Re: [PATCH net-next v8 1/3] net: dsa: microchip: implement KSZ87xx Module 3 low-loss cable errata

[Nicolai Buchwitz <nb@tipi-net.de>]

Hi Fidelio

On 1.6.2026 13:54, Fidelio Lawson wrote:
> Implement the KSZ87xx short cable workaround.
> 
> This patch implements the KSZ87xx short cable erratum
> described in Microchip document DS80000687C for KSZ87xx switches
> and the following support article:
> 
> Link: 
> https://support.microchip.com/s/article/Solution-for-Using-CAT-5E-or-CAT-6-Short-Cable-with-a-Link-Issue-for-the-KSZ8795-Family
> 
> 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.
> 
> This change models the erratum handling as vendor-specific Clause 22 
> PHY
> registers, virtualized by the KSZ8 DSA driver and accessed via
> ksz8_r_phy() / ksz8_w_phy(). The following controls are provided:
> 
> - A boolean “short-cable” preset, which applies a documented and
>   conservative configuration (LPF 62 MHz bandwidth and DSP EQ initial
>   value 0), and is the recommended interface for typical use cases.
> 
> - Separate LPF bandwidth and DSP EQ initial value controls intended for
>   advanced or experimental tuning. These are orthogonal and 
> independent,
>   and override the corresponding settings without requiring any 
> specific
>   ordering.
> 
> The preset and tunables act as simple setters with no implicit state
> machine or invalid combinations, keeping the API predictable and 
> aligned
> with the KISS principle.
> 
> The erratum affects the shared PHY analog front-end and therefore 
> applies
> globally to the switch.
> 
> Fixes: e66f840c08a2 ("net: dsa: ksz: Add Microchip KSZ8795 DSA driver")

Do we need/want a Fixes: tag for a net-next patch?  AFAIU the only thing
actually fixed is the TABLE_LINK_MD vs TABLE_LINK_MD_V macro, and that
macro was not referenced anywhere before this patch.

> Signed-off-by: Fidelio Lawson <fidelio.lawson@exotec.com>
> ---
>  drivers/net/dsa/microchip/ksz8.c       | 89 
> ++++++++++++++++++++++++++++++++++
>  drivers/net/dsa/microchip/ksz8_reg.h   | 23 ++++++++-
>  drivers/net/dsa/microchip/ksz_common.h |  4 ++
>  3 files changed, 115 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/dsa/microchip/ksz8.c 
> b/drivers/net/dsa/microchip/ksz8.c
> index 0f84b2e7d25d..b8a7b9004142 100644
> --- a/drivers/net/dsa/microchip/ksz8.c
> +++ b/drivers/net/dsa/microchip/ksz8.c

> [...]

> @@ -2070,6 +2155,10 @@ static int ksz8_setup(struct dsa_switch *ds)
>  			return ret;
>  	}
> 
> +	/* Initialize KS87xx short-cable preset control */

Should be KSZ87xx.

> [...]

Besides from the above:

Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>

Thanks
Nicolai