Re: [PATCH v2 1/2] dt-bindings: hwmon: amc6821: add PWM polarity

[Guenter Roeck]

On 3/19/25 03:12, Francesco Dolcini wrote:
> Hello Rob and all,
>
> On Wed, Feb 26, 2025 at 02:58:06PM +0100, Francesco Dolcini wrote:
> > On Wed, Feb 26, 2025 at 07:49:22AM -0600, Rob Herring wrote:
> > > On Mon, Feb 24, 2025 at 07:08:00PM +0100, Francesco Dolcini wrote:
> > > > From: Francesco Dolcini <francesco.dolcini@xxxxxxxxxxx>
> > > >
> > > > Add property to describe the PWM-Out pin polarity.
> > >
> > > Why doesn't the invert support in the pwm binding work for you? Yes, I
> > > read the discussion, but don't remember the conclusion and you need to
> > > justify it here.
> >
> > This chip is not a PWM controller, it is a FAN controller.
> >
> > The HW has a PWM pin output that is used to control the fan, but the
> > device is not modelled as a PWM controller (correctly, given that is not
> > such a device) and the OS does not control the PWM, the chip reads the
> > temperature and decide the PWM duty cycle accordingly in an autonomous
> > way.
>
> Can you advise on how to move this forward? Is my explanation good
> enough or some more clarification is needed? Should I send a v3
> incorporating such a comment into the commit message? Anything else?

DT maintainers insist that pwm properties are described using pwm cells.
That does not mean that the driver has to implement a pwm controller
(and I would resist doing that, because it is pointless), just that
the chip's pwm properties are described this way. That is indeed a deviation
from older devicetree files, where "inverted" properties were acceptable.

I don't know if there is a means to avoid that. Some devicetree files
don't mention pwm in the property name. See nxp,inverted-out or
atmel,lcdcon-backlight-inverted for examples. I suspect that is no longer
acceptable, though. The easiest would probably be to define optional
minimal pwm bindings for the chip. Unless I am missing something, that
would just be the pwm polarity, so you would have a single pwm cell.
Something like

  '#pwm-cells':
    const: 1
    description: |
      Number of cells in a PWM specifier.
      - cell 0: The PWM polarity: 0 or PWM_POLARITY_INVERTED

and then something like

    fan_controller: fan@18 {
	compatible = "ti,amc6821";
	reg = <0x18>;
	#pwm-cells = <1>;

	pwms = <&fan_controller PWM_POLARITY_INVERTED>;
    };	

That may require some tweaking though; I think #pwm-cells may apply to the
children of a property, not to the property itself.

Guenter