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

[Guenter Roeck <linux@roeck-us.net>]

On Fri, Dec 19, 2014 at 04:32:24PM +0100, Lukasz Majewski wrote:
> 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@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) 
> 
> 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.
> 
FWIW, devicetree information is supposed to be implementation independent.
So this is a poor argument.

> 
> 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.
> 
Why and how does that all suggest that the current default behavior
should be changed ?

Guenter