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

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Date:   Wed, 26 Sep 2018 20:51:21 +0200
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Rob Herring <robh@...nel.org>
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 26/09/2018 10:47:08-0500, Rob Herring wrote:
> > > +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?
> 

For that particular RTC, yes but the isl1208 has 0.5pF steps and the
x1205 has 0.25pf steps. I'd prefer having a single generic property for
all the RTCs: quartz-load-femtofarads.

-- 
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com