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

From: Guenter Roeck <linux@roeck-us.net>
To: Chanh Nguyen <chanh@amperemail.onmicrosoft.com>,
	Conor Dooley <conor@kernel.org>
Cc: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>,
	Chanh Nguyen <chanh@os.amperecomputing.com>,
	Jean Delvare <jdelvare@suse.com>,
	Rob Herring <robh+dt@kernel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
	Conor Dooley <conor+dt@kernel.org>,
	Justin Ledford <justinledford@google.com>,
	devicetree@vger.kernel.org, linux-hwmon@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	OpenBMC Maillist <openbmc@lists.ozlabs.org>,
	Open Source Submission <patches@amperecomputing.com>,
	Phong Vo <phong@os.amperecomputing.com>,
	Thang Nguyen <thang@os.amperecomputing.com>,
	Quan Nguyen <quan@os.amperecomputing.com>
Subject: Re: [PATCH v2 3/3] dt-bindings: hwmon: max31790: Add maxim,pwmout-pin-as-tach-input property
Date: Thu, 25 Apr 2024 07:05:31 -0700	[thread overview]
Message-ID: <396b47f5-9604-44ab-881f-94d0664bcab8@roeck-us.net> (raw)
In-Reply-To: <ab5cfd8c-0e88-4194-a77e-5ffbb6890319@amperemail.onmicrosoft.com>

On 4/25/24 03:33, Chanh Nguyen wrote:
> 
> 
> On 24/04/2024 00:02, Conor Dooley wrote:
>> [EXTERNAL EMAIL NOTICE: This email originated from an external sender. Please be mindful of safe email handling and proprietary information protection practices.]
>>
> 

The quote doesn't make much sense.

> Sorry Conor, there may be confusion here. I mean the mapping of the PWM output to the TACH input, which is on the MAX31790, and it is not sure a common feature on all fan controllers.
> 

I think the term "mapping" is a bit confusing here.

tach-ch, as I understand it, is supposed to associate a tachometer input
with a pwm output, meaning the fan speed measured with the tachometer input
is expected to change if the pwm output changes.

On MAX31790, it is possible to configure a pwm output pin as tachometer input pin.
That is something completely different. Also, the association is fixed.
If the first pwm channel is used as tachometer channel, it would show up as 7th
tachometer channel. If the 6th pwm channel is configured to be used as tachometer
input, it would show up as 12th tachometer channel.

Overall, the total number of channels on MAX31790 is always 12. 6 of them
are always tachometer inputs, the others can be configured to either be a
pwm output or a tachometer input.

pwm outputs on MAX31790 are always tied to the matching tachometer inputs
(pwm1 <--> tach1 etc) and can not be reconfigured, meaning tach-ch for
channel X would always be X.

> I would like to open a discussion about whether we should use the tach-ch property on the fan-common.yaml
> 
> I'm looking forward to hearing comments from everyone. For me, both tach-ch and vendor property are good.
> 

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.

Guenter