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

[Marek Vasut <marex@nabladev.com>]

On 4/14/26 4:54 PM, Andrew Lunn wrote:
> On Tue, Apr 14, 2026 at 03:48:33PM +0200, Fidelio LAWSON wrote:
>> On 4/14/26 14:40, Andrew Lunn wrote:
>>> 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?
>> We were seeing random link dropouts with the default settings, and since
>> enabling the workaround 2, the link has remained stable and we have not
>> observed any further issues.
> 
> So for you, a boolean which enables workaround 2 would be sufficient.
I agree with the observation from Fidelio, the hardware behaves that 
way. As for the rest format of the tunables, I now replied to previous 
email.