Re: [PATCH RFC v2 0/9] Add System Controller support for RZ/T2H and RZ/N2H SoCs

[Geert Uytterhoeven]

Hi Prabhakar,

On Tue, 14 Jul 2026 at 15:27, Lad, Prabhakar <prabhakar.csengg@xxxxxxxxx> wrote:
> On Mon, Jul 13, 2026 at 5:00 PM Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> > On Thu, 2 Jul 2026 at 14:31, Prabhakar <prabhakar.csengg@xxxxxxxxx> wrote:
> > > From: Lad Prabhakar <prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx>
> > > This series adds support for the System Controller (SYSC) blocks found on
> > > the Renesas RZ/T2H and RZ/N2H SoCs. These blocks handle critical low-power
> > > management and access control functions.
> > >
> > > Hardware Architecture & Dependency Challenges:
> > > ----------------------------------------------
> > > The SYSC in these SoCs is a multi-functional block responsible for:
> > >     - Clock & Reset Control
> > >     - Low Power Management
> > >     - Clock Monitor (CLMA)
> > >     - Access Control
> > >
> > > A key architectural detail is that these SYSC blocks are physically located
> > > within the CPG/MSSR (Clock Pulse Generator / Module Standby Software Reset)
> > > address space. While the CPG/MSSR driver is already implemented and
> > > functional for these SoCs, the integration of SYSC adds a layer of
> > > complexity due to a cyclic dependency:
> > >     - SYSC requires CPG: The system controller needs a clock to operate.
> > >     - CPG requires SYSC: Access control registers within the SYSC contain
> > >       bits necessary to control the PLLs managed by the CPG.
> > >
> > > If implemented as a completely separate top-level system controller node, we
> > > would face a cyclic dependency between the CPG and SYSC drivers during the
> > > boot process.
> > >
> > > Proposed Implementation
> > > ----------------------------
> > > To resolve this, the SYSC blocks are represented as child nodes of the
> > > CPG/MSSR controller in the device tree. The SYSC regmap is registered
> > > directly against the CPG device node. This hierarchy correctly models the
> > > hardware address space while allowing the drivers to share resources
> > > without deadlock.
> > >
> > > I am sending this as an RFC specifically to get feedback on the
> > > implementation of the SYSC as child nodes of the CPG to resolve the
> > > dependency cycle.
> >
> > Personally, I am not a big fan of subnodes.  I assume you are using
> > subnodes because you can register only a single regmap per syscon node?
> Yes so that the consumers don't have to specify it by indexing.
>
> > Would it be possible to just extend the existing clock-controller
> > node with two more reg entries, and expose them through a single
> > combined regmap?
> >
> That should be possible. Or would you prefer just to extend the sizes
> and create a single regmap for it?
>
>                cpg: clock-controller@80280000 {
>                         compatible = "renesas,r9a09g077-cpg-mssr";
> -                       reg = <0 0x80280000 0 0x10000>,
> -                             <0 0x81280000 0 0x10000>;
> +                       reg = <0 0x80280000 0 0x20000>,
> +                             <0 0x81280000 0 0x20000>;

Yes, as they are contiguous.
These registers are not accessed unless there's a user specified in
the DTB, right? So there is no issue when booting an old DTB that has
the short reg blocks.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds