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

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Message-ID: <20251215140330.GA2360845-robh@kernel.org>
Date: Mon, 15 Dec 2025 08:03:30 -0600
From: Rob Herring <robh@...nel.org>
To: Stefan Eichenberger <eichest@...il.com>
Cc: andrew+netdev@...n.ch, davem@...emloft.net, edumazet@...gle.com,
	kuba@...nel.org, pabeni@...hat.com, krzk+dt@...nel.org,
	conor+dt@...nel.org, hkallweit1@...il.com, linux@...linux.org.uk,
	geert+renesas@...der.be, ben.dooks@...ethink.co.uk,
	netdev@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, francesco.dolcini@...adex.com,
	rafael.beims@...adex.com,
	Stefan Eichenberger <stefan.eichenberger@...adex.com>
Subject: Re: [PATCH net-next v1 2/3] dt-bindings: net: micrel: Add
 keep-preamble-before-sfd

On Fri, Dec 12, 2025 at 09:46:17AM +0100, Stefan Eichenberger wrote:
> From: Stefan Eichenberger <stefan.eichenberger@...adex.com>
> 
> 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.