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

[Fidelio LAWSON <lawson.fidelio@gmail.com>]

On 4/14/26 13:05, 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 ?
You are right that EQ training and LPF bandwidth control
are orthogonal from a hardware point of view.

In this case, the interface is intentionally modeled after the erratum
guidance rather than exposing all possible combinations. Microchip
documents the workarounds as alternative solutions:

"If work around 1 does not solve the short cable issue in a CAT-5E or 
CAT-6 application, change the work around 1 register (0x3C) to its 
default value (0x0A), and use the following settings"
from: 
https://ww1.microchip.com/downloads/aemDocuments/documents/OTH/ProductDocuments/Errata/KSZ87xx-Errata-DS80000687C.pdf

If you’d prefer exposing these as orthogonal controls, I can revise the
interface in the next iteration.