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

[Stefan Eichenberger <eichest@gmail.com>]

Hi Maxime,

On Mon, Jan 05, 2026 at 01:23:46PM +0100, Maxime Chevallier wrote:
> 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 ?

This is what Rob also suggested:
https://lore.kernel.org/all/20251215140330.GA2360845-robh@kernel.org/

Unfortunately, I'm afraid of breaking something on the platforms that
are already working, as this is an Ethernet controller issue. As I
understand it, the PHY works according to the standard. Since the bit is
undocumented, it seemed safer to only apply it to the i.MX8MP.

However, if this is preferred, I am also happy to always enable the
preamble. In theory, I don't see any reason why it should break
anything.

Regards,
Stefan