Re: [EXT] Re: [PATCH v4 1/2] dt-bindings: rtc: Add pcf85053 support

[Conor Dooley <conor@kernel.org>]

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On Thu, Oct 09, 2025 at 08:13:49AM +0000, Lakshay Piplani wrote:
> 
> > -----Original Message-----
> > From: Conor Dooley <conor@kernel.org>
> > Sent: Wednesday, September 24, 2025 12:28 AM
> > To: Lakshay Piplani <lakshay.piplani@nxp.com>
> > Cc: alexandre.belloni@bootlin.com; linux-rtc@vger.kernel.org; linux-
> > kernel@vger.kernel.org; robh@kernel.org; krzk+dt@kernel.org;
> > conor+dt@kernel.org; devicetree@vger.kernel.org; Pankit Garg
> > <pankit.garg@nxp.com>; Vikash Bansal <vikash.bansal@nxp.com>; Priyanka
> > Jain <priyanka.jain@nxp.com>; Shashank Rebbapragada
> > <shashank.rebbapragada@nxp.com>
> > Subject: [EXT] Re: [PATCH v4 1/2] dt-bindings: rtc: Add pcf85053 support
> > 
> > On Tue, Sep 23, 2025 at 05:04:40PM +0530, Lakshay Piplani wrote:
> > > Add device tree bindings for NXP PCF85053 RTC chip.
> > >
> > > Signed-off-by: Pankit Garg <pankit.garg@nxp.com>
> > > Signed-off-by: Lakshay Piplani <lakshay.piplani@nxp.com>
> > > ---
> > > V3 -> V4: Add dedicated nxp,pcf85053.yaml.
> > >           Remove entry from trivial-rtc.yaml.
> > > V2 -> V3: Moved MAINTAINERS file changes to the driver patch
> > > V1 -> V2: Handled dt-bindings by trivial-rtc.yaml
> > >
> > >  .../devicetree/bindings/rtc/nxp,pcf85053.yaml | 128
> > > ++++++++++++++++++
> > >  1 file changed, 128 insertions(+)
> > >  create mode 100644
> > > Documentation/devicetree/bindings/rtc/nxp,pcf85053.yaml
> > >
> > > diff --git a/Documentation/devicetree/bindings/rtc/nxp,pcf85053.yaml
> > > b/Documentation/devicetree/bindings/rtc/nxp,pcf85053.yaml
> > > new file mode 100644
> > > index 000000000000..6b1c97358486
> > > --- /dev/null
> > > +++ b/Documentation/devicetree/bindings/rtc/nxp,pcf85053.yaml
> > > @@ -0,0 +1,128 @@
> > > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause) # Copyright
> > > +2025 NXP %YAML 1.2
> > > +---
> > > +$id: http://devicetree.org/schemas/rtc/nxp,pcf85053.yaml#
> > > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > > +
> > > +title: NXP PCF85053 Real Time Clock
> > > +
> > > +maintainers:
> > > +  - Pankit Garg <pankit.garg@nxp.com>
> > > +  - Lakshay Piplani <lakshay.piplani@nxp.com>
> > > +
> > > +properties:
> > > +  compatible:
> > > +    enum:
> > > +      - nxp,pcf85053
> > > +
> > > +  reg:
> > > +    maxItems: 1
> > > +
> > > +  interrupts:
> > > +    maxItems: 1
> > > +
> > > +  nxp,interface:
> > > +    $ref: /schemas/types.yaml#/definitions/string
> > > +    enum: [ primary, secondary ]
> > > +    description: |
> > > +      Identifies this host's logical role in a multi-host topology for the
> > > +      PCF85053 RTC. The device exposes a "TWO" ownership bit in the CTRL
> > > +      register that gates which host may write time/alarm registers.
> > > +        - "primary": Designated host that *may* claim write ownership (set
> > > +          CTRL.TWO=1) **if** write-access is explicitly requested.
> > > +        - "secondary": Peer host that writes only when CTRL.TWO=0 (default).
> > > +
> > > +  nxp,write-access:
> > > +    type: boolean
> > > +    description: |
> > > +      Request the driver to claim write ownership at probe time by setting
> > > +      CTRL.TWO=1. This property is only valid when nxp,interface="primary".
> > > +      The driver will not modify any other CTRL bits (HF/DM/etc.) and will
> > not
> > > +      clear any status/interrupt flags at probe.
> > > +
> > > +required:
> > > +  - compatible
> > > +  - reg
> > > +  - nxp,interface
> > > +
> > > +additionalProperties: false
> > > +
> > > +# Schema constraints matching driver:
> > > +# 1) If nxp,write-access is present, nxp,interface must be "primary".
> > > +#    Rationale: only the primary may claim ownership; driver will set
> > TWO=1.
> > > +# 2) If nxp,interface is "secondary", nxp,write-access must not be present.
> > > +#    Rationale: secondary never claims ownership and cannot write
> > CTRL/ST/alarm.
> > > +#
> > > +# Practical effect:
> > > +# - Primary without 'nxp,write-access'; primary is read only; secondary
> > may
> > > +#   write time registers.
> > > +# - Primary with 'nxp,write-access'; primary owns writes, secondary is read
> > only.
> > > +allOf:
> > > +  - $ref: rtc.yaml#
> > > +  - oneOf:
> > > +      # Case 1: primary with write-access
> > > +      - required: [ "nxp,write-access" ]
> > > +        properties:
> > > +          nxp,interface:
> > > +            const: primary
> > > +
> > > +      # Case 2: primary without write-access
> > > +      - properties:
> > > +          nxp,interface:
> > > +            const: primary
> > > +        not:
> > > +          required: [ "nxp,write-access" ]
> > 
> > Aren't case 1 and case 2 here redundant? All you need to do is block interface
> > == secondary when nxp,write-access is present, which your case
> > 3 should be able to be modified to do via
> > 
> > if:
> >   properties:
> >     nxp,interface:
> >       const: secondary
> > then:
> >   properties:
> >    nxp,write-access: false
> > 
> > I think your description for nxp,write-access gets the point across about when
> > it can be used, and the additional commentary is not really helpful.
> > 
> Thanks for reviewing the patch.
> 
> We kept both cases: primary with write-access and primary without write-access, because the hardware 
> supports three different ways it can be used, and we want to show that clearly in the bindings:
>  
> Primary with nxp,write-access: primary host can write to the device.
> Primary without nxp,write-access - primary host is read-only, and the secondary host can write.
> Secondary - default role, with write access; when no primary host is claiming it.
>  
> Even though both case 1 and 2 use nxp,interface = "primary", they behave differently.
> Keeping both cases separate makes it easier to understand whether Primary host can write or not.

Just explain it properly in the property description, creating
if/then/else stuff with additional commentary like this makes it more,
rather than less, confusing.

> 
> > > +
> > > +      # Case 3: secondary (must not have write-access)
> > > +      - properties:
> > > +          nxp,interface:
> > > +            const: secondary
> > > +        not:
> > > +          required: [ "nxp,write-access" ]
> > > +
> > > +examples:
> > > +  # Single host example.
> > > +  - |
> > > +    #include <dt-bindings/interrupt-controller/irq.h>
> > > +    i2c {
> > > +      #address-cells = <1>;
> > > +      #size-cells = <0>;
> > > +
> > > +      rtc@6f {
> > > +        compatible = "nxp,pcf85053";
> > > +        reg = <0x6f>;
> > > +        nxp,interface = "primary";
> > > +        nxp,write-access;
> > > +        interrupt-parent = <&gpio2>;
> > > +        interrupts = <3 IRQ_TYPE_EDGE_FALLING>;
> > > +      };
> > > +    };
> > > +
> > > +  # Dual-host example: one primary that claims writes; one secondary that
> > never claims writes.
> > > +  - |
> > > +    #include <dt-bindings/interrupt-controller/irq.h>
> > > +    i2c0 {
> > > +      #address-cells = <1>;
> > > +      #size-cells = <0>;
> > > +
> > > +      rtc@6f {
> > > +        compatible = "nxp,pcf85053";
> > > +        reg = <0x6f>;
> > > +        nxp,interface = "primary";
> > > +        nxp,write-access;
> > > +        interrupt-parent = <&gpio2>;
> > > +        interrupts = <3 IRQ_TYPE_EDGE_FALLING>;
> > > +      };
> > > +    };
> > > +
> > > +    i2c1 {
> > > +      #address-cells = <1>;
> > > +      #size-cells = <0>;
> > > +
> > > +      rtc@6f {
> > > +        compatible = "nxp,pcf85053";
> > > +        reg = <0x6f>;
> > > +        nxp,interface = "secondary";
> > 
> > Maybe a silly question, but if you have a system that wants to have two pairs
> > of RTCs, how would you determine which primary a secondary belongs to? I
> > notice you have no link between these devices in dt so I am curious. Would it
> > be better to eschew nxp,interface and have a phandle from the secondary to
> > the primary?
> > 
> > I don't know anything about your use case or features, so maybe knowing the
> > relationship just is not relevant at all, or it can be determined at runtime.
> 
> This device can connect to two independent hosts via separate I²C buses. 
> Each host sees the same hardware instance through its own I²C address. The nxp,interface
> property simply declares the host's role, so the driver knows whether to attempt write
> access or not.

That doesn't really answer what I was looking for. Is knowing the
relationships either unimportant or determinable at runtime?

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