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

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Date: Thu, 21 Dec 2023 09:07:36 -0500
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 Thu, Dec 21, 2023 at 1:28 PM Emil Renner Berthing
> <emil.renner.berthing@...onical.com> wrote:
> > 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
>
> Yeah hehe elementary electronics beats me, of course it is in parallel.
>
> > The problem is that the documentation doesn't actually mention what will happen
> > if you combine the strong pull-up with the regular bias.
>
> So why even allow it then?
>
> Do the people designing boards using this have better documentation than what
> you have? Then either get that documentation or just don't give them
> too much rope.

We can certainly prevent Linux from ever combining the strong pull-up with the
regular bias, but that doesn't mean that the vendor u-boot can't find a use for
it and might hand over pins in such states Linux then wouldn't know how to
handle.

If you think its better we could just postpone that problem to when/if it ever
happens.