net: dsa: microchip: implement KSZ87xx Module 3 low-loss cable errata

Implement the KSZ87xx short cable workaround.

This patch implements the KSZ87xx short cable erratum
described in Microchip document DS80000687C for KSZ87xx switches
and the following support article:

Link: https://support.microchip.com/s/article/Solution-for-Using-CAT-5E-or-CAT-6-Short-Cable-with-a-Link-Issue-for-the-KSZ8795-Family

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.

Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>
Reviewed-by: Marek Vasut <marex@nabladev.com>
Signed-off-by: Fidelio Lawson <fidelio.lawson@exotec.com>
Link: https://patch.msgid.link/20260609-ksz87xx_errata_low_loss_connections-v10-1-9ba4418cf3db@exotec.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions