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

On 7/16/21 12:44 AM, Sa, Nuno wrote:
[ ... ]

quoted

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