Re: [PATCH v4 0/3] ksz87xx: add support for low-loss cable equalizer errata

[Marek Vasut <marex@nabladev.com>]

On 4/17/26 2:44 PM, Fidelio Lawson wrote:
> Hello,
> 
> This patch implements the “Module 3: Equalizer fix for short cables” erratum
> described in Microchip document DS80000687C for KSZ87xx switches.
> 
> According to the erratum, the embedded PHY receiver in KSZ87xx switches is
> tuned by default for long, high-loss Ethernet cables. When operating with
> short or low-loss cables (for example CAT5e or CAT6), the PHY equalizer may
> over-amplify the incoming signal, leading to internal distortion and link
> establishment failures.
> 
> Microchip documents two independent mechanisms to mitigate this issue:
> adjusting the receiver low‑pass filter bandwidth and reducing the DSP
> equalizer initial value. These registers are located in the switch’s
> internal LinkMD table and cannot be accessed directly through a
> stand‑alone PHY driver.
> 
> To keep the PHY‑facing API clean, this series models the erratum handling
> as vendor‑specific Clause 22 PHY registers, virtualized by the KSZ8 DSA
> driver. Accesses are intercepted by ksz8_r_phy() / ksz8_w_phy() and
> translated into the appropriate indirect LinkMD register writes. The
> erratum affects the shared PHY analog front‑end and therefore applies
> globally to the switch.
> 
> Based on review feedback, the user‑visible interface is kept deliberately
> simple and predictable:
> 
> - A boolean “short‑cable” PHY tunable applies a documented and
>    conservative preset (LPF bandwidth 62MHz, DSP EQ initial value 0).
>    This is the recommended KISS interface for the common short‑cable
>    scenario.
> 
> - Two additional integer PHY tunables allow advanced or experimental
>    tuning of the LPF bandwidth and the DSP EQ initial value. These
>    controls are orthogonal, have no ordering requirements, and simply
>    override the corresponding setting when written.
> 
> The tunables act as simple setters with no implicit state machine or
> invalid combinations, avoiding surprises for userspace and not relying
> on extended error reporting or netlink ethtool support.
> 
> This series contains:
> 
>    1. Support for the KSZ87xx low‑loss cable erratum in the KSZ8 DSA driver,
>       including the short‑cable preset and orthogonal tuning controls.
> 
>    2. Addition of vendor‑specific PHY tunable identifiers for the
>       short‑cable preset, LPF bandwidth, and DSP EQ initial value.
> 
>    3. Exposure of these tunables through the Micrel PHY driver via
>       get_tunable / set_tunable callbacks.
> 
> This version follows the design agreed upon during v3 review and
> reworks the interface accordingly.
> 
> This series is based on Linux v7.0-rc1.
> 
> Signed-off-by: Fidelio Lawson <fidelio.lawson@exotec.com>
Thank you for working on this, except for that one nitpick on 1/3, this 
looks really good !