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

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Date: Mon, 11 Mar 2024 10:52:17 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Rob Herring <robh@...nel.org>, Chanh Nguyen <chanh@...amperecomputing.com>
Cc: Jean Delvare <jdelvare@...e.com>,
 Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
 Conor Dooley <conor+dt@...nel.org>, Justin Ledford
 <justinledford@...gle.com>, devicetree@...r.kernel.org,
 linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org,
 OpenBMC Maillist <openbmc@...ts.ozlabs.org>,
 Open Source Submission <patches@...erecomputing.com>,
 Phong Vo <phong@...amperecomputing.com>,
 Thang Nguyen <thang@...amperecomputing.com>,
 Quan Nguyen <quan@...amperecomputing.com>
Subject: Re: [PATCH 3/3] dt-bindings: hwmon: max31790: Add
 pwmout-pin-as-tach-input property

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@...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