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

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

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

> [...]
> 
> +static int ksz87xx_apply_low_loss_preset(struct ksz_device *dev, bool 
> enable)
> +{
> +	/* Apply the Microchip erratum short-cable preset (LPF 62 MHz, EQ 
> init 0)
> +	 * providing a conservative configuration for short or low-loss 
> cables.
> +	 */
> +	u8 lpf_bw, eq_init;
> +	int ret;
> +
> +	lpf_bw = KSZ87XX_PHY_LPF_62MHZ;
> +	eq_init = KSZ87XX_DSP_EQ_INIT_LOW_LOSS;
> +
> +	if (!ksz_is_ksz87xx(dev))
> +		return -EOPNOTSUPP;
> +
> +	if (!enable)
> +		return 0;

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?

> [...]

Thanks
Nicolai