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

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Date: Thu, 21 Dec 2023 04:28:46 -0800
From: Emil Renner Berthing <emil.renner.berthing@...onical.com>
To: Linus Walleij <linus.walleij@...aro.org>, 
	Emil Renner Berthing <emil.renner.berthing@...onical.com>
Cc: Rob Herring <robh@...nel.org>, linux-gpio@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org, 
	Hoan Tran <hoan@...amperecomputing.com>, Serge Semin <fancer.lancer@...il.com>, 
	Bartosz Golaszewski <brgl@...ev.pl>, Andy Shevchenko <andy@...nel.org>, 
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>, Conor Dooley <conor+dt@...nel.org>, 
	Jisheng Zhang <jszhang@...nel.org>, Guo Ren <guoren@...nel.org>, Fu Wei <wefu@...hat.com>, 
	Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>
Subject: Re: [PATCH v1 1/8] dt-bindings: pinctrl: Add thead,th1520-pinctrl bindings

Linus Walleij wrote:
> On Sat, Dec 16, 2023 at 2:57 PM Emil Renner Berthing
> <emil.renner.berthing@...onical.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 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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