Re: [PATCH 1/4] dt-bindings: rtc: add schema for NXP S32G2/S32G3 SoCs

[Alexandre Belloni]

On 12/09/2024 13:50:25+0300, Ciprian Marian Costea wrote:
> On 9/11/2024 9:21 PM, Conor Dooley wrote:
> > On Wed, Sep 11, 2024 at 10:00:25AM +0300, Ciprian Costea wrote:
> > > From: Ciprian Marian Costea <ciprianmarian.costea@xxxxxxxxxxx>
> > > 
> > > This patch adds the dt-bindings for NXP S32G2/S32G3 SoCs RTC driver.
> > > 
> > > Signed-off-by: Bogdan-Gabriel Roman <bogdan-gabriel.roman@xxxxxxx>
> > > Signed-off-by: Ghennadi Procopciuc <ghennadi.procopciuc@xxxxxxx>
> > > Signed-off-by: Ciprian Marian Costea <ciprianmarian.costea@xxxxxxxxxxx>
> > > ---
> > >   .../devicetree/bindings/rtc/nxp,s32g-rtc.yaml | 79 +++++++++++++++++++
> > >   1 file changed, 79 insertions(+)
> > >   create mode 100644 Documentation/devicetree/bindings/rtc/nxp,s32g-rtc.yaml
> > > 
> > > diff --git a/Documentation/devicetree/bindings/rtc/nxp,s32g-rtc.yaml b/Documentation/devicetree/bindings/rtc/nxp,s32g-rtc.yaml
> > > new file mode 100644
> > > index 000000000000..8f78bce6470a
> > > --- /dev/null
> > > +++ b/Documentation/devicetree/bindings/rtc/nxp,s32g-rtc.yaml
> > > @@ -0,0 +1,79 @@
> > > +# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
> > > +%YAML 1.2
> > > +---
> > > +$id: http://devicetree.org/schemas/rtc/nxp,s32g-rtc.yaml#
> > > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > > +
> > > +title: NXP S32G2/S32G3 Real Time Clock (RTC)
> > > +
> > > +maintainers:
> > > +  - Bogdan Hamciuc <bogdan.hamciuc@xxxxxxx>
> > > +  - Ciprian Marian Costea <ciprianmarian.costea@xxxxxxx>
> > > +
> > > +properties:
> > > +  compatible:
> > > +    const: nxp,s32g-rtc
> > > +
> > > +  reg:
> > > +    maxItems: 1
> > > +
> > > +  interrupts:
> > > +    maxItems: 1
> > > +
> > > +  nxp,clksel:
> > > +    $ref: /schemas/types.yaml#/definitions/uint32
> > > +    description:
> > > +      Input clock selector. Choose between 0-SIRC and 2-FIRC.
> > > +      The reason for these IDs not being consecutive is because
> > > +      they are hardware coupled.
> > > +    enum:
> > > +      - 0  # SIRC
> > > +      - 2  # FIRC
> > 
> > Could you please explain why, given both clocks must be provided by
> > the hardware for there to be a choice, why choosing between them is a
> > property of the hardware?
> > 
> 
> Hello Conor,
> 
> Thanks for your review.
> 
> According to RTC module's clocking scheme for S32G2/S32G3 SoCs, it has three
> potential clock sources to select between:
>   1. FIRC:
>     - fast clock - ~48 MHz output
>     - chosen by default because it is proven to be more reliable (e.g:
> temperature drift).
>   2. SIRC:
>     - slow clock - ~32 kHz output
>     - When in Standby mode, SIRC_CLK is the only available clock for RTC.
> This is important because RTC module is used as a wakeup source from Suspend
> to RAM on S32G2/S32G3 SoC. Therefore, a temporary switch to SIRC clock is
> performed when entering Suspend to RAM.
> 
>   3. EXT_CLK:
>     - has not been tested/validated for those SoCs within NXP's downstream
> Linux. Therefore, I did not treat it, nor mention it, for the moment.
> 
> Now to answer your question, all above clocks are entering a RTCC[CLKSEL]
> (RTCC - RTC Control Register) mux. Therefore, a selection can be made,
> according to one's needs.
> 

Then should this mux be registered in the CCF so you can use the usual
clock node properties?

> I will add a shorter version of above information in the bindings
> documentation in the V2 of this patchset.
> 
> > > +
> > > +  nxp,dividers:
> > > +    $ref: /schemas/types.yaml#/definitions/uint32-array
> > > +    description:
> > > +      An array of two u32 elements, the former encoding DIV512,
> > > +      the latter encoding DIV32. These are dividers that can be enabled
> > > +      individually, or cascaded. Use 0 to disable the respective divider,
> > > +      and 1 to enable it.
> > 
> > Please explain to me what makes this a property of the hardware and how
> > someone would go about choosing the divider settings for their hardware.
> > 
> 
> As per hardware RTC module clocking scheme, the output of the clock mux can
> be optionally divided by a combination of 512 and 32 (via other two input
> cascaded muxes) to give various count periods for different clock sources.
> 
> With respect to choosing the divider settings for custom hardware, it
> depends on the clock source being selected and the desired rollover time.
> For example, on S32G2 or S32G3 SoC based boards, using FIRC (~48-51 MHz)
> with DIV512 enabled results in a rollover time of aprox. 13 hours.
> 
> > > +    items:
> > > +      - description: div512
> > > +      - description: div32
> > > +
> > > +  clocks:
> > > +    maxItems: 3
> > 
> > I'd rather you provided an explicit items list here, explaining what
> > each of the tree clocks do.
> > 
> > Cheers,
> > Conor.
> > 
> 
> I will add such information in the V2 of this patchset.
> 
> Regards,
> Ciprian
> 
> > > +
> > > +  clock-names:
> > > +    items:
> > > +      - const: ipg
> > > +      - const: sirc
> > > +      - const: firc
> > > +
> > > +required:
> > > +  - clock-names
> > > +  - clocks
> > > +  - compatible
> > > +  - interrupts
> > > +  - nxp,clksel
> > > +  - nxp,dividers
> > > +  - reg
> > > +
> > > +additionalProperties: false
> > > +
> > > +examples:
> > > +  - |
> > > +    #include <dt-bindings/interrupt-controller/arm-gic.h>
> > > +    #include <dt-bindings/interrupt-controller/irq.h>
> > > +
> > > +    rtc0: rtc@40060000 {
> > > +        compatible = "nxp,s32g-rtc";
> > > +        reg = <0x40060000 0x1000>;
> > > +        interrupts = <GIC_SPI 121 IRQ_TYPE_LEVEL_HIGH>;
> > > +        clocks = <&clks 54>,
> > > +                 <&clks 55>,
> > > +                 <&clks 56>;
> > > +        clock-names = "ipg", "sirc", "firc";
> > > +        nxp,clksel = <2>;
> > > +        nxp,dividers = <1 0>;
> > > +    };
> > > -- 
> > > 2.45.2
> > > 
> 

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