Re: [PATCH net-next 1/4] dt-bindings: net: ethernet-controller: update descriptions of RGMII modes

[Siddharth Vadapalli]

On Tue, Apr 15, 2025 at 12:18:01PM +0200, Matthias Schiffer wrote:
> As discussed [1], the comments for the different rgmii(-*id) modes do not
> accurately describe what these values mean.
> 
> As the Device Tree is primarily supposed to describe the hardware and not
> its configuration, the different modes need to distinguish board designs

If the Ethernet-Controller (MAC) is integrated in an SoC (as is the case
with CPSW Ethernet Switch), and, given that "phy-mode" is a property
added within the device-tree node of the MAC, I fail to understand how
the device-tree can continue "describing" hardware for different board
designs using the same SoC (unchanged MAC HW).

How do we handle situations where a given MAC supports various
"phy-modes" in HW? Shouldn't "phy-modes" then be a "list" to technically
descibe the HW? Even if we set aside the "rgmii" variants that this
series is attempting to address, the CPSW MAC supports "sgmii", "qsgmii"
and "usxgmii/xfi" as well.

> (if a delay is built into the PCB using different trace lengths); whether
> a delay is added on the MAC or the PHY side when needed should not matter.
> 
> Unfortunately, implementation in MAC drivers is somewhat inconsistent
> where a delay is fixed or configurable on the MAC side. As a first step
> towards sorting this out, improve the documentation.
> 
> Link: https://lore.kernel.org/lkml/d25b1447-c28b-4998-b238-92672434dc28@xxxxxxx/ [1]
> Signed-off-by: Matthias Schiffer <matthias.schiffer@xxxxxxxxxxxxxxx>
> ---
>  .../bindings/net/ethernet-controller.yaml        | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/Documentation/devicetree/bindings/net/ethernet-controller.yaml b/Documentation/devicetree/bindings/net/ethernet-controller.yaml
> index 45819b2358002..2ddc1ce2439a6 100644
> --- a/Documentation/devicetree/bindings/net/ethernet-controller.yaml
> +++ b/Documentation/devicetree/bindings/net/ethernet-controller.yaml
> @@ -74,19 +74,21 @@ properties:
>        - rev-rmii
>        - moca
>  
> -      # RX and TX delays are added by the MAC when required
> +      # RX and TX delays are part of the board design (through PCB traces). MAC
> +      # and PHY must not add delays.
>        - rgmii
>  
> -      # RGMII with internal RX and TX delays provided by the PHY,
> -      # the MAC should not add the RX or TX delays in this case
> +      # RGMII with internal RX and TX delays provided by the MAC or PHY. No
> +      # delays are included in the board design; this is the most common case
> +      # in modern designs.
>        - rgmii-id
>  
> -      # RGMII with internal RX delay provided by the PHY, the MAC
> -      # should not add an RX delay in this case
> +      # RGMII with internal RX delay provided by the MAC or PHY. TX delay is
> +      # part of the board design.
>        - rgmii-rxid
>  
> -      # RGMII with internal TX delay provided by the PHY, the MAC
> -      # should not add an TX delay in this case
> +      # RGMII with internal TX delay provided by the MAC or PHY. RX delay is
> +      # part of the board design.

Since all of the above is documented in "ethernet-controller.yaml" and
not "ethernet-phy.yaml", describing what the "MAC" should or shouldn't
do seems accurate, and modifying it to describe what the "PHY" should or
shouldn't do seems wrong.

>        - rgmii-txid
>        - rtbi
>        - smii

Regards,
Siddharth.