Re: [RFC PATCH 3/6] dt-bindings: axi-fan-control: add tacho properties

[Guenter Roeck <linux@roeck-us.net>]

On 7/16/21 12:44 AM, Sa, Nuno wrote:
[ ... ]
>>
>> Are you sure you can ever get this stable ? Each fan has its own
>> properties
>> and tolerances. If you replace a fan in a given system, you might get
>> different RPM numbers. The RPM will differ widely from system to
>> system
>> and from fan to fan. Anything that assumes a specific RPM in
>> devicetree
>> data seems to be quite vulnerable to failures. I have experienced that
>> recently with a different chip which also tries to correlate RPM and
>> PWM
>> and fails quite miserably.
>>
>> In my experience, anything other than minimum fan speed is really a
>> recipe
>> for instability and sporadic false failures. Even setting a minimum fan
>> speed
>> is tricky because it depends a lot on the fan.
> 
> I see what you mean. So, I had to go through this process when testing
> this changes because the fan I'm using is different from the default one
> used to develop and stablish the default values in the IP core. The core

Exactly my point.

> provides you with a register which contains the tacho measurements in
> clock cycles. You can read that for all the PWM points of interest
> (with devmem2 for example) and make your own "calibration". I assume
> that people have to go through this process before putting some values
> in the devicetree. I'm aware this is not the neatest process but I guess it's
> acceptable...
> 

Do you really expect everyone using a system with this chip to go through
this process and update its devicetree configuration, and then repeat it
whenever a fan is changed ? Given how dynamic this is, I really wonder
if that information should be in devicetree in the first place.

Guenter