linux-kernel - Re: [PATCH v2 1/3] dt-bindings: rtc: pcf8523: add DSM pm option for battery switch-over

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Message-ID: <20200727094553.GH1551@shell.armlinux.org.uk>
Date:   Mon, 27 Jul 2020 10:45:53 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     Rob Herring <robh@...nel.org>, miguelborgesdefreitas@...il.com,
        a.zummo@...ertech.it, baruch@...s.co.il, shawnguo@...nel.org,
        s.hauer@...gutronix.de, kernel@...gutronix.de, festevam@...il.com,
        linux-imx@....com, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] dt-bindings: rtc: pcf8523: add DSM pm option for
 battery switch-over

On Thu, Jul 23, 2020 at 09:57:55PM +0200, Alexandre Belloni wrote:
> On 23/07/2020 11:49:05-0600, Rob Herring wrote:
> > On Mon, Jul 20, 2020 at 12:23:59PM +0100, miguelborgesdefreitas@...il.com wrote:
> > > From: Miguel Borges de Freitas <miguelborgesdefreitas@...il.com>
> > > 
> > > This adds direct-switching mode as a configurable DT flag for
> > > RTC modules supporting it (e.g. nxp pcf8523).
> > > DSM switches the power source to the battery supply whenever the
> > > VDD drops below VBAT. The option is recommended for hw designs
> > > where VDD is always expected to be higher than VBAT.
> > > 
> > > Signed-off-by: Miguel Borges de Freitas <miguelborgesdefreitas@...il.com>
> > > ---
> > > Changes in v2:
> > > - Added extended commit message for git history
> > > - Separate dt bindings documentation into a single patch
> > > 
> > >  Documentation/devicetree/bindings/rtc/nxp,pcf8523.txt | 7 ++++++-
> > >  Documentation/devicetree/bindings/rtc/rtc.yaml        | 7 +++++++
> > >  2 files changed, 13 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/Documentation/devicetree/bindings/rtc/nxp,pcf8523.txt b/Documentation/devicetree/bindings/rtc/nxp,pcf8523.txt
> > > index 0b1080c..f715a8f 100644
> > > --- a/Documentation/devicetree/bindings/rtc/nxp,pcf8523.txt
> > > +++ b/Documentation/devicetree/bindings/rtc/nxp,pcf8523.txt
> > > @@ -4,10 +4,14 @@ Required properties:
> > >  - compatible: Should contain "nxp,pcf8523".
> > >  - reg: I2C address for chip.
> > >  
> > > -Optional property:
> > > +Optional properties:
> > >  - quartz-load-femtofarads: The capacitive load of the quartz(x-tal),
> > >    expressed in femto Farad (fF). Valid values are 7000 and 12500.
> > >    Default value (if no value is specified) is 12500fF.
> > > +- pm-enable-dsm: battery switch-over function is enabled in direct
> > > +  switching mode. The power failure condition happens when VDD < VBAT,
> > > +  without requiring VDD to drop below Vth(sw)bat.
> > > +  Default value (if not provided) is the standard mode.
> > >  
> > >  Example:
> > >  
> > > @@ -15,4 +19,5 @@ pcf8523: rtc@68 {
> > >  	compatible = "nxp,pcf8523";
> > >  	reg = <0x68>;
> > >  	quartz-load-femtofarads = <7000>;
> > > +	pm-enable-dsm;
> > >  };
> > > diff --git a/Documentation/devicetree/bindings/rtc/rtc.yaml b/Documentation/devicetree/bindings/rtc/rtc.yaml
> > > index ee237b2..a0048f4 100644
> > > --- a/Documentation/devicetree/bindings/rtc/rtc.yaml
> > > +++ b/Documentation/devicetree/bindings/rtc/rtc.yaml
> > > @@ -47,4 +47,11 @@ properties:
> > >      description:
> > >        Enables wake up of host system on alarm.
> > >  
> > > +  pm-enable-dsm:
> > > +    $ref: /schemas/types.yaml#/definitions/flag
> > > +    description:
> > > +      Enables the battery switch-over function in direct switching
> > > +      mode. Should be set in systems where VDD is higher than VBAT
> > > +      at all times.
> > 
> > I'm all for common properties, but is this common across vendors?
> > 
> 
> This is but this shouldn't be a DT property as it has to be changed
> dynamically. I'm working on an ioctl interface to change this
> configuration.

Why does it need to be changed dynamically?  If the hardware components
are not fitted to allow the RTC to be safely used without DSM, then
why should userspace be able to disable DSM?

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