Message-ID:
 <AS4PR04MB93620F7CAD21F745B95FAF1CFBEEA@AS4PR04MB9362.eurprd04.prod.outlook.com>
Date: Thu, 9 Oct 2025 08:13:49 +0000
From: Lakshay Piplani <lakshay.piplani@....com>
To: Conor Dooley <conor@...nel.org>
CC: "alexandre.belloni@...tlin.com" <alexandre.belloni@...tlin.com>,
	"linux-rtc@...r.kernel.org" <linux-rtc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"robh@...nel.org" <robh@...nel.org>, "krzk+dt@...nel.org"
	<krzk+dt@...nel.org>, "conor+dt@...nel.org" <conor+dt@...nel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>, Pankit Garg
	<pankit.garg@....com>, Vikash Bansal <vikash.bansal@....com>, Priyanka Jain
	<priyanka.jain@....com>, Shashank Rebbapragada
	<shashank.rebbapragada@....com>
Subject: RE: [EXT] Re: [PATCH v4 1/2] dt-bindings: rtc: Add pcf85053 support


> -----Original Message-----
> From: Conor Dooley <conor@...nel.org>
> Sent: Wednesday, September 24, 2025 12:28 AM
> To: Lakshay Piplani <lakshay.piplani@....com>
> Cc: alexandre.belloni@...tlin.com; linux-rtc@...r.kernel.org; linux-
> kernel@...r.kernel.org; robh@...nel.org; krzk+dt@...nel.org;
> conor+dt@...nel.org; devicetree@...r.kernel.org; Pankit Garg
> <pankit.garg@....com>; Vikash Bansal <vikash.bansal@....com>; Priyanka
> Jain <priyanka.jain@....com>; Shashank Rebbapragada
> <shashank.rebbapragada@....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@....com>
> > Signed-off-by: Lakshay Piplani <lakshay.piplani@....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@....com>
> > +  - Lakshay Piplani <lakshay.piplani@....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.

> > +
> > +      # 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.
> 
> Cheers,
> Conor.

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.

Thanks
Lakshay

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