Subject: Re: [PATCH net-next v1 2/3] dt-bindings: net: micrel: Add keep-preamble-before-sfd

From: Rob Herring <[email-masked]>
Date: Mon, 15 Dec 2025 08:03:30 -0600

On Fri, Dec 12, 2025 at 09:46:17AM +0100, Stefan Eichenberger wrote:
> From: Stefan Eichenberger <[email-masked]>
> 
> Add a property to activate a Micrel PHY feature that keeps the preamble
> enabled before the SFD (Start Frame Delimiter) is transmitted.
> 
> This allows to workaround broken Ethernet controllers as found on the
> NXP i.MX8MP. Specifically, errata ERR050694 that states:
> ENET_QOS: MAC incorrectly discards the received packets when Preamble
> Byte does not precede SFD or SMD.

It doesn't really work right if you have to change the DT to work-around 
a quirk in the kernel. You should have all the information needed 
already in the DT. The compatible string for the i.MX8MP ethernet 
controller is not sufficient? 

> 
> 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. Withouth this property set, no packets are received. With this
> property set, reception works fine.

What's the impact of just unconditionally setting this bit? Seems like 
any impact would be minimal given 10MBit is probably pretty rare now.