linux-kernel - Re: [PATCH 1/2] dt-bindings: i2c: Add the clock stretching property

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Date:   Sun, 12 Mar 2023 14:43:11 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Andi Shyti <andi.shyti@...nel.org>, linux-i2c@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Wolfram Sang <wsa@...nel.org>, Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Chris Packham <chris.packham@...iedtelesis.co.nz>,
        Ryan Chen <ryan_chen@...eedtech.com>
Subject: Re: [PATCH 1/2] dt-bindings: i2c: Add the clock stretching property

On 12/03/2023 14:19, Andi Shyti wrote:
> The I2C specification allows for the clock line to be held low
> for a specified timeout to force the slave device into a 'wait'
> mode. This feature is known as 'Clock stretching' and is
> optional.
> 
> In the NXP I2C specification, clock stretching is described as
> the process of pausing a transaction by holding the SCL line LOW.
> The transaction can only continue when the line is released HIGH
> again.[*] However, most target devices do not include an SCL
> driver and are therefore unable to stretch the clock.
> 
> Add the following properties:
> 
>  - i2c-scl-clk-low-timeout-ms: This property specifies the
>    duration, in milliseconds, for which the clock is kept low and
>    a client needs to detect a forced waiting state.
> 
>  - i2c-scl-has-clk-low-timeout: This property specifies whether
>     the I2C controller implements the clock stretching property.
> 
> It's important to note that this feature should not be confused
> with the SMBUS clock timeout, which serves a similar function but
> specifies a timeout of 25-35ms. The I2C specification does not
> recommend any specific timeout.
> 
> [*] NXP, UM10204 - I2C-bus specification and user manual
>     Rev. 7.0, 1 October 2021, chapter 3.1.9.
> 
> Signed-off-by: Andi Shyti <andi.shyti@...nel.org>
> ---
>  Documentation/devicetree/bindings/i2c/i2c.txt | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/i2c/i2c.txt b/Documentation/devicetree/bindings/i2c/i2c.txt
> index fc3dd7ec0445..12c311f0e831 100644
> --- a/Documentation/devicetree/bindings/i2c/i2c.txt
> +++ b/Documentation/devicetree/bindings/i2c/i2c.txt

Although the properties are still sometimes added here, but the bindings
are now in dtschema, so rather this should be updated:

https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/i2c/i2c-controller.yaml

> @@ -45,6 +45,15 @@ wants to support one of the below features, it should adapt these bindings.
>  	Number of nanoseconds the SCL signal takes to rise; t(r) in the I2C
>  	specification.
>  
> +- i2c-scl-clk-low-timeout-ms
> +	Number of miliseconds the clock line needs to be pulled down in order
> +	to force a waiting state.
> +
> +- i2c-scl-has-clk-low-timeout
> +	Boolean value that indicates whether the controller implements the
> +	feature of wait induction through SCL low, with the timeout being
> +	implemented internally by the controller.
> +
>  - i2c-sda-falling-time-ns
>  	Number of nanoseconds the SDA signal takes to fall; t(f) in the I2C
>  	specification.

Best regards,
Krzysztof