linux-kernel - Re: [PATCH v7 3/5] ARM: dts: stm32: Add I2C1 support for STM32F429 SoC

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Date:   Fri, 30 Dec 2016 10:07:01 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     "M'boumba Cedric Madianga" <cedric.madianga@...il.com>
Cc:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, Wolfram Sang <wsa@...-dreams.de>,
        Rob Herring <robh+dt@...nel.org>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Patrice Chotard <patrice.chotard@...com>,
        Russell King <linux@...linux.org.uk>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7 3/5] ARM: dts: stm32: Add I2C1 support for STM32F429 SoC

On Fri, Dec 23, 2016 at 2:09 PM, M'boumba Cedric Madianga
<cedric.madianga@...il.com> wrote:
> 2016-12-22 20:11 GMT+01:00 Uwe Kleine-König <u.kleine-koenig@...gutronix.de>:
>> On Thu, Dec 22, 2016 at 02:35:02PM +0100, M'boumba Cedric Madianga wrote:
>>> @@ -337,6 +350,16 @@
>>>                                       slew-rate = <2>;
>>>                               };
>>>                       };
>>> +
>>> +                     i2c1_pins_b: i2c1@0 {
>>> +                             pins1 {
>>> +                                     pinmux = <STM32F429_PB9_FUNC_I2C1_SDA>;
>>> +                                     drive-open-drain;
>>> +                             };
>>> +                             pins2 {
>>> +                                     pinmux = <STM32F429_PB6_FUNC_I2C1_SCL>;
>>> +                             };
>>
>> the second doesn't need the open-drain property? Why?
>
> I thought that open-drain was only needed for SDA line.
> But after double-checking I2C specification, it seems that SDA and SCL
> lines need open-drain or open-collector to perform the wired-AND
> function.

I think I2C SDA/SCL must be open drain by definition.

It also requires pull-up resistors, I guess you have these mounted on the board
so you do not need pull-up from the pin controller?

Yours,
Linus Walleij