Re: [PATCH v3 1/3] net: dsa: microchip: implement KSZ87xx Module 3 low-loss cable errata
From: Marek Vasut
Date: Tue Apr 14 2026 - 11:50:38 EST
On 4/14/26 4:54 PM, Andrew Lunn wrote:
On Tue, Apr 14, 2026 at 03:48:33PM +0200, Fidelio LAWSON wrote: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.
On 4/14/26 14:40, Andrew Lunn wrote:
On Tue, Apr 14, 2026 at 01:05:49PM +0200, Marek Vasut wrote:We were seeing random link dropouts with the default settings, and since
On 4/14/26 11:12 AM, Fidelio Lawson wrote:
Implement the "Module 3: Equalizer fix for short cables" erratum fromI may not fully understand this, but aren't the EQ and LPF settings
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
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?
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.