linux-kernel - Re: [PATCH 8/9] arm64: dts: mediatek: mt8183-kukui: Fix pull-down/up-adv values

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Message-ID: <8be063910b553945da12263818bd05c4e90e27f6.camel@collabora.com>
Date: Tue, 19 Aug 2025 11:12:58 +0200
From: Julien Massot <julien.massot@...labora.com>
To: Chen-Yu Tsai <wenst@...omium.org>, Linus Walleij
 <linus.walleij@...aro.org>
Cc: kernel@...labora.com, Michael Turquette <mturquette@...libre.com>, 
 Stephen Boyd <sboyd@...nel.org>, Rob Herring <robh@...nel.org>, Krzysztof
 Kozlowski <krzk+dt@...nel.org>,  Conor Dooley <conor+dt@...nel.org>,
 Matthias Brugger <matthias.bgg@...il.com>, AngeloGioacchino Del Regno	
 <angelogioacchino.delregno@...labora.com>, Ikjoon Jang <ikjn@...omium.org>,
  Enric Balletbo i Serra	 <eballetbo@...nel.org>, Weiyi Lu
 <weiyi.lu@...iatek.com>, Eugen Hristev	 <eugen.hristev@...aro.org>, Liam
 Girdwood <lgirdwood@...il.com>, Mark Brown	 <broonie@...nel.org>, Julien
 Massot <jmassot@...labora.com>, Sean Wang	 <sean.wang@...nel.org>,
 linux-clk@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	linux-mediatek@...ts.infradead.org, linux-sound@...r.kernel.org, 
	linux-gpio@...r.kernel.org
Subject: Re: [PATCH 8/9] arm64: dts: mediatek: mt8183-kukui: Fix
 pull-down/up-adv values

Hi,
On Tue, 2025-08-19 at 13:29 +0800, Chen-Yu Tsai wrote:
> On Tue, Aug 19, 2025 at 1:27 PM Chen-Yu Tsai <wenst@...omium.org> wrote:
> > 
> > On Mon, Aug 18, 2025 at 11:22 PM Linus Walleij <linus.walleij@...aro.org> wrote:
> > > 
> > > On Wed, Aug 6, 2025 at 8:38 AM Chen-Yu Tsai <wenst@...omium.org> wrote:
> > > > On Fri, Aug 1, 2025 at 7:18 PM Julien Massot wrote
> > > 
> > > > >                 pins-clk {
> > > > >                         pinmux = <PINMUX_GPIO124__FUNC_MSDC0_CLK>;
> > > > >                         drive-strength = <MTK_DRIVE_14mA>;
> > > > > -                       mediatek,pull-down-adv = <10>;
> > > > > +                       mediatek,pull-down-adv = <2>;
> > > > 
> > > >         bias-pull-down = <MTK_PUPD_SET_R1R0_10>;
> > > > 
> > > > and so on.
> > > > 
> > > > ChenYu
> > > 
> > > I agree with ChenYu, the more standardized properties are the better it is.
> > > 
> > > All the custom properties makes sense for an engineer working with just
> > > that one SoC (like the SoC vendor...) but for field engineers who have
> > > to use different SoCs every day this is just a big mess for the mind.
> > > 
> > > The standard properties are clear, concise and tell you exactly what
> > > they are about.
> > > 
> > > The argument should be in Ohms though, according to the standard
> > > bindings, but maybe the value of MTK_PUPD_SET_R1R0_10 is
> > > something like that?
> > 
> > For reasons I can't recall clearly these are just placeholder values
> > that the driver then maps to the R1 and R0 settings. But at least they
> > use the standard properties.
> > 
> > The reason was either one of the following or both:
> > 
> >   a. not every group of pins had the same resistance values for R1 & R0
> >   b. there are no known precise values; the values depend on the process
> >      and batch
> 
> I don't know for (b), but no there is a lot of different values for R1 & R0
> 

>From what I saw in the register table
We can have for the pull up resistors
75K / 200K
2K  / 75K
5K  / 20K
50k / 10K 

And for the pull down ones:

75k/2k
75k/75k
10k/50k

And we can have a combination of both resistors that will give odd values 
(e.g 1948 Ohm for 2k/75K, 545454 Ohm for 75k/200k) to express in the device tree 


> 
> Also, their customers seemed more accustomed to dealing with toggling
> R1 & R0 vs setting some actual value. I presume that comes with the
> uncertainty of the actual hardware value, and they just try which
> combination works better.
> 
> ChenYu

Regards,
Julien