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

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Message-ID: <d11093bb-230b-4918-a8cd-4f4eb760ccf3@alliedtelesis.co.nz>
Date: Thu, 9 May 2024 18:19:12 +0000
From: Chris Packham <Chris.Packham@...iedtelesis.co.nz>
To: Krzysztof Kozlowski <krzk@...nel.org>, "jdelvare@...e.com"
	<jdelvare@...e.com>, "linux@...ck-us.net" <linux@...ck-us.net>,
	"robh@...nel.org" <robh@...nel.org>, "krzk+dt@...nel.org"
	<krzk+dt@...nel.org>, "conor+dt@...nel.org" <conor+dt@...nel.org>
CC: "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

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.

>
> Best regards,
> Krzysztof
>