linux-kernel - Re: [PATCH v2 1/2] dt-bindings: hwmon: Document adt7475 PWM initial duty cycle

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Date: Thu, 9 May 2024 20:36:10 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Chris Packham <Chris.Packham@...iedtelesis.co.nz>
Cc: Krzysztof Kozlowski <krzk@...nel.org>,
	"jdelvare@...e.com" <jdelvare@...e.com>,
	"robh@...nel.org" <robh@...nel.org>,
	"krzk+dt@...nel.org" <krzk+dt@...nel.org>,
	"conor+dt@...nel.org" <conor+dt@...nel.org>,
	"linux-hwmon@...r.kernel.org" <linux-hwmon@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] dt-bindings: hwmon: Document adt7475 PWM initial
 duty cycle

Chris,

On Thu, May 09, 2024 at 06:19:12PM +0000, Chris Packham wrote:
> Hi Krzysztof,
> 
> On 9/05/24 19:06, Krzysztof Kozlowski wrote:
> > On 08/05/2024 23:55, Chris Packham wrote:
> >> Add documentation for the pwm-initial-duty-cycle and
> >> pwm-initial-frequency properties. These allow the starting state of the
> >> PWM outputs to be set to cater for hardware designs where undesirable
> >> amounts of noise is created by the default hardware state.
> >>
> >> Signed-off-by: Chris Packham <chris.packham@...iedtelesis.co.nz>
> >> ---
> >>
> >> Notes:
> >>      Changes in v2:
> >>      - Document 0 as a valid value (leaves hardware as-is)
> >>
> >>   .../devicetree/bindings/hwmon/adt7475.yaml    | 27 ++++++++++++++++++-
> >>   1 file changed, 26 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/Documentation/devicetree/bindings/hwmon/adt7475.yaml b/Documentation/devicetree/bindings/hwmon/adt7475.yaml
> >> index 051c976ab711..97deda082b4a 100644
> >> --- a/Documentation/devicetree/bindings/hwmon/adt7475.yaml
> >> +++ b/Documentation/devicetree/bindings/hwmon/adt7475.yaml
> >> @@ -51,6 +51,30 @@ properties:
> >>         enum: [0, 1]
> >>         default: 1
> >>   
> >> +  adi,pwm-initial-duty-cycle:
> >> +    description: |
> >> +      Configures the initial duty cycle for the PWM outputs. The hardware
> >> +      default is 100% but this may cause unwanted fan noise at startup. Set
> >> +      this to a value from 0 (0% duty cycle) to 255 (100% duty cycle).
> >> +    $ref: /schemas/types.yaml#/definitions/uint32-array
> >> +    minItems: 3
> >> +    maxItems: 3
> >> +    items:
> >> +      minimum: 0
> >> +      maximum: 255
> >> +      default: 255
> >> +
> >> +  adi,pwm-initial-frequency:
> > Frequency usually has some units, so use appropriate unit suffix and
> > drop $ref.  Maybe that's just target-rpm property?
> >
> > But isn't this duplicating previous property? This is fan controller,
> > not PWM provider (in any case you miss proper $refs to pwm.yaml or
> > fan-common.yaml), so the only thing you initially want to configure is
> > the fan rotation, not specific PWM waveform. If you you want to
> > configure specific PWM waveform, then it's a PWM provider... but it is
> > not... Confused.
> 
> There's two things going on here. There's a PWM duty cycle which is 
> configurable from 0% to 100%. It might be nice if this was expressed as 
> a percentage instead of 0-255 but I went with the latter because that's 
> how the sysfs ABI for the duty cycle works.
> 
> The frequency (which I'll call adi,pwm-initial-frequency-hz in v3) 
> affects how that duty cycle is presented to the fans. So you could still 
> have a duty cycle of 50% at any frequency. What frequency is best 
> depends on the kind of fans being used. In my particular case the lower 
> frequencies end up with the fans oscillating annoyingly so I use the 
> highest setting.
> 

My udnerstanding is that we are supposed to use standard pwm provider
properties. The property description is provider specicic, so I think
we can pretty much just make it up.

Essentially you'd first define a pwm provider which defines all the
pwm parameters needed, such as pwm freqency, default duty cycle,
and flags such as PWM_POLARITY_INVERTED. You'd then add something like

	pwms = <&pwm index frequency duty_cycle ... flags>;

to the node for each fan, and be done.

That doesn't mean that we would actually have to register the chip
as pwm provider with the pwm subsystem; all we would have to do is to
interpret the property values.

Hope thie helps,
Guenter