Re: [PATCH 2/9] hwmon: dts: Doc: Add DTS doc to explain how to use PWM FAN as a cooling device

Sjoerd Simons
2014-12-18T10:42:25Z

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On Thu, 2014-12-18 at 11:13 +0100, Lukasz Majewski wrote:
> Several new properties to allow PWM fan working as a cooling device have been
> combined into this single commit.
>
> Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
> ---
> .../devicetree/bindings/hwmon/pwm-fan.txt | 28 ++++++++++++++++++++++
> 1 file changed, 28 insertions(+)
>
> diff --git a/Documentation/devicetree/bindings/hwmon/pwm-fan.txt b/Documentation/devicetree/bindings/hwmon/pwm-fan.txt
> index 610757c..3877810 100644
> --- a/Documentation/devicetree/bindings/hwmon/pwm-fan.txt
> +++ b/Documentation/devicetree/bindings/hwmon/pwm-fan.txt
> @@ -3,10 +3,38 @@ Bindings for a fan connected to the PWM lines
> Required properties:
> - compatible : "pwm-fan"
> - pwms : the PWM that is used to control the PWM fan
> +- cooling-pwm-values : PWM duty cycle values relative to
> + cooling-max-pwm-value correspondig to
> + proper cooling states
> +- default-pulse-width : Property specifying default pulse width for FAN
> + at system boot (zero to disable FAN on boot).
> + Allowed range is 0 to 255

The 0..255 range seems somewhat random. Would be nicer to either use the
approach of pwm-backlight (iotw, have the range go from the first to the
last entry of cooling-pwm-values) or simply have be the duty lenght in
NS as entries instead of the current indirection.

I assumed your cooling-pwm-values are a [0..255] range as well instead
of nanoseconds (would be good to make that more clear)?

Also having more consistent names would be nice.. To take pwm-backlight
as inspiration, cooling-levels and default-cooling-level would make it
more clear the second property picks a default setting from the first
one.

One thing i do wonder, is having an explicit default setting useful?
Should it not default to maximum cooling unless otherwise configured by
either the thermal framework or sysfs ?

> +Thorough description of the following bindings:
> + cooling-min-state = <0>;
> + cooling-max-state = <3>;
> + #cooling-cells = <2>;
> + thermal-zone {
> + cpu_thermal: cpu-thermal {
> + cooling-maps {
> + map0 {
> + trip = <&cpu_alert1>;
> + cooling-device = <&fan0 0 1>;
> + };
> + };
> + };
> +
> +for PWM FAN used as cooling device can be found at:
> +./Documentation/devicetree/bindings/thermal/thermal.txt
>
> Example:
> pwm-fan {
> compatible = "pwm-fan";
> status = "okay";
> pwms = <&pwm 0 10000 0>;
> + cooling-min-state = <0>;
> + cooling-max-state = <3>;
> + #cooling-cells = <2>;
> + cooling-pwm-values = <0 102 170 255>;
> + default-pulse-width = <0>;
> };

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