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

[Matthias Schiffer]

On Tue, 2025-04-15 at 16:06 +0530, Siddharth Vadapalli wrote:
> 
> 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).

The setting is part of the MAC node, but it is always set in the board DTS,
together with assigning a PHY to the MAC.

> 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.

This is not about PHY mode support of the MAC, but the mode to be used on a
particular board. I would not expect a board to use multiple different
interfaces with a single PHY (and if such cases exist, I consider them out of
scope for this patch series).

> 
> > (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.

The settings describe the connection between MAC and PHY, thus their explanation
must mention both to make sense. See the linked discussion with Andrew for
details.

Best,
Matthias



> 
> >        - rgmii-txid
> >        - rtbi
> >        - smii
> 
> Regards,
> Siddharth.

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