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

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Message-ID: <aUKgP4Hi-8tP9eaK@eichest-laptop>
Date: Wed, 17 Dec 2025 13:21:19 +0100
From: Stefan Eichenberger <eichest@...il.com>
To: Rob Herring <robh@...nel.org>
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 Wed, Dec 17, 2025 at 10:58:54AM +0100, Stefan Eichenberger wrote:
> On Mon, Dec 15, 2025 at 08:03:30AM -0600, Rob Herring wrote:
> > 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? 
> 
> Is doing something like this acceptable in a phy driver?
> if (of_machine_is_compatible("fsl,imx8mp")) {
> ...
> }
> 
> That would be a different option, rather than having to add a new DT
> property. Unfortunately, the workaround affects the PHY rather than the
> MAC driver. This is why we considered adding a DT property.

Francesco made a good point about this. The i.MX8MP has two MACs, but
only one of them is affected. Therefore, checking the machine's
compatible string would not be correct. As far as I know, checking the
MAC's compatible string from within the PHY driver is also not good
practice, is it?