Re: [PATCH 3/3] dt-bindings: hwmon: max31790: Add pwmout-pin-as-tach-input property

[Chanh Nguyen <chanh@amperemail.onmicrosoft.com>]

On 12/03/2024 00:52, Guenter Roeck wrote:
> On 3/11/24 10:34, Rob Herring wrote:
>> On Mon, Mar 11, 2024 at 06:13:47PM +0700, Chanh Nguyen wrote:
>>> Add pwmout-pin-as-tach-input property.
>>>
>>> Signed-off-by: Chanh Nguyen <chanh@os.amperecomputing.com>
>>> ---
>>>   Documentation/devicetree/bindings/hwmon/max31790.yaml | 11 +++++++++++
>>>   1 file changed, 11 insertions(+)
>>>
>>> diff --git a/Documentation/devicetree/bindings/hwmon/max31790.yaml 
>>> b/Documentation/devicetree/bindings/hwmon/max31790.yaml
>>> index 5a93e6bdebda..447cac17053a 100644
>>> --- a/Documentation/devicetree/bindings/hwmon/max31790.yaml
>>> +++ b/Documentation/devicetree/bindings/hwmon/max31790.yaml
>>> @@ -25,6 +25,16 @@ properties:
>>>     reg:
>>>       maxItems: 1
>>> +  pwmout-pin-as-tach-input:
>>> +    description: |
>>> +      An array of six integers responds to six PWM channels for
>>> +      configuring the pwm to tach mode.
>>> +      When set to 0, the associated PWMOUT produces a PWM waveform for
>>> +      control of fan speed. When set to 1, PWMOUT becomes a TACH input
>>> +    $ref: /schemas/types.yaml#/definitions/uint8-array
>>> +    maxItems: 6
>>> +    minItems: 6
>>
>> Seems incomplete. For example, fan tachs have different number of
>> pulses per revolution, don't you need to know that too?
>>
> 
> Per Documentation/ABI/testing/sysfs-class-hwmon:
> 
> What:           /sys/class/hwmon/hwmonX/fanY_pulses
> Description:
>                  Number of tachometer pulses per fan revolution.
> 
>                  Integer value, typically between 1 and 4.
> 
>                  RW
> 
>                  This value is a characteristic of the fan connected to the
>                  device's input, so it has to be set in accordance with 
> the fan
>                  model.
> 
>                  Should only be created if the chip has a register to 
> configure
>                  the number of pulses. In the absence of such a register 
> (and
>                  thus attribute) the value assumed by all devices is 2 
> pulses
>                  per fan revolution.
> 
> We only expect the property (and attribute) to exist if the controller
> supports it.
> 
> Guenter
> 

Hi Guenter and Rob,
Most general-purpose brushless DC fans produce two tachometer pulses per 
revolution. My fan devices also produce two tachometer pulses per 
revolution.

In max31790.c, I saw the formula that is used to calculate TACH Count 
Registers, which defines the pulses per revolution as 2

	#define RPM_TO_REG(rpm, sr)             ((60 * (sr) * 8192) / ((rpm) * 2))

Do you think we should support the pulses-per-revolution property in 
this case?