Re: [RFC net-next v2 1/6] ethtool: Add loopback netlink UAPI definitions

[Maxime Chevallier <maxime.chevallier@bootlin.com>]

Hi,

On 08/03/2026 13:40, Björn Töpel wrote:
> Add the netlink YAML spec and auto-generated UAPI header for a unified
> loopback interface covering MAC, PCS, PHY, and pluggable module
> components.
> 
> Each loopback point is described by a nested entry attribute
> containing:
> 
>  - component  where in the path (MAC, PCS, PHY, MODULE)
>  - name       subsystem label, e.g. "cmis-host" or "cmis-media"
>  - id         optional instance selector (e.g. PHY id, port id)
>  - supported  bitmask of supported directions
>  - direction  NEAR_END, FAR_END, or 0 (disabled)
> 
> Signed-off-by: Björn Töpel <bjorn@kernel.org>
> ---
>  Documentation/netlink/specs/ethtool.yaml      | 115 ++++++++++++++++++
>  .../uapi/linux/ethtool_netlink_generated.h    |  52 ++++++++
>  2 files changed, 167 insertions(+)
> 
> diff --git a/Documentation/netlink/specs/ethtool.yaml b/Documentation/netlink/specs/ethtool.yaml
> index 4707063af3b4..05ebad6ae4e0 100644
> --- a/Documentation/netlink/specs/ethtool.yaml
> +++ b/Documentation/netlink/specs/ethtool.yaml
> @@ -211,6 +211,39 @@ definitions:
>          name: discard
>          value: 31
>  
> +  -
> +    name: loopback-component
> +    type: enum
> +    doc: |
> +      Loopback component. Identifies where in the network path the
> +      loopback is applied.
> +    entries:
> +      -
> +        name: mac
> +        doc: MAC loopback
> +      -
> +        name: pcs
> +        doc: PCS loopback
> +      -
> +        name: phy
> +        doc: PHY loopback
> +      -
> +        name: module
> +        doc: Pluggable module (e.g. CMIS (Q)SFP) loopback

Should we also add "serdes" ?

> +  -
> +    name: loopback-direction
> +    type: flags
> +    doc: |
> +      Loopback direction flags. Used as a bitmask in supported, and as
> +      a single value in direction.
> +    entries:
> +      -
> +        name: near-end
> +        doc: Near-end loopback; host-loop-host
> +      -
> +        name: far-end
> +        doc: Far-end loopback; line-loop-line
> +
>  attribute-sets:
>    -
>      name: header
> @@ -1903,6 +1936,60 @@ attribute-sets:
>          name: link
>          type: nest
>          nested-attributes: mse-snapshot
> +  -
> +    name: loopback-entry
> +    doc: Per-component loopback configuration entry.
> +    attr-cnt-name: __ethtool-a-loopback-entry-cnt
> +    attributes:
> +      -
> +        name: unspec
> +        type: unused
> +        value: 0
> +      -
> +        name: component
> +        type: u32
> +        enum: loopback-component
> +        doc: Loopback component
> +      -
> +        name: id
> +        type: u32
> +        doc: |
> +          Optional component instance identifier. Required for PHY,
> +          optional for MODULE, omitted for MAC and PCS.

it doesn't have to be required for PHY. The current idea is that if you
don't pass any PHY index when issueing a PHY-targetting command, then it
means you're targetting net_device->phydev, that is the PHY device
attached to the netdev (if any).

I think we can keep that behaviour, as systems with multiple PHYs are
not very common.

> +      -
> +        name: name
> +        type: string
> +        doc: |
> +          Subsystem-specific name for the loopback point within the
> +          component.

We'll need to be careful about keeping this subsystem-specific and not
driver-specific :)

> +      -
> +        name: supported
> +        type: u32
> +        enum: loopback-direction
> +        enum-as-flags: true
> +        doc: Bitmask of supported loopback directions
> +      -
> +        name: direction
> +        type: u32
> +        enum: loopback-direction
> +        doc: Current loopback direction, 0 means disabled

no need for an u32 for 3 different values I think :)

Maxime