linux-kernel - Re: [PATCH v10 1/2] dt-bindings: pinctrl: mt8195: add rsel define

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Message-ID: <CACRpkdYkwXr76Kq5WYdz=1KkLTpaByAL1vJFo8V+2mncqs8-3Q@mail.gmail.com>
Date:   Tue, 17 Aug 2021 22:09:21 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     "zhiyong.tao" <zhiyong.tao@...iatek.com>
Cc:     Chen-Yu Tsai <wenst@...omium.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Sean Wang <sean.wang@...nel.org>,
        srv_heupstream <srv_heupstream@...iatek.com>,
        hui.liu@...iatek.com, Eddie Huang <eddie.huang@...iatek.com>,
        Light Hsieh <light.hsieh@...iatek.com>,
        Biao Huang <biao.huang@...iatek.com>,
        Hongzhou Yang <hongzhou.yang@...iatek.com>,
        Sean Wang <sean.wang@...iatek.com>,
        Seiya Wang <seiya.wang@...iatek.com>,
        Devicetree List <devicetree@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-mediatek@...ts.infradead.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>
Subject: Re: [PATCH v10 1/2] dt-bindings: pinctrl: mt8195: add rsel define

On Tue, Aug 17, 2021 at 9:51 AM zhiyong.tao <zhiyong.tao@...iatek.com> wrote:

> In one chip, If GPIO is different, the MTXXXX_PULL_UP_RSEL_001 may
> means different actual bias resistance setting.
>
> For example,
>
> KPROW IO
> Paramters       Descriptions                   Min      Typ     Max
>  UNIT
> Rpd     Input pull-down resistance      40      75      190     Kohm
> Rpu     Input pull-up resistance        40      75      190     Kohm
> Rpd     Input pull-down resistance      0.8     1.6     2       Kohm
> Rpu     Input pull-up resistance        0.8     1.6     2       Kohm

This is exactly why we should try to use SI units in the device tree.
I assume that the software can eventually configure which resistance
it gets?

The electronics people will say make sure it is pulled down by around
80 kOhm, they can put that on the device tree and your code can
say, "hm 40 < 80 < 190 this is OK" and let the value pass.

We do not define these exact semantics, it is up to the driver code
to decide what to do with the Ohm value 80000 in this case, but
it makes perfect sent for me to let it pass and fail if someone
for example requests 20 kOhm, or at least print a helpful warning:

dev_warn(dev, "the requested resistance %d is out of range, supported
range %d to %d kOhm\n",
                 val, low, high);

This is what makes the SI units really helpful for people writing device
trees: solve real integration tasks and make it easy to do the right thing.

Yours,
Linus Walleij