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

Théo Lebrun
2024-01-24T17:40:19Z

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@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.

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