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

[Sjoerd Simons]

Hey Lukasz,

Blame the holiday season for my late reply ;)

On Fri, 2014-12-19 at 17:13 +0100, Lukasz Majewski wrote:
> 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@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) 
> > > 
> > > 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.

I don't specifically mind the indirection, i was just thinking out loud
whether it added value (but if it's quite common, might indeed be good
to keep the pattern). What i do dislike is the number of levels is being
set to an arbitrary levels, as that will very rarely match the actual
number of distinct pwm levels you 

One thing though, when following the pattern of the pwm-backlight
driver; In pwm-backlight the highest index of brightness-levels is
always 100% duty cycle.. On e.g. XU3 the vendor kernel never drives the
fan at 100% duty (maximum of 91%). So it would be nice if the dt
bindigns could model that e.g. by having:

pwm-levels = <20>; // 21 distinct pwm levels
valid-pwm-level = <5 15 18>; /* 5 15 and 18 are usable levels - pwm will
                                default to highest level */


> > > 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.

Yeah, what happens on my XU3 is that u-boot sets the pwm to 100% duty
and the thermal infrastructure turns it off as soon as it gets into
control, which works quite nicely (and keeps my sanity as that fan at
100% is *loud*)...  So if you want to avoid unpleasant sounds, just
build with thermal :p


-- 
Sjoerd Simons <sjoerd.simons@xxxxxxxxxxxxxxx>
Collabora Ltd.

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