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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Message-id: <20141219171301.1527d81e@amdc2363>
Date:	Fri, 19 Dec 2014 17:13:01 +0100
From:	Lukasz Majewski <l.majewski@...sung.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Sjoerd Simons <sjoerd.simons@...labora.co.uk>,
	Eduardo Valentin <edubezval@...il.com>,
	Kamil Debski <k.debski@...sung.com>,
	Jean Delvare <jdelvare@...e.de>, 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 Guenter,

> 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@...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.
> > 
> FWIW, devicetree information is supposed to be implementation
> independent. So this is a poor argument.

Many other pwm drivers use the indirection - e.g. mentioned
pwm-backlight.


> 
> > 
> > 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 ?

I wanted to avoid the unpleasant sound for full speed fan when thermal
is not enabled by default.

But as I said, I fully understand the policy and I would be happy to
comply with it as thermal should reduce the fan speed anyway at boot
time.

> 
> Guenter



-- 
Best regards,

Lukasz Majewski

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