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

[Sjoerd Simons]

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@xxxxxxxxxxx>
> ---
>  .../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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