On 1/3/25 11:58, Rob Herring wrote:
On Fri, Jan 03, 2025 at 11:14:47AM +0100, Peter Korsgaard wrote:
The pwm-fan driver uses full PWM (255) duty cycle at startup, which may not
always be desirable because of noise or power consumption peaks, so add an
optional "default-pwm" property that can be used to specify a custom default
PWM duty cycle (0..255).
This is somewhat similar to target-rpm from fan-common.yaml, but that cannot
be used here as target-rpm specifies the target fan speed, whereas this is
the default pwm to set when the device is instantiated - And the resulting
fan RPM resulting from a given PWM duty cycle is fan dependent.
I still don't agree. Quoting Guenter:
The two values are also orthogonal. The fan rpm is fan dependent.
Each fan will require a different pwm value to reach the target speed.
Trying to use target-rpm to set a default pwm value would really
not make much if any sense.
That is a mis-quote, really. I was talking about target-rpm. The property
to be added here is default-pwm, which is completely different.
But RPM is ultimately what you care about and is the fan parameter
that's universal yet independent of the underlying control. RPM is what
determines noise and power consumption.
There's 2 cases to consider: you have a tach signal and know the fan RPM
or you don't know the RPM. If you have a tach signal, we probably
wouldn't be discussing this because target-rpm would be enough. So I'm
assuming this is the case and you have no idea what RPM the fan runs at.
The fan-common.yaml binding is a bit incomplete for this. What you need
is some map of fan speed to PWM duty cycle as most likely it is not
linear response. I think there are 2 options here:
Use the 'cooling-levels' property. Fan "speed" is the index of the
array. So you just need a 'default-cooling-level' property that's the
default index.
The other option is define an array of (fan RPM, PWM duty cycle) tuples.
Then target-rpm can be used to select the entry. We already have
something like this with 'gpio-fan,speed-map'.
That won't work. Literally each individual fan (even the same model)
runs at a different rpm for a given pwm value, and I am sure the rpm
for a given pwm input changes depending on fan age and temperature.
Quoting from the NCT6796D-S datasheet, for an example:
"The default duty cycle is 7Fh, or 50% for SYSFANOUT, CPUFANOUT, AUXFANOUT0, AUXFANOUT1,
AUXFANOUT2, AUXFANOUT3 and AUXFANOUT4.
Note.The default speed of fan output is specified in registers CR[E0h] ~ CR[E4h] and CR[E7h] of Logical Device B,
and the AUXFANOUT4 default speed of fan output is specified in register CR[E3h] of Logical Device D.
"
Note that the term "default speed" as used in the datasheet refers to pwm.
Again, from the datasheet:
CR E3h. AUXFAN4 Duty Cycle Register
^^^^^^^^^^^^^^^^^^^
Setting the default value for such a register is what we are talking about here.
No rpm involved, just default pwm values. This is completely independent of rpm.
Guenter
