Re: [PATCH net-next v3 06/12] dt-bindings: net: dsa: lantiq,gswip: add support for MII delay properties

[Vladimir Oltean <olteanv@gmail.com>]

On Sun, Oct 26, 2025 at 11:45:19PM +0000, Daniel Golle wrote:
> Add support for standard tx-internal-delay-ps and rx-internal-delay-ps
> properties on port nodes to allow fine-tuning of RGMII clock delays.
> 
> The GSWIP switch hardware supports delay values in 500 picosecond
> increments from 0 to 3500 picoseconds, with a default of 2000
> picoseconds for both TX and RX delays.
> 
> This corresponds to the driver changes that allow adjusting MII delays
> using Device Tree properties instead of relying solely on the PHY
> interface mode.
> 
> Signed-off-by: Daniel Golle <daniel@makrotopia.org>
> ---
> v3:
>  * redefine ports node so properties are defined actually apply
>  * RGMII port with 2ps delay is 'rgmii-id' mode
> 
>  .../bindings/net/dsa/lantiq,gswip.yaml        | 29 +++++++++++++++++--
>  1 file changed, 26 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/devicetree/bindings/net/dsa/lantiq,gswip.yaml b/Documentation/devicetree/bindings/net/dsa/lantiq,gswip.yaml
> index f3154b19af78..b0227b80716c 100644
> --- a/Documentation/devicetree/bindings/net/dsa/lantiq,gswip.yaml
> +++ b/Documentation/devicetree/bindings/net/dsa/lantiq,gswip.yaml
> @@ -6,8 +6,29 @@ $schema: http://devicetree.org/meta-schemas/core.yaml#
>  
>  title: Lantiq GSWIP Ethernet switches
>  
> -allOf:
> -  - $ref: dsa.yaml#/$defs/ethernet-ports
> +$ref: dsa.yaml#
> +
> +patternProperties:
> +  "^(ethernet-)?ports$":
> +    type: object
> +    patternProperties:
> +      "^(ethernet-)?port@[0-6]$":
> +        $ref: dsa-port.yaml#
> +        unevaluatedProperties: false
> +
> +        properties:
> +          tx-internal-delay-ps:
> +            enum: [0, 500, 1000, 1500, 2000, 2500, 3000, 3500]
> +            default: 2000

No. This is confusing and wrong. I looked at the driver implementation
code, wanting to note that it has the potential of being a breaking
change for device trees without the "tx-internal-delay-ps" and
"rx-internal-delay-ps" properties.

But then I saw that the driver implementation is subtly different.
"tx-internal-delay-ps" defaults to 2000 only if "rx-internal-delay-ps" is set, and
"rx-internal-delay-ps" defaults to 2000 only if "tx-internal-delay-ps" is set.

So when implemented in this way, it won't cause the regressions I was
concerned about, but it is misrepresented in the schema.

Why overcomplicate this and just not set a default? Modify the RX clock
skew if set, and the TX clock skew if set.

> +            description:
> +              RGMII TX Clock Delay defined in pico seconds.
> +              The delay lines adjust the MII clock vs. data timing.
> +          rx-internal-delay-ps:
> +            enum: [0, 500, 1000, 1500, 2000, 2500, 3000, 3500]
> +            default: 2000
> +            description:
> +              RGMII RX Clock Delay defined in pico seconds.
> +              The delay lines adjust the MII clock vs. data timing.
>  
>  maintainers:
>    - Hauke Mehrtens <hauke@hauke-m.de>
> @@ -113,8 +134,10 @@ examples:
>                      port@0 {
>                              reg = <0>;
>                              label = "lan3";
> -                            phy-mode = "rgmii";
> +                            phy-mode = "rgmii-id";
>                              phy-handle = <&phy0>;
> +                            tx-internal-delay-ps = <2000>;
> +                            rx-internal-delay-ps = <2000>;
>                      };
>  
>                      port@1 {
> -- 
> 2.51.1