linux-kernel - Re: [PATCH 4/6] dt-bindings: clock: st,stm32-rcc: support spread spectrum clocking

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Message-ID: <CABGWkvpgGNitRfe+qxT9faZ=Oa5+OzbE06dr-0O+w31JOmOQfw@mail.gmail.com>
Date: Thu, 9 Jan 2025 22:18:05 +0100
From: Dario Binacchi <dario.binacchi@...rulasolutions.com>
To: Rob Herring <robh@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-amarula@...rulasolutions.com, 
	Alexandre Torgue <alexandre.torgue@...s.st.com>, Conor Dooley <conor+dt@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Maxime Coquelin <mcoquelin.stm32@...il.com>, 
	Michael Turquette <mturquette@...libre.com>, Stephen Boyd <sboyd@...nel.org>, devicetree@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-clk@...r.kernel.org, 
	linux-stm32@...md-mailman.stormreply.com
Subject: Re: [PATCH 4/6] dt-bindings: clock: st,stm32-rcc: support spread
 spectrum clocking

Hello Rob,

On Thu, Jan 9, 2025 at 12:48 AM Rob Herring <robh@...nel.org> wrote:
>
> On Sun, Jan 05, 2025 at 07:14:16PM +0100, Dario Binacchi wrote:
> > The addition of DT bindings for enabling and tuning spread spectrum
> > clocking generation is available only for the main PLL.
> >
> > Signed-off-by: Dario Binacchi <dario.binacchi@...rulasolutions.com>
> > ---
> >
> >  .../bindings/clock/st,stm32-rcc.yaml          | 27 +++++++++++++++++++
> >  1 file changed, 27 insertions(+)
> >
> > diff --git a/Documentation/devicetree/bindings/clock/st,stm32-rcc.yaml b/Documentation/devicetree/bindings/clock/st,stm32-rcc.yaml
> > index ae9e5b26d876..c345d3ff3fc4 100644
> > --- a/Documentation/devicetree/bindings/clock/st,stm32-rcc.yaml
> > +++ b/Documentation/devicetree/bindings/clock/st,stm32-rcc.yaml
> > @@ -77,6 +77,26 @@ properties:
> >        Phandle to system configuration controller. It can be used to control the
> >        power domain circuitry.
> >
> > +  st,ssc-modfreq-hz:
> > +    description:
> > +      The modulation frequency for main PLL (in Hz)
>
> No constraints?

They vary depending on the HSE clock frequency used.

>
> > +
> > +  st,ssc-moddepth-permyriad:
> > +    $ref: /schemas/types.yaml#/definitions/uint32
> > +    description:
> > +      The modulation rate for main PLL (in permyriad, i.e. 0.01%)
> > +    minimum: 25
> > +    maximum: 200
> > +
> > +  st,ssc-modmethod:
> > +    $ref: /schemas/types.yaml#/definitions/non-unique-string-array
> > +    description:
> > +      The modulation techniques for main PLL.
> > +    items:
> > +      enum:
> > +        - center-spread
> > +        - down-spread
>
> What's the default? If there's only 2 possibilities, then you can use a
> boolean instead. Though I assume you want to support spread-spectrum
> disabled.

Generally, there can also be more than two options. For example, in
the case of the imx8m platform, there are three possibilities:
- down-spread
- center-spread
- up-spread
In our case, using a boolean would hide one of the two pieces of information,
meaning that if I'm not using center-spread, what am I using?

IMHO, I would keep the string type, changing it from
on-unique-string-array to string.

>
> Are there dependencies between these properties?

The values moddepth and modfreq are used to calculate, according to specific
relationships, two values MODPER and INCSTEP, which are bit fields of
a suitable
register for managing the SSCG. There is a constraint between these two values,
namely that MODPER * INCSTEP < 32768. This constraint will be managed
at the driver level.

Thanks and regards,
Dario
>
> > +
> >  required:
> >    - compatible
> >    - reg
> > @@ -97,6 +117,10 @@ allOf:
> >            const: 1
> >            description: |
> >              The clock index for the specified type.
> > +        st,ssc-modfreq-hz: false
> > +        st,ssc-moddepth-permyriad: false
> > +        st,ssc-modmethod: false
> > +
> >      else:
> >        properties:
> >          '#clock-cells':
> > @@ -118,6 +142,9 @@ examples:
> >          reg = <0x40023800 0x400>;
> >          clocks = <&clk_hse>, <&clk_i2s_ckin>;
> >          st,syscfg = <&pwrcfg>;
> > +        st,ssc-modfreq-hz = <10000>;
> > +        st,ssc-moddepth-permyriad = <200>;
> > +        st,ssc-modmethod = "center-spread";
> >      };
> >
> >    - |
> > --
> > 2.43.0
> >



-- 

Dario Binacchi

Senior Embedded Linux Developer

dario.binacchi@...rulasolutions.com

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