linux-kernel - Re: [PATCH 1/2] dt-bindings: gpio: nxp,pcf8575: add reset GPIO

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Message-ID: <20250220122424.GB20111@pendragon.ideasonboard.com>
Date: Thu, 20 Feb 2025 14:24:24 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Quentin Schulz <foss+kernel@...il.net>
Cc: Linus Walleij <linus.walleij@...aro.org>,
	Bartosz Golaszewski <brgl@...ev.pl>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Heiko Stuebner <heiko@...ech.de>, linux-gpio@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	Quentin Schulz <quentin.schulz@...rry.de>
Subject: Re: [PATCH 1/2] dt-bindings: gpio: nxp,pcf8575: add reset GPIO

Hi Quentin,

Thank you for the patch.

On Thu, Feb 20, 2025 at 10:56:51AM +0100, Quentin Schulz wrote:
> From: Quentin Schulz <quentin.schulz@...rry.de>
> 
> A few of the I2C GPIO expander chips supported by this binding have a
> RESETN pin to be able to reset the chip. The chip is held in reset while
> the pin is low, therefore the polarity of reset-gpios is expected to
> reflect that, i.e. a GPIO_ACTIVE_HIGH means the GPIO will be held low
> for reset and released high, GPIO_ACTIVE_LOW means the GPIO will be held
> high for reset and released low.

I think the convention in DT is the opposite. The DT property is
"reset-gpios", no "resetn-gpio", so the polarity should indicate how to
drive the GPIO to assert a logical "reset". GPIO_ACTIVE_LOW should mean
that the chip will be in reset when the physical GPIO is 0.

Could DT maintainers confirm this ?

> Out of the supported chips, only PCA9670, PCA9671, PCA9672 and PCA9673
> show a RESETN pin in their datasheets. They all share the same reset
> timings, that is 4+us reset pulse[0] and 100+us reset time[0].
> 
> When performing a reset, "The PCA9670 registers and I2C-bus state
> machine will be held in their default state until the RESET input is
> once again HIGH."[1] meaning we now know the state of each line
> controlled by the GPIO expander. Therefore, setting lines-initial-states
> and reset-gpios both does not make sense and their presence is XOR'ed.
> 
> [0] https://www.nxp.com/docs/en/data-sheet/PCA9670.pdf Fig 22.
> [1] https://www.nxp.com/docs/en/data-sheet/PCA9670.pdf 8.5
> 
> Signed-off-by: Quentin Schulz <quentin.schulz@...rry.de>
> ---
>  .../devicetree/bindings/gpio/nxp,pcf8575.yaml      | 33 ++++++++++++++++++++++
>  1 file changed, 33 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/gpio/nxp,pcf8575.yaml b/Documentation/devicetree/bindings/gpio/nxp,pcf8575.yaml
> index 3718103e966a13e1d77f73335ff73c18a3199469..d08d3f848f82e74de949da16d26a810dc52a74e5 100644
> --- a/Documentation/devicetree/bindings/gpio/nxp,pcf8575.yaml
> +++ b/Documentation/devicetree/bindings/gpio/nxp,pcf8575.yaml
> @@ -73,6 +73,39 @@ properties:
>  
>    wakeup-source: true
>  
> +  reset-gpios:
> +    maxItems: 1
> +    description:
> +      GPIO controlling the (reset active LOW) RESET# pin.
> +
> +      Performing a reset makes all lines initialized to their input (pulled-up)
> +      state.
> +
> +allOf:
> +  - if:
> +      properties:
> +        compatible:
> +          not:
> +            contains:
> +              enum:
> +                - nxp,pca9670
> +                - nxp,pca9671
> +                - nxp,pca9672
> +                - nxp,pca9673
> +    then:
> +      properties:
> +        reset-gpios: false
> +
> +  # lines-initial-states XOR reset-gpios
> +  # Performing a reset reinitializes all lines to a known state which
> +  # may not match passed lines-initial-states
> +  - if:
> +      required:
> +        - lines-initial-states
> +    then:
> +      properties:
> +        reset-gpios: false
> +
>  patternProperties:
>    "^(.+-hog(-[0-9]+)?)$":
>      type: object
> 

-- 
Regards,

Laurent Pinchart