Re: [PATCH v3 1/2] dt-bindings: hwmon: amc6821: add fan and PWM output

[Guenter Roeck]

On 3/31/25 09:16, Francesco Dolcini wrote:
> Hello Guenter,
>
>
> On Mon, Mar 31, 2025 at 09:01:08AM -0700, Guenter Roeck wrote:
> > On 3/31/25 08:52, Francesco Dolcini wrote:
> > > From: Francesco Dolcini <francesco.dolcini@xxxxxxxxxxx>
> > >
> > > Add properties to describe the fan and the PWM controller output.
> > >
> > > Link: https://www.ti.com/lit/gpn/amc6821
> > > Signed-off-by: Francesco Dolcini <francesco.dolcini@xxxxxxxxxxx>
> > > ---
> > > v3:
> > >    - explicitly describe the fan, use standard PWM and FAN bindings
> > >    - pwm.yaml cannot be referenced, because of the $nodename pattern that is
> > >      enforced there
> > > v2: https://lore.kernel.org/all/20250224180801.128685-2-francesco@xxxxxxxxxx/
> > >    - no changes
> > > v1: https://lore.kernel.org/all/20250218165633.106867-2-francesco@xxxxxxxxxx/
> > > ---
> > >    .../devicetree/bindings/hwmon/ti,amc6821.yaml      | 14 +++++++++++++-
> > >    1 file changed, 13 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/Documentation/devicetree/bindings/hwmon/ti,amc6821.yaml b/Documentation/devicetree/bindings/hwmon/ti,amc6821.yaml
> > > index 5d33f1a23d03..94aca9c378e6 100644
> > > --- a/Documentation/devicetree/bindings/hwmon/ti,amc6821.yaml
> > > +++ b/Documentation/devicetree/bindings/hwmon/ti,amc6821.yaml
> > > @@ -28,6 +28,13 @@ properties:
> > >      i2c-mux:
> > >        type: object
> > > +  fan:
> > > +    $ref: fan-common.yaml#
> > > +    unevaluatedProperties: false
> > > +
> > > +  "#pwm-cells":
> > > +    const: 3
> > > +
> > >    required:
> > >      - compatible
> > >      - reg
> > > @@ -50,9 +57,14 @@ examples:
> > >            #address-cells = <1>;
> > >            #size-cells = <0>;
> > > -        fan@18 {
> > > +        fan_controller: fan@18 {
> > >                compatible = "ti,amc6821";
> > >                reg = <0x18>;
> > > +            #pwm-cells = <3>;
> > > +
> > > +            fan {
> > > +                pwms = <&fan_controller 0 40000 0>;
> >
> > There is no explanation for the parameters. I guess that the first is the index and the
> > second is the frequency. The index is not used and can be dropped (there is just a single
> > channel), and the frequency only makes sense if it can actually be configured.
>
> The PWM period can be configured, the HW supports it, so I think is a
> good idea to properly describe the HW now that I am modifying the
> binding and not have the situation in which we need it in 1 month.
>
> The instance is just one, it's correct, but there is an assumption
> pretty much everywhere that these 3 #cells are
>   - PWM instance
>   - PWM period in ns
>   - PWM flags
>
> I decided to keep the first cell there, with the PWM instance, for this
> reason, even as you correctly wrote it would not be required here.
>
> With that said, looking at existing binding, the description is almost
> never present.

That is not a valid argument.

> I would proceed this way for v4:
>   - keep the first cell as PWM instance
>   - add the description

We just had another driver where we dropped the pwm instance number because
it is unnecessary. See commit a25633a00906. I very much prefer to keep it that
way if it is not needed. Everything else is just confusing. It is bad enough
that the sub-node is needed. We should not make it worse.

Thanks,
Guenter