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SubjectRe: [PATCH v2 1/2] dt-bindings: hwmon: Document adt7475 PWM initial duty cycle
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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@alliedtelesis.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
>
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