linux-kernel - Re: [PATCH v3 1/2] dt-bindings: iio: adf4371: add rdiv2 and doubler

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Message-ID: <20241130163948.269289dc@jic23-huawei>
Date: Sat, 30 Nov 2024 16:39:48 +0000
From: Jonathan Cameron <jic23@...nel.org>
To: Antoniu Miclaus <antoniu.miclaus@...log.com>
Cc: <robh@...nel.org>, <conor+dt@...nel.org>, <linux-iio@...r.kernel.org>,
 <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
 <linux-pwm@...r.kernel.org>
Subject: Re: [PATCH v3 1/2] dt-bindings: iio: adf4371: add rdiv2 and doubler

On Fri, 29 Nov 2024 17:33:52 +0200
Antoniu Miclaus <antoniu.miclaus@...log.com> wrote:

> Add support for reference doubler enable and reference divide by 2
> clock.
> 
> Both of these blocks are optional on the frequency path within the
> chip and can be adjusted depending on the custom needs of the
> applications.
Thanks for the additional info!
> 
> The doubler is useful for increasing the PFD comparison frequency
> which will result in a noise performance of the system.

So I'll play devil's advocate. Improved noise performance sounds
good. If it doesn't take me out of range of allowed frequencies, why
would I not turn it on?  What is it about the surrounding circuitry
etc that would make this a bad idea for some uses of this chip
but not others?

> 
> The reference divide by 2 divides the reference signal by 2,
> resulting in a 50% duty cycle PFD frequency.

why would I want one of those? My 'guess' is this makes sense
if the reference frequency is too high after the application of
the scaling done by the 5 bit counter.  In effect it means the
division circuitry does divide by 1-31, 2-64 in steps of 2.

That could all be wrapped up in the existing control of the
frequency, and so far I'm still not seeing a strong reason why
it belongs in DT.

The 50% cycle thing is a bit of a red herring as assuming it
is triggered on say the rising edge of the high frequency signal
to toggle the divided signal, that will always be a 50% duty cycle.

Jonathan

> 
> Signed-off-by: Antoniu Miclaus <antoniu.miclaus@...log.com>
> ---
> changes in v3:
>  - add explanation in commit body
>  .../devicetree/bindings/iio/frequency/adf4371.yaml    | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/iio/frequency/adf4371.yaml b/Documentation/devicetree/bindings/iio/frequency/adf4371.yaml
> index 1cb2adaf66f9..ef241c38520c 100644
> --- a/Documentation/devicetree/bindings/iio/frequency/adf4371.yaml
> +++ b/Documentation/devicetree/bindings/iio/frequency/adf4371.yaml
> @@ -40,6 +40,17 @@ properties:
>        output stage will shut down until the ADF4371/ADF4372 achieves lock as
>        measured by the digital lock detect circuitry.
>  
> +  adi,reference-doubler-enable:
> +    type: boolean
> +    description:
> +      If this property is present, the reference doubler block is enabled.
> +
> +  adi,adi,reference-div2-enable:
> +    type: boolean
> +    description:
> +      If this property is present, the reference divide by 2 clock is enabled.
> +      This feature can be used to provide a 50% duty cycle signal to the PFD.
> +
>  required:
>    - compatible
>    - reg