm (figure =
out what
>   resistance it actually is....)
> bias-pull-up =3D <21000000>; <- strong pull up
> bias-pull-up =3D <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 ha=
ppen
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 v=
alue?

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

> > So the idea here was just to make sure that you can do eg.
> >
> >         thead,strong-pull-up =3D <0>;
> >
> > to make sure the bit is cleared.
>
> No use bias-disable; for this.
>
> Yours,
> Linus Walleij
>
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