linux-kernel - Re: [PATCH v6 1/2] dt-bindings: i2c: add bus-reset-gpios property

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Date:   Wed, 15 Nov 2023 22:29:55 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Chris Packham <chris.packham@...iedtelesis.co.nz>, wsa@...nel.org,
        andi.shyti@...nel.org, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, conor+dt@...nel.org,
        gregory.clement@...tlin.com
Cc:     linux-i2c@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 1/2] dt-bindings: i2c: add bus-reset-gpios property

On 15/11/2023 04:57, Chris Packham wrote:
> Add bus-reset-gpios and bus-reset-duration-us properties to the binding
> description for i2c busses. These can be used to describe hardware where
> a common reset GPIO is connected to all downstream devices on and I2C
> bus. This reset will be asserted then released before the downstream
> devices on the bus are probed.
> 
> Signed-off-by: Chris Packham <chris.packham@...iedtelesis.co.nz>
> ---
> 

...

> 
>  Documentation/devicetree/bindings/i2c/i2c.txt | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/i2c/i2c.txt b/Documentation/devicetree/bindings/i2c/i2c.txt
> index fc3dd7ec0445..3f95d71b9985 100644
> --- a/Documentation/devicetree/bindings/i2c/i2c.txt
> +++ b/Documentation/devicetree/bindings/i2c/i2c.txt
> @@ -99,6 +99,14 @@ wants to support one of the below features, it should adapt these bindings.
>  	indicates that the system is accessible via this bus as an endpoint for
>  	MCTP over I2C transport.
>  
> +- bus-reset-gpios:
> +	GPIO pin providing a common reset for all downstream devices. This GPIO
> +	will be asserted then released before the downstream devices are probed.

I initially reviewed it, but did not think enough about it. After more
consideration, I believe this is not a property of the I2C bus
controller. This is a property of each device, even if the GPIO is the same.

Linux kernel already supports shared GPIO, so you only need
enable-ref-counting on it.

Putting it into the controller bindings looks like solving OS issue with
incorrect hardware description.

Best regards,
Krzysztof