Re: [PATCH v6 1/3] net: dsa: microchip: implement KSZ87xx Module 3 low-loss cable errata

[Marek Vasut]

On 5/20/26 10:48 AM, Fidelio Lawson wrote:
> Implement the "Module 3: Equalizer fix for short cables" erratum from
> Microchip document DS80000687C for KSZ87xx switches.
>
> The issue affects short or low-loss cable links (e.g. CAT5e/CAT6),
> where the PHY receiver equalizer may amplify high-amplitude signals
> excessively, resulting in internal distortion and link establishment
> failures.
>
> KSZ87xx devices require a workaround for the Module 3 low-loss cable
> condition, controlled through the switch TABLE_LINK_MD_V indirect
> registers.
>
> This change models the erratum handling as vendor-specific Clause 22 PHY
> registers, virtualized by the KSZ8 DSA driver and accessed via
> ksz8_r_phy() / ksz8_w_phy(). The following controls are provided:
>
> - A boolean “short-cable” preset, which applies a documented and
>    conservative configuration (LPF 62 MHz bandwidth and DSP EQ initial
>    value 0), and is the recommended interface for typical use cases.
>
> - Separate LPF bandwidth and DSP EQ initial value controls intended for
>    advanced or experimental tuning. These are orthogonal and independent,
>    and override the corresponding settings without requiring any specific
>    ordering.
>
> The preset and tunables act as simple setters with no implicit state
> machine or invalid combinations, keeping the API predictable and aligned
> with the KISS principle.
>
> The erratum affects the shared PHY analog front-end and therefore applies
> globally to the switch.
>
> Fixes: e66f840c08a2 ("net: dsa: ksz: Add Microchip KSZ8795 DSA driver")
> Signed-off-by: Fidelio Lawson <fidelio.lawson@xxxxxxxxxx>
Reviewed-by: Marek Vasut <marex@xxxxxxxxxxxx>