Re: [PATCH v2 1/4] dt-bindings: PCI: renesas,r9a08g045-pcie: Add RZ/V2H(P) support

[Geert Uytterhoeven]

Hi Prabhakar,

On Mon, 18 May 2026 at 17:53, Prabhakar <prabhakar.csengg@xxxxxxxxx> wrote:
> From: Lad Prabhakar <prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx>
>
> Add support for the PCIe controller found on the Renesas RZ/V2H(P) SoC.
>
> The RZ/V2H(P) controller is similar to the RZ/G3E variant but includes
> additional registers and configuration bits for PCIe lane control. It
> supports multilink operation configured as either a single x4 link
> or two independent x2 link controllers.
>
> Unlike earlier SoCs supported by this driver which only feature a single
> PCIe controller, the RZ/V2H(P) SoC implements two controllers. Both
> controllers rely on the system controller (`sysc`) for configuration,
> but the required registers reside at different offsets for each instance.
> To correctly identify the controller instance and map the corresponding
> system controller registers, make the "linux,pci-domain" and "num-lanes"
> properties mandatory for this SoC and restrict their values according to
> the hardware capabilities.
>
> Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx>

Thanks for your patch!

> --- a/Documentation/devicetree/bindings/pci/renesas,r9a08g045-pcie.yaml
> +++ b/Documentation/devicetree/bindings/pci/renesas,r9a08g045-pcie.yaml

> @@ -235,6 +238,20 @@ allOf:
>            maxItems: 1
>          reset-names:
>            maxItems: 1
> +  - if:
> +      properties:
> +        compatible:
> +          contains:
> +            const: renesas,r9a09g057-pcie
> +    then:
> +      properties:
> +        linux,pci-domain:
> +          enum: [0, 1]

Sashiko has the following comments about this property[1]:

| Is it appropriate to use linux,pci-domain to identify the hardware
| controller instance?
| Device Tree is designed to describe hardware rather than software policy
| or configuration. The linux,pci-domain property is an OS-specific hint
| used to assign logical PCI domain numbers.
| Using it to identify hardware instances strictly couples the hardware
| register mapping to an OS numbering scheme.

and

| Does limiting this to [0, 1] cause a regression for valid software
| configurations?
| For example, if a board with multiple PCIe controllers assigns domains 2
| and 3 to these instances to avoid conflicts, the binding will fail
| validation and the driver will fail to probe.
| Could the hardware instances instead be identified via standard hardware
| descriptive mechanisms, such as their physical base address (reg),
| standard aliases, or by adding a phandle specifier argument to
| renesas,sysc?

dtschema/schemas/pci/pci-host-bridge.yaml states:

| linux,pci-domain:
|   description:
|     If present this property assigns a fixed PCI domain number to a
host bridge,
|     otherwise an unstable (across boots) unique number will be assigned.
|     It is required to either not set this property at all or set it for all
|     host bridges in the system, otherwise potentially conflicting
domain numbers
|     may be assigned to root buses behind different host bridges.  The domain
|     number for each host bridge in the system must be unique.

In the RZ/V2H case, the number is local to the SYSC instance referenced
by the renesas,sysc property, while linux,pci-domain is global to the
full system by definition. Fixing the numbers to zero and one works,
until some other controller in the system has conflicting requirements.

I do see a future case where this may break: if Renesas would release a
new SoC containing two RZ/V2H dies, there would be four PCIe controllers
and two SYSC controllers.  This would require two PCIe controllers to
have linux,pci-domain = <0>, and two having linux,pci-domain = <1>,
which is not permitted.

Hence adding a second cell to the renesas,sysc property, to specify
the base offset of the PCIe-related registers inside SYSC seems like
the best way to go?

[1] https://sashiko.dev/#/patchset/20260518155324.168948-1-prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx?part=1

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