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

[Théo Lebrun <theo.lebrun@xxxxxxxxxxx>]

Hello,

On Wed Jan 24, 2024 at 6:28 PM CET, Théo Lebrun wrote:
> 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-olb.yaml
> > > @@ -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.

I had forgotten one other reason:

 - Reusability does count for something. Other Mobileye platforms exist,
   and the system controller stuff is more complex on those. Multiple
   different OLB blocks, etc. But my understanding is that
   per-peripheral logic is reused across versions.

>
> 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