Re: [PATCH net-next v7 1/3] net: dsa: microchip: implement KSZ87xx Module 3 low-loss cable errata

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

On 5/28/26 09:19, Nicolai Buchwitz wrote:
> Hi Fidelio
> 
> On 24.5.2026 12:44, Fidelio Lawson wrote:
>> Implement the "Module 3: Equalizer fix for short cables" erratum from
>> Microchip document DS80000687C for KSZ87xx switches.
> 
> The commit message says this implements Module 3 from DS80000687C, but
> the errata's Work Around 1 writes 0x15 to indirect register 0x3C
> ("PHY equalizer register"), and the patch never touches 0x3C. Could you
> please clarify which document the register addresses
> (KSZ87XX_REG_DSP_EQ = 0x08, KSZ87XX_REG_PHY_LPF = 0x4C) come from?
> 
> I checked against the the following document:
> https://ww1.microchip.com/downloads/aemDocuments/documents/OTH/ 
> ProductDocuments/Errata/KSZ87xx-Errata-DS80000687C.pdf
> 

Hi Nicolai,

Thanks for the review.
Regarding the register addresses the definitions used in the 
implementation are based on the following Microchip support article:

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

I'll add that clarification to the commit message.

> This silently returns when enable = false and nothing is written to the 
> registers.
> Either I'm missing something or the short-cable preset cannot be cleared 
> through this tunable?
> 

You are correct that the current implementation does not restore any
configuration when 'enable' is set to false.

In the next revision, I will update this to apply the default
settings when disabling the preset, so that the tunable behaves as a
proper reversible configuration.

Thanks again for pointing these out.

Best regards,
Fidelio