linux-kernel - Re: [PATCH v2 3/3] dt-bindings: hwmon: max31790: Add maxim,pwmout-pin-as-tach-input property

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Date: Thu, 9 May 2024 10:29:31 +0200
From: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To: Chanh Nguyen <chanh@...eremail.onmicrosoft.com>,
 Guenter Roeck <linux@...ck-us.net>, Conor Dooley <conor@...nel.org>
Cc: Chanh Nguyen <chanh@...amperecomputing.com>,
 Jean Delvare <jdelvare@...e.com>, Rob Herring <robh+dt@...nel.org>,
 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 v2 3/3] dt-bindings: hwmon: max31790: Add
 maxim,pwmout-pin-as-tach-input property

On 08/05/2024 05:44, Chanh Nguyen wrote:
>>>>>
>>>>
>>>> I am not even sure how to define tach-ch to mean "use the pwm output pin
>>>> associated with this tachometer input channel not as pwm output
>>>> but as tachometer input". That would be a boolean, not a number.
>>>>
>>>
>>> Thank Guenter,
>>>
>>> I reviewed again the "tach-ch" property, which is used in the 
>>> https://elixir.bootlin.com/linux/v6.9-rc6/source/Documentation/devicetree/bindings/hwmon/aspeed,g6-pwm-tach.yaml#L68 and https://elixir.bootlin.com/linux/v6.9-rc6/source/drivers/hwmon/aspeed-g6-pwm-tach.c#L434
>>>
>>> That is something completely different from my purpose.
>>>
>>
>> Based on its definition, tach-ch is associated with fans, and it looks
>> like the .yaml file groups multiple sets of fans into a single
>> fan node.
>>
>> In the simple case that would be
>>      tach-ch = <1>
>> ...
>>      tach-ch = <12>
>>
>> or, if all fans are controlled by a single pwm
>>      tach-ch = <1 2 3 4 5 6 8 9 10 11 12>
>>
>> The existence of tachometer channel 7..12 implies that pwm channel 
>> (tachometer
>> channel - 6) is used as tachometer channel. That should be sufficient to 
>> program
>> the chip for that channel. All you'd have to do is to ensure that pwm 
>> channel
>> "X" is not listed as tachometer channel "X + 6", and program pwm channel 
>> "X - 6"
>> for tachometer channels 7..12 as tachometer channels.
>>
> 
> Hi Guenter,
> 
> I applied the patch [2/3] in my patch series 
> (https://lore.kernel.org/lkml/20240414042246.8681-3-chanh@os.amperecomputing.com/)
> 
> My device tree is configured as below, I would like to configure PWMOUT 
> pins 5 and 6 to become the tachometer input pins.
> 

And what is wrong in described common tach-ch property? I think we
explained it three times and you did not provide any arguments, what's
missing. Instead you say "I want something like this in DTS" which is
not an argument and does not help discussion.

Best regards,
Krzysztof