linux-kernel - Re: [PATCH 3/6] dt-bindings: HID: i2c-hid: elan: add 'no-reset-on-power-off' property

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Date: Tue, 23 Apr 2024 18:29:44 +0200
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Johan Hovold <johan+linaro@...nel.org>, Jiri Kosina <jikos@...nel.org>,
 Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc: Dmitry Torokhov <dmitry.torokhov@...il.com>, Rob Herring
 <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>, Bjorn Andersson <andersson@...nel.org>,
 Konrad Dybcio <konrad.dybcio@...aro.org>,
 Linus Walleij <linus.walleij@...aro.org>,
 Douglas Anderson <dianders@...omium.org>, linux-input@...r.kernel.org,
 devicetree@...r.kernel.org, linux-arm-msm@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] dt-bindings: HID: i2c-hid: elan: add
 'no-reset-on-power-off' property

On 23/04/2024 15:46, Johan Hovold wrote:
> When the power supply is shared with other peripherals the reset line
> can be wired in such a way that it can remain deasserted regardless of
> whether the supply is on or not.

To clarify: the reset line is still present and working in such case?

> 
> This is important as it can be used to avoid holding the controller in
> reset for extended periods of time when it remains powered, something
> which can lead to increased power consumption. Leaving reset deasserted
> also avoids leaking current through the reset circuitry pull-up
> resistors.
> 
> Add a new 'no-reset-on-power-off' devicetree property which can be used
> by the OS to determine when reset needs to be asserted on power down.
> 
> Note that this property can also be used when the supply cannot be
> turned off by the OS at all.
> 
> Signed-off-by: Johan Hovold <johan+linaro@...nel.org>
> ---
>  Documentation/devicetree/bindings/input/elan,ekth6915.yaml | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/input/elan,ekth6915.yaml b/Documentation/devicetree/bindings/input/elan,ekth6915.yaml
> index c3a6f901ff45..3d20673f10b2 100644
> --- a/Documentation/devicetree/bindings/input/elan,ekth6915.yaml
> +++ b/Documentation/devicetree/bindings/input/elan,ekth6915.yaml
> @@ -37,6 +37,12 @@ properties:
>    reset-gpios:
>      description: Reset GPIO; not all touchscreens using eKTH6915 hook this up.
>  
> +  no-reset-on-power-off:

Missing vendor prefix. Unless you want to re-use existing property
"keep-power-in-suspend", but the case here mentions power off, not suspend.

Anyway, the property sounds like what the OS should be doing, which is
not what we want. You basically instruct driver what to do. We want a
described hardware configuration or hardware specifics.

Reset is pulled to something? What is exactly different in this hardware
configuration comparing to other hardware setup (regular)?

> +    type: boolean
> +    description:
> +      Reset line is wired so that it can be left deasserted when the power
> +      supply is off.

Best regards,
Krzysztof