linux-kernel - Re: [PATCH v1 1/5] dt-binding: rtci-pcf8523: add quartz

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Date:   Wed, 26 Sep 2018 10:47:08 -0500
From:   Rob Herring <robh@...nel.org>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     Sam Ravnborg <sam@...nborg.org>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Andrew Jeffery <andrew@...id.au>,
        Fabio Estevam <fabio.estevam@....com>,
        Joel Stanley <joel@....id.au>,
        Mark Rutland <mark.rutland@....com>,
        Russell King <linux@...linux.org.uk>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Shawn Guo <shawnguo@...nel.org>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-rtc@...r.kernel.org,
        Søren Andersen <san@...v.dk>
Subject: Re: [PATCH v1 1/5] dt-binding: rtci-pcf8523: add quartz_load property

On Thu, Sep 13, 2018 at 09:05:16PM +0200, Alexandre Belloni wrote:
> Hi,
> 
> You can remove 'rtci-' from the subject.
> 
> On 07/09/2018 21:35:04+0200, Sam Ravnborg wrote:
> > From: Søren Andersen <san@...v.dk>
> > 
> > The NXP pcf8523 supports two different quartz loads.
> > - 7 pF (default)
> > - 12.5 pF (minimum power consumption)
> > 
> > The pcf8523 needs to know the size of the quartz load,
> > otherwise the the RTC will have a bad precision.
> > 
> > The default for the rtc (after power-on) is 7 pF.
> > Add a property that tells if the external capacitor is 12.5 pF.
> > 
> > Signed-off-by: Søren Andersen <san@...v.dk>
> > Signed-off-by: Sam Ravnborg <sam@...nborg.org>
> > Cc: Alessandro Zummo <a.zummo@...ertech.it>
> > Cc: Alexandre Belloni <alexandre.belloni@...tlin.com>
> > Cc: Rob Herring <robh+dt@...nel.org>
> > Cc: Mark Rutland <mark.rutland@....com>
> > ---
> >  Documentation/devicetree/bindings/rtc/nxp,pcf8523.txt | 19 +++++++++++++++++++
> >  Documentation/devicetree/bindings/trivial-devices.txt |  1 -
> >  2 files changed, 19 insertions(+), 1 deletion(-)
> >  create mode 100644 Documentation/devicetree/bindings/rtc/nxp,pcf8523.txt
> > 
> > diff --git a/Documentation/devicetree/bindings/rtc/nxp,pcf8523.txt b/Documentation/devicetree/bindings/rtc/nxp,pcf8523.txt
> > new file mode 100644
> > index 000000000000..7c5e93f5077c
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/rtc/nxp,pcf8523.txt
> > @@ -0,0 +1,19 @@
> > +* NXP PCF8523 Real Time Clock
> > +
> > +NXP PCF8523 Real Time Clock
> > +
> > +Required properties:
> > +- compatible: Should contain "nxp,pcf8523".
> > +- reg: I2C address for chip.
> > +
> > +Optional property:
> > +- nxp,quartz_load_12.5pF: The capacitive load on the quartz is 12.5 pF,
> > +  which differ from the default value of 7 pF
> > +
> 
> The boolean properties usually don't work well for RTCs because people
> usually want to keep any previous configuration that may have been done
> at the factory or in the bootloader so I would use:
> 
> nxp,quartz_load_fF and this would be either 7000 or 12500.

Use '-' rather than '_'.

There's already a defined unit suffix of '-picofarads'. If you need 
'-femtofarads', then please add to property-units.txt. Though I'd prefer 
not as wouldn't a value of 12 (or 13) be close enough for the needs 
here?

Rob