linux-kernel - Re: [PATCH v2 2/4] dt-bindings: pinctrl: qcom,tlmm-common: document i2c pull property

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Message-ID: <8602cacd-f552-e843-5c17-681b099069a3@linaro.org>
Date:   Fri, 25 Nov 2022 13:40:09 +0100
From:   neil.armstrong@...aro.org
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Abel Vesa <abel.vesa@...aro.org>, Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        devicetree@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>
Subject: Re: [PATCH v2 2/4] dt-bindings: pinctrl: qcom,tlmm-common: document
 i2c pull property

On 25/11/2022 10:47, Linus Walleij wrote:
> On Thu, Nov 24, 2022 at 2:24 PM Neil Armstrong
> <neil.armstrong@...aro.org> wrote:
>> On 24/11/2022 14:11, Linus Walleij wrote:
>>> On Wed, Nov 23, 2022 at 4:20 PM Abel Vesa <abel.vesa@...aro.org> wrote:
>>>
>>>> From: Neil Armstrong <neil.armstrong@...aro.org>
>>>>
>>>> Document the new i2c_pull property introduced for SM8550 setting
>>>> an I2C specific pull mode on I2C able pins.
>>>>
>>>> Signed-off-by: Neil Armstrong <neil.armstrong@...aro.org>
>>>> Signed-off-by: Abel Vesa <abel.vesa@...aro.org>
>>> (...)
>>>> +      qcom,i2c-pull:
>>>> +        type: boolean
>>>> +        description: enable bias pull feature designed for I2C on pin
>>>
>>> But what is this?
>>>
>>> I2C buses are usually just plain old bias-high-impedance, high-z
>>> or open drain, wire-or or whatever you want to call it.
>>>
>>> But now there is some special i2c mode, huh?
>>>
>>> The description is pretty much "it is what it is"... can we have
>>> some explanation about what this means electrically speaking
>>> and why you cannot use bias-high-impedance?
>>
>> I'll try to get some more info, but so far I only found what I wrote in the bindings.
> 
> Björn: can you see if you can get some clarity about the i2c
> bias thing?

As I understood, it enables an "I2C resistor" on the pin, removing the need
of an external pull-up resistor on the line.

I assume the classical pull-up bias is not strong enough to replace an actual
resistor on the PCB.

Neil

> 
> Yours,
> Linus Walleij