Re: [PATCH RESEND net-next v2] net: stmmac: dwmac: Add a fixup for the Micrel KSZ9131 PHY

[Maxime Chevallier <maxime.chevallier@bootlin.com>]

Hi Stefan,

On 05/01/2026 11:02, Stefan Eichenberger wrote:
> From: Stefan Eichenberger <stefan.eichenberger@toradex.com>
> 
> Add a fixup to the stmmac driver to keep the preamble before the SFD
> (Start Frame Delimiter) on the Micrel KSZ9131 PHY when the driver is
> used on an NXP i.MX8MP SoC.
> 
> This allows to workaround errata ERR050694 of the NXP i.MX8MP that
> states:
> ENET_QOS: MAC incorrectly discards the received packets when Preamble
> Byte does not precede SFD or SMD.
> 
> The bit which disables this feature is not documented in the datasheet
> from Micrel, but has been found by NXP and Micrel following this
> discussion:
> https://community.nxp.com/t5/i-MX-Processors/iMX8MP-eqos-not-working-for-10base-t/m-p/2151032
> 
> It has been tested on Verdin iMX8MP from Toradex by forcing the PHY to
> 10MBit. Without bit 2 being set in the remote loopback register, no
> packets are received. With the bit set, reception works fine.
> 
> Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>

I've also faced this issue, however I'm wondering wether this is the
correct approach to fix this. It seems that all Micrel / Microchip PHYs
have this behaviour of discaring the preamble at 10Mbps.

Some of these phys have accessible control registers to re-enable it,
however this register/bit changes depending on the PHY model. For
example, on KSZ8041, this is register 0x14 bit 6.

We may end-up with many many more fixups for this, basically for every
micrel/microchip PHY.

Wouldn't it be safer to just always enable preamble at 10M for these
PHYs, regardless of the MAC that's connected to it ? Is there any risk
always having the preamble there ?

Maxime