linux-kernel - Re: [PATCH v4 1/2] dt-bindings: hwmon: pwm-fan: Document default-pwm property

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Message-ID: <6059a0f8-bd53-47da-8fe9-dab115c40b52@roeck-us.net>
Date: Mon, 6 Jan 2025 10:32:28 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Rob Herring <robh@...nel.org>, Peter Korsgaard <peter@...sgaard.com>
Cc: devicetree@...r.kernel.org, linux-hwmon@...r.kernel.org,
 Jean Delvare <jdelvare@...e.com>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 1/2] dt-bindings: hwmon: pwm-fan: Document default-pwm
 property

On 1/6/25 09:38, Rob Herring wrote:

>> The minimum PWM is presumably 0, E.G. signal always low?
> 
> I'm talking about the duty cycle needed to start the fan spinning and to
> keep it spinning. I'm sure that value is not 1 for any fan except one in
> a physics textbook (the only place friction does not exist).
> 
> Maybe the minimum is index 0 of cooling-levels?
> 

Fan controllers often have a separate value for the lowest cooling level
and for the PWM necessary to start a fan. The pwm associated with the lowest
cooling level may be lower than the pwm necessary to start a fan.

The fan controller would start a fan by applying the "start fan" pwm value
for a period of time, then reduce the pwm to the desired pwm value for
the running fan.

I'll be happy to dig up datasheet references if needed.

Guenter