Re: [PATCH v3 1/3] net: dsa: microchip: implement KSZ87xx Module 3 low-loss cable errata

[Andrew Lunn <andrew@lunn.ch>]

On Tue, Apr 14, 2026 at 01:05:49PM +0200, Marek Vasut wrote:
> On 4/14/26 11:12 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.
> > 
> > The affected registers are part of the switch address space and are not
> > directly accessible from the PHY driver. To keep the PHY-facing API
> > clean and avoid leaking switch-specific details, model this errata
> > control as vendor-specific Clause 22 PHY registers.
> > 
> > A vendor-specific Clause 22 PHY register is introduced as a mode
> > selector in PHY_REG_LOW_LOSS_CTRL, and ksz8_r_phy() / ksz8_w_phy()
> > translate accesses to these bits into the appropriate indirect
> > TABLE_LINK_MD_V accesses.
> > 
> > The control register defines the following modes:
> > 0: disabled (default behavior)
> > 1: EQ training workaround
> > 2: LPF 90 MHz
> > 3: LPF 62 MHz
> > 4: LPF 55 MHz
> > 5: LPF 44 MHz
> I may not fully understand this, but aren't the EQ and LPF settings
> orthogonal ?

What is the real life experience using this feature? Is it needed for
1cm cables, but most > 1m cables are O.K with the defaults? Do we need
all these configuration options? How is a user supposed to discover
the different options? Can we simplify it down to a Boolean?

Ethernet is just supposed to work with any valid length of cable,
KISS. So maybe we should try to keep this feature KISS. Just tell the
driver it is a short cable, pick different defaults which should work
with any short cable?

A boolean should also help with making this tunable reusable with
other devices. It is unlikely any other devices have these same
configuration options, unless it is from the same vendor.

     Andrew