Re: [PATCH phy 2/8] dt-bindings: phy: lynx-28g: add compatible strings per SerDes and instantiation

[Rob Herring (Arm)]

On Wed, 14 Jan 2026 17:21:05 +0200, Vladimir Oltean wrote:
> The 28G Lynx SerDes is instantiated 3 times in the NXP LX2160A SoC and
> twice in the NXP LX2162A. All these instances share the same register
> map, but the number of lanes and the protocols supported by each lane
> differs in a way that isn't detectable by the programming model.
> 
> Going by the generic "fsl,lynx-28g" compatible string and expecting all
> SerDes instantiations to use it was a mistake that needs to be fixed.
> 
> The two major options considered are
> (a) encode the SoC and the SerDes instance in the compatible string,
>     everything else is the responsibility of the driver to derive based
>     on this sufficient information
> (b) add sufficient device tree properties to describe the per-lane
>     differences, as well as the different lane count
> 
> Another important consideration is that any decision made here should
> be consistent with the decisions taken for the yet-to-be-introduced
> 10G Lynx SerDes (older generation for older SoCs), because of how
> similar they are.
> 
> I've seen option (b) at play in this unmerged patch set for the 10G Lynx
> here, and I didn't like it:
> https://lore.kernel.org/linux-phy/20230413160607.4128315-3-sean.anderson@xxxxxxxx/
> 
> This is because there, we have a higher degree of variability in the
> PCCR register values that need to be written per protocol. This makes
> that approach more drawn-out and more prone to errors, compared to (a)
> which is more succinct and obviously correct.
> 
> So I've chosen option (a) through elimination, and this also reflects
> how the SoC reference manual provides different tables with protocol
> combinations for each SerDes. NXP clearly documents these as not
> identical, and refers to them as such (SerDes 1, 2, etc).
> 
> The per-SoC compatible string is prepended to the "fsl,lynx-28g" generic
> compatible, which is left there for compatibility with old kernels. An
> exception would be LX2160A SerDes #3, which at the time of writing is
> not described in fsl-lx2160a.dtsi, and is a non-networking SerDes, so
> the existing Linux driver is useless for it. So there is no practical
> reason to put the "fsl,lynx-28g" fallback for "fsl,lx2160a-serdes3".
> 
> The specific compatible strings give us the opportunity to express more
> constraints in the schema that we weren't able to express before:
> - We allow #phy-cells in the top-level SerDes node only for
>   compatibility with old kernels that don't know how to translate
>   "phys = <&serdes_1_lane_a>" to a PHY. We don't need that feature for
>   the not-yet-introduced LX2160A SerDes #3, so make the presence of
>   #phy-cells at the top level be dependent on the presence of the
>   "fsl,lynx-28g" fallback compatible.
> - The modernization of the compatible string should come together with
>   per-lane OF nodes.
> - LX2162A SerDes 1 has fewer lanes than the others, and trying to use
>   lanes 0-3 would be a mistake that could be caught by the schema.
> 
> Cc: Rob Herring <robh@xxxxxxxxxx>
> Cc: Krzysztof Kozlowski <krzk+dt@xxxxxxxxxx>
> Cc: Conor Dooley <conor+dt@xxxxxxxxxx>
> Cc: devicetree@xxxxxxxxxxxxxxx
> Signed-off-by: Vladimir Oltean <vladimir.oltean@xxxxxxx>
> ---
> part 1 -> part 2:
> - drop everything having to do with constraints (on #phy-cells,
>   #address-cells, #size-cells) based on new compatible strings.
> 
> Patch made its last appearance in v4 from part 1:
> https://lore.kernel.org/linux-phy/20251110092241.1306838-16-vladimir.oltean@xxxxxxx/
> 
> v3->v4:
> - OF nodes per lane broken out as a separate "[PATCH v4 phy 01/16]
>   dt-bindings: phy: lynx-28g: permit lane OF PHY providers"
> - rewritten commit message
> - s|"^phy@[0-9a-f]+$"|"^phy@[0-7]$"|g in patternProperties
> - define "#address-cells" and "#size-cells" as part of common
>   properties, only leave the part which marks them required in the allOf
>   constraints area
> v2->v3:
> - re-add "fsl,lynx-28g" as fallback compatible, and #phy-cells = <1> in
>   top-level "serdes" node
> - drop useless description texts
> - fix text formatting
> - schema is more lax to allow overlaying old and new required properties
> v1->v2:
> - drop the usage of "fsl,lynx-28g" as a fallback compatible
> - mark "fsl,lynx-28g" as deprecated
> - implement Josua's request for per-lane OF nodes for the new compatible
>   strings
> 
>  .../devicetree/bindings/phy/fsl,lynx-28g.yaml | 33 +++++++++++++++++--
>  1 file changed, 30 insertions(+), 3 deletions(-)
> 

Reviewed-by: Rob Herring (Arm) <robh@xxxxxxxxxx>