linux-kernel - Re: [PATCH v2 1/2] dt-bindings: rtc: pcf2127: Add nxp,battery-switch-over property

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Message-ID: <pj3tgtsdsjsqqznxgzzmxrcozibqie6ubtythou7t23tfgde5w@t6nwxob4rjah>
Date: Thu, 24 Oct 2024 09:25:29 +0200
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Philipp Rosenberger <p.rosenberger@...bus.com>
Cc: Conor Dooley <conor@...nel.org>, 
	Alexandre Belloni <alexandre.belloni@...tlin.com>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, linux-rtc@...r.kernel.org, 
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Lino Sanfilippo <l.sanfilippo@...bus.com>
Subject: Re: [PATCH v2 1/2] dt-bindings: rtc: pcf2127: Add
 nxp,battery-switch-over property

On Thu, Oct 24, 2024 at 09:11:04AM +0200, Philipp Rosenberger wrote:
> On 22.10.24 18:35, Conor Dooley wrote:
> > On Tue, Oct 22, 2024 at 11:28:54AM +0200, Philipp Rosenberger wrote:
> > > The nxp,battery-switch-over property is used to control the switch-over,
> > > battery low detection and extra power fail detection functions.
> > > 
> > > The PCF2131 has a different default value for the PWRMNG bits. It is set
> > > to 0x7: battery switch-over function is disabled, only one power supply
> > > (VDD); battery low detection function is disabled.
> > > This is the opposite of the default of the PCF2127/PCA2129 and PCF2129.
> > > With the nxp,battery-switch-over the behavior can be controlled through
> > > the device tree.
> > > 
> > > Signed-off-by: Philipp Rosenberger <p.rosenberger@...bus.com>
> > > ---
> > >   Documentation/devicetree/bindings/rtc/nxp,pcf2127.yaml | 10 ++++++++++
> > >   1 file changed, 10 insertions(+)
> > > 
> > > diff --git a/Documentation/devicetree/bindings/rtc/nxp,pcf2127.yaml b/Documentation/devicetree/bindings/rtc/nxp,pcf2127.yaml
> > > index 2d9fe5a75b06..5739c3e371e7 100644
> > > --- a/Documentation/devicetree/bindings/rtc/nxp,pcf2127.yaml
> > > +++ b/Documentation/devicetree/bindings/rtc/nxp,pcf2127.yaml
> > > @@ -30,6 +30,16 @@ properties:
> > >     reset-source: true
> > > +  nxp,battery-switch-over:
> > > +    description:
> > > +      Battery and power related configuration. This property is used to set the
> > > +      PWRMNG bits of the Control_3 register to control the battery switch-over,
> > > +      battery low detection and extra power fail detection functions.
> > > +      The actual supported functions depend on the device capabilities.
> > > +    $ref: /schemas/types.yaml#/definitions/uint8
> > > +    minimum: 0
> > > +    maximum: 7
> > 
> > Beyond the fact that I dislike register-content properties like this, where
> > it is not possible to grok the meaning by reading the property, what
> 
> Yes, I'm not satisfied with this solution myself.
> There are three different functions, which can be configured in the
> register:
> - battery switch-over mode: standard; direct; disabled
> - battery low detection: enabled; disabled
> - extra power fail detection: enabled; disabled
> 
> I'm not sure what a proper way is to implement this in the devicetree.
> 
> > even makes this suitable for DT in the first place? Reading the commit
> > message this sounds like software policy, and that different users of
> > the same board might want to configure these register bits in different
> > ways.
> 
> It is less a software policy, but a configuration how the hardware is
> implemented. If the device has no battery, it is possible to disable the
> battery switch-over function. In this case the V_BAT must be connected to
> ground.

monitored-battery property already tells you this.

> If a battery is connected, the battery switchover will only work if the
> battery switch-over function is in standard mode or direct switching mode.
> Until now the driver has just ignored the PWRMNG bits. As the default was
> battery switching in standard mode. Thus all use cases worked good enough.
> Battery switching was working if a battery was connected. If no battery was
> connected it did no real harm (the rtc may have used a tiny bit more power
> then needed, I guess).

Why driver cannot use standard mode always? Or other way?

> With the new PCF2131 the default has changed to battery switch-over
> disabled. Now even with a battery attached, the rtc will lose time after a
> power cycle.
> I guess I should describe this better in the commit message.

In any case this is pcf2131 related, right? So compatible implies it.

Best regards,
Krzysztof