Re: [PATCH v3 04/17] dt-bindings: soc: mobileye: add EyeQ5 OLB system controller

[Théo Lebrun]

Hello,

On Wed Jan 24, 2024 at 4:14 PM CET, Rob Herring wrote:
> On Tue, Jan 23, 2024 at 07:46:49PM +0100, Théo Lebrun wrote:
> > Add documentation to describe the "Other Logic Block" syscon.
> > 
> > Signed-off-by: Théo Lebrun <theo.lebrun@xxxxxxxxxxx>
> > ---
> >  .../bindings/soc/mobileye/mobileye,eyeq5-olb.yaml  | 77 ++++++++++++++++++++++
> >  MAINTAINERS                                        |  1 +
> >  2 files changed, 78 insertions(+)
> > 
> > diff --git a/Documentation/devicetree/bindings/soc/mobileye/mobileye,eyeq5-olb.yaml b/Documentation/devicetree/bindings/soc/mobileye/mobileye,eyeq5-olb.yaml
> > new file mode 100644
> > index 000000000000..031ef6a532c1
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/soc/mobileye/mobileye,eyeq5-olbyaml
> > @@ -0,0 +1,77 @@
> > +# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/soc/mobileye/mobileye,eyeq5-olb.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: Mobileye EyeQ5 SoC system controller
> > +
> > +maintainers:
> > +  - Grégory Clement <gregory.clement@xxxxxxxxxxx>
> > +  - Théo Lebrun <theo.lebrun@xxxxxxxxxxx>
> > +  - Vladimir Kondratiev <vladimir.kondratiev@xxxxxxxxxxxx>
> > +
> > +description:
> > +  OLB ("Other Logic Block") is a hardware block grouping smaller blocks. Clocks,
> > +  resets, pinctrl are being handled from here.
> > +
> > +properties:
> > +  compatible:
> > +    items:
> > +      - const: mobileye,eyeq5-olb
> > +      - const: syscon
> > +      - const: simple-mfd
> > +
> > +  reg:
> > +    maxItems: 1
> > +
> > +  clock-controller:
> > +    $ref: /schemas/clock/mobileye,eyeq5-clk.yaml#
> > +    type: object
> > +
> > +  reset-controller:
> > +    $ref: /schemas/reset/mobileye,eyeq5-reset.yaml#
> > +    type: object
> > +
> > +  pinctrl-a:
> > +    $ref: /schemas/pinctrl/mobileye,eyeq5-pinctrl.yaml#
> > +    type: object
> > +
> > +  pinctrl-b:
> > +    $ref: /schemas/pinctrl/mobileye,eyeq5-pinctrl.yaml#
> > +    type: object
> > +
> > +required:
> > +  - compatible
> > +  - reg
> > +
> > +additionalProperties: false
> > +
> > +examples:
> > +  - |
> > +    system-controller@e00000 {
> > +      compatible = "mobileye,eyeq5-olb", "syscon", "simple-mfd";
> > +      reg = <0xe00000 0x400>;
> > +
> > +      clock-controller {
> > +        compatible = "mobileye,eyeq5-clk";
> > +        #clock-cells = <1>;
> > +        clocks = <&xtal>;
> > +        clock-names = "ref";
> > +      };
> > +
> > +      reset-controller {
> > +        compatible = "mobileye,eyeq5-reset";
> > +        #reset-cells = <2>;
> > +      };
> > +
> > +      pinctrl-a {
> > +        compatible = "mobileye,eyeq5-a-pinctrl";
> > +        #pinctrl-cells = <1>;
>
> Sure you need this? Generally only pinctrl-single uses this.

You are completely right, it is useless. I naively expected it in the
same vein as other subsystems.

>
> > +      };
> > +
> > +      pinctrl-b {
> > +        compatible = "mobileye,eyeq5-b-pinctrl";
> > +        #pinctrl-cells = <1>;
> > +      };
> > +    };
>
> This can all be simplified to:
>
> system-controller@e00000 {
>     compatible = "mobileye,eyeq5-olb", "syscon";
>     reg = <0xe00000 0x400>;
>     #reset-cells = <2>;
>     #clock-cells = <1>;
>     clocks = <&xtal>;
>     clock-names = "ref";
>
>     pins { ... };
> };
>
> There is no need for sub nodes unless you have reusable blocks or each 
> block has its own resources in DT.

That is right, and it does simplify the devicetree as you have shown.
However, the split nodes gives the following advantages:

 - Devicetree-wise, it allows for one alias per function.
   `clocks = <&clocks EQ5C_PLL_CPU>` is surely more intuitive
   than `clocks = <&olb EQ5C_PLL_CPU>;`. Same for reset.

 - It means an MFD driver must be implemented, adding between 100 to 200
   lines of boilerplate code to the kernel.

 - It means one pinctrl device for the two banks. That addresses your
   comment on [PATCH v3 10/17]. This is often done and would be doable
   on this platform. However it means added logic to each individual
   function of pinctrl-eyeq5.

   Overall it makes for less readable code, for code that already looks
   more complex than it really is.

   My initial non-public version of pinctrl-eyeq5 was using this method
   (a device handling both banks) and I've leaned away from it.

Those are all minor, but I don't have the feeling a few lines and nodes
less in devicetree compensate for those.

Thanks,

--
Théo Lebrun, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com