linux-kernel - Re: [PATCH v2 1/2] dt-bindings: i2c: add property to avoid device detection

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Date:   Tue, 12 Apr 2022 16:22:41 -0500
From:   Rob Herring <robh@...nel.org>
To:     Vincent Whitchurch <vincent.whitchurch@...s.com>
Cc:     wsa@...nel.org, kernel@...s.com, linux-i2c@...r.kernel.org,
        devicetree@...r.kernel.org, krzk+dt@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] dt-bindings: i2c: add property to avoid device
 detection

On Tue, Apr 12, 2022 at 10:50:45AM +0200, Vincent Whitchurch wrote:
> When drivers with ->detect callbacks are loaded, the I2C core does a
> bunch of transactions to try to probe for these devices, regardless of
> whether they are specified in the devicetree or not.  (This only happens
> on I2C controllers whose drivers enable the I2C_CLASS* flags, but this
> is the case for generic drivers like i2c-gpio.)
> 
> These kinds of transactions are unnecessary on systems where the
> devicetree specifies all the devices on the I2C bus, so add a property
> to indicate that the devicetree description of the hardware is complete
> and thus allow this discovery to be disabled.
> 
> Signed-off-by: Vincent Whitchurch <vincent.whitchurch@...s.com>
> ---
> 
> Notes:
>     v2:
>     - Change subject prefix
>     - Reword description of property
> 
>  Documentation/devicetree/bindings/i2c/i2c.txt | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/i2c/i2c.txt b/Documentation/devicetree/bindings/i2c/i2c.txt
> index fc3dd7ec0445..960d1d5c9362 100644
> --- a/Documentation/devicetree/bindings/i2c/i2c.txt
> +++ b/Documentation/devicetree/bindings/i2c/i2c.txt
> @@ -72,6 +72,10 @@ wants to support one of the below features, it should adapt these bindings.
>  	this information to adapt power management to keep the arbitration awake
>  	all the time, for example. Can not be combined with 'single-master'.
>  
> +- no-detect
> +	states that no other devices are present on this bus other than the
> +	ones listed in the devicetree.

This belongs in the schema instead:

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

Rob