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

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Date: Fri, 10 May 2024 15:51:54 +0000
From: Chris Packham <Chris.Packham@...iedtelesis.co.nz>
To: Guenter Roeck <linux@...ck-us.net>
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


On 10/05/24 15:36, Guenter Roeck wrote:
> 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.

We've already got the pwm-active-state as a separate property so that 
might be tricky to deal with, I guess it could be deprecated in favour 
of something else. Looking at pwm.yaml and fan-common.yaml I can't quite 
see how that'd help here. Were you thinking maybe something like

pwm: hwmon@2e {
     compatible = "adi,adt7476";
     reg = <0x2e>;
     #pwm-cells = <4>;
     fan-0 {
         pwms = <&pwm 0 255 22500 PWM_POLARITY_INVERTED>;
         pwm-names = "PWM1";
         tach-ch = <0>;
     };
     fan-1 {
         // controlled by pwm 0
         tach-ch = <1>
     };
     fan-0 {
         pwms = <&pwm 2 255 22500 PWM_POLARITY_INVERTED>;
         pwm-names = "PWM3";
         tach-ch <2>;
     };
     fan-1 {
         // controlled by pwm 2
         tach-ch = <3>
     };
};