linux-kernel - Re: [RFC 1/2] dt-bindings: clk: versaclock5: Add load capacitance properties

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Date:   Fri, 8 Jan 2021 20:48:42 -0600
From:   Adam Ford <aford173@...il.com>
To:     Luca Ceresoli <luca@...aceresoli.net>
Cc:     linux-clk <linux-clk@...r.kernel.org>,
        Adam Ford-BE <aford@...conembedded.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        devicetree <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 1/2] dt-bindings: clk: versaclock5: Add load capacitance properties

On Fri, Jan 8, 2021 at 4:49 PM Luca Ceresoli <luca@...aceresoli.net> wrote:
>
> Hi Adam,
>
> On 06/01/21 18:38, Adam Ford wrote:
> > There are two registers which can set the load capacitance for
> > XTAL1 and XTAL2. These are optional registers when using an
> > external crystal.  Update the bindings to support them.
> >
> > Signed-off-by: Adam Ford <aford173@...il.com>
> > ---
> >  .../devicetree/bindings/clock/idt,versaclock5.yaml   | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> >
> > diff --git a/Documentation/devicetree/bindings/clock/idt,versaclock5.yaml b/Documentation/devicetree/bindings/clock/idt,versaclock5.yaml
> > index 2ac1131fd922..e5e55ffb266e 100644
> > --- a/Documentation/devicetree/bindings/clock/idt,versaclock5.yaml
> > +++ b/Documentation/devicetree/bindings/clock/idt,versaclock5.yaml
> > @@ -59,6 +59,18 @@ properties:
> >      minItems: 1
> >      maxItems: 2
> >
> > +  idt,xtal1-load-femtofarads:
>
> I wonder whether we should have a common, vendor independent property.

That would be nice.

> In mainline we have xtal-load-pf (ti,cdce925.txt bindings) which has no
> vendor prefix. However I don't know how much common it is to need

rtc-pcf85063.c uses  quartz-load-femtofarads, so there is already some
discrepancy.

Since the unit of measure here is femtofarads, using pF in the name seems wrong.
We need to read the data as a u32, so femtofarads works better than
pF, which would require a decimal point.

> different loads for x1 and x2. Any hardware engineer around?

I talked to a hardware engineer where I work, and he said it makes
sense to keep them the same.  I only separated them because there are
two registers, and I assumed there might be a reason to have X1 and X2
be different, but I'm ok with reading one value and writing it to two
different registers.

adam
>
> > +    $ref: /schemas/types.yaml#/definitions/uint32
> > +    minimum: 9000
> > +    maximum: 25000
> > +    description: Optional loading capacitor for XTAL1
>
> Nit: I think the common wording is "load capacitor", not "loading
> capacitor".
>
> --
> Luca