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From: Emil Renner Berthing <emil.renner.berthing@canonical.com>
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Date: Thu, 21 Dec 2023 09:07:36 -0500
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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 Thu, Dec 21, 2023 at 1:28=E2=80=AFPM Emil Renner Berthing
> <emil.renner.berthing@canonical.com> wrote:
> > 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 independen=
tly from the
> > > > regular pull-up/down, so in theory you could enable both the regula=
r pull-up
> > > > and the strong pull-up at the same time, or even the regular poll-d=
own 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 (fig=
ure 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_u=
nits_2
>
> Yeah hehe elementary electronics beats me, of course it is in parallel.
>
> > The problem is that the documentation doesn't actually mention what wil=
l 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 e=
ver
happens.