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

Guenter Roeck
2025-01-11T17:15:08Z

On 1/10/25 12:06, Peter Korsgaard wrote:
>>>>>> "Rob" == Rob Herring <robh@kernel.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@korsgaard.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@korsgaard.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