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

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Message-ID: <f6ee420b-7c3f-4a21-831b-619fe38408b5@roeck-us.net>
Date: Sat, 11 Jan 2025 09:15:05 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Peter Korsgaard <peter@...sgaard.com>, Rob Herring <robh@...nel.org>
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 1/10/25 12:06, Peter Korsgaard wrote:
>>>>>> "Rob" == Rob Herring <robh@...nel.org> writes:
> On 1/6/25 18:38, Rob Herring wrote:
> 
>>> I am not sure I what you mean with the RPM reference here? The
>>> cooling-levels support in the fan-pwm.c driver is a mapping between cooling
>>> levels and PWM values, NOT RPM value.
>>
>> Did I say RPM anywhere for this option?
>>
>> It is the index of the array that is meaningful to anything outside of
>> the driver. The values are opaque. They are duty cycle in some cases
>> and RPMs in other cases. The thermal subsystem knows nothing about PWM
>> duty cycle nor RPMs.
>>
>> Defining a default-cooling-level would be useful to anyone, not just
>> your usecase.
>>
>> IOW, you are proposing:
>>
>> default-pwm = <123>;
>>
>> I'm proposing doing this instead:
>>
>> cooling-levels = <0 123 255>;
>> default-cooling-level = <1>;
> 
> I don't have CONFIG_THERMAL enabled in my builds (and don't know the
> subsystem), but I see the pwm-fan driver has some logic to default to
> the highest cooling level, it just forgets to actually set the PWM to
> match it, so perhaps we can just fix that?
> 
> E.G. something like:
> 
> commit 02c8ba74eb7dddf210ceefa253385bc8e40f49ae
> Author: Peter Korsgaard <peter@...sgaard.com>
> Date:   Thu Jan 2 18:26:45 2025 +0100
> 
>      hwmon: (pwm-fan): Default to the Maximum cooling level if provided
>      
>      The pwm-fan driver uses full PWM (255) duty cycle at startup, which may not
>      always be desirable because of noise or power consumption peaks.
>      
>      The driver optionally accept a list of "cooling-levels" for the thermal
>      subsystem.  If provided, use the PWM value corresponding to the maximum
>      cooling level rather than the full level.
>      
>      Signed-off-by: Peter Korsgaard <peter@...sgaard.com>
> 
> diff --git a/drivers/hwmon/pwm-fan.c b/drivers/hwmon/pwm-fan.c
> index 53a1a968d00d..33525096f1e7 100644
> --- a/drivers/hwmon/pwm-fan.c
> +++ b/drivers/hwmon/pwm-fan.c
> @@ -499,6 +499,7 @@ static int pwm_fan_probe(struct platform_device *pdev)
>   	const struct hwmon_channel_info **channels;
>   	u32 pwm_min_from_stopped = 0;
>   	u32 *fan_channel_config;
> +	u32 default_pwm = MAX_PWM;
>   	int channel_count = 1;	/* We always have a PWM channel. */
>   	int i;
>   
> @@ -545,11 +546,18 @@ static int pwm_fan_probe(struct platform_device *pdev)
>   
>   	ctx->enable_mode = pwm_disable_reg_enable;
>   
> +	ret = pwm_fan_get_cooling_data(dev, ctx);
> +	if (ret)
> +		return ret;
> +
> +	if (ctx->pwm_fan_cooling_levels)
> +		default_pwm = ctx->pwm_fan_cooling_levels[ctx->pwm_fan_max_state];
> +
>   	/*
> -	 * Set duty cycle to maximum allowed and enable PWM output as well as
> +	 * Set duty cycle to default and enable PWM output as well as
>   	 * the regulator. In case of error nothing is changed
>   	 */
> -	ret = set_pwm(ctx, MAX_PWM);
> +	ret = set_pwm(ctx, default_pwm);
>   	if (ret) {
>   		dev_err(dev, "Failed to configure PWM: %d\n", ret);
>   		return ret;
> @@ -661,10 +669,6 @@ static int pwm_fan_probe(struct platform_device *pdev)
>   		return PTR_ERR(hwmon);
>   	}
>   
> -	ret = pwm_fan_get_cooling_data(dev, ctx);
> -	if (ret)
> -		return ret;
> -
>   	ctx->pwm_fan_state = ctx->pwm_fan_max_state;
>   	if (IS_ENABLED(CONFIG_THERMAL)) {
>   		cdev = devm_thermal_of_cooling_device_register(dev,
> 
> 
> Guenter, what do you say? This way we don't need any new device tree
> properties. I personally find it less clear than a default-pwm property,
> but oh well.
> 

I would not call that "default". It is more along the line of
"If available, use highest cooling level as maximum allowed".

Other than that, I don't like it, but since it looks like we
won't get approval for the devicetree property, I'd say go for it.

Guenter