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

Théo Lebrun
2024-01-24T17:28:18Z

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@bootlin.com>
> > ---
> > .../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@bootlin.com>
> > + - Théo Lebrun <theo.lebrun@bootlin.com>
> > + - Vladimir Kondratiev <vladimir.kondratiev@mobileye.com>
> > +
> > +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