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

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Date:	Fri, 19 Dec 2014 16:32:24 +0100
From:	Lukasz Majewski <l.majewski@...sung.com>
To:	Sjoerd Simons <sjoerd.simons@...labora.co.uk>
Cc:	Eduardo Valentin <edubezval@...il.com>,
	Kamil Debski <k.debski@...sung.com>,
	Jean Delvare <jdelvare@...e.de>,
	Guenter Roeck <linux@...ck-us.net>, lm-sensors@...sensors.org,
	Linux PM list <linux-pm@...r.kernel.org>,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>, devicetree@...r.kernel.org,
	Lukasz Majewski <l.majewski@...ess.pl>,
	Kukjin Kim <kgene.kim@...sung.com>,
	linux-kernel@...r.kernel.org,
	Sylwester Nawrocki <s.nawrocki@...sung.com>
Subject: Re: [PATCH 2/9] hwmon: dts: Doc: Add DTS doc to explain how to use PWM
 FAN as a cooling device

Hi Sjoerd,

Thanks for your feedback and sorry for a late reply.

> 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@...sung.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) 

I'm OK to change the default-pulse-width to be similar to
"default-brightness-level" (as it is in
Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt)

> or simply have be the duty
> lenght in NS as entries instead of the current indirection.

I'd prefer to keep the indirection - as it is utilized in the current
pwm-fan.c driver.

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

Your assumption is correct. cooling-pwm-values are indeed in the
[0..255] range. I will add this information in v2.

> 
> 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.

I agree.

> 
> 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 ?

Enabling pan to full RPM was the default behaviour in the current
pwm-fan.c file.

To be honest, there is no need to enable fan to full RPM speed in this
board for following reasons:
1. In Odroid the FAN is optional (stacked on top of a heat sink) - very
often it is just enough to only have the heat sink.

2. Odroid has thermal enabled by default and IMHO it would be more
feasible to allow thermal to control fan from the very beginning.

However, I can also understand if the policy for hwmon implies a rule
to enable by default all fans to full RPM speed.

> 
> 
> > +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>;
> >  	};
> 
> 



-- 
Best regards,

Lukasz Majewski

Samsung R&D Institute Poland (SRPOL) | Linux Platform Group
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