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

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Message-ID: <aVvp70S2Lr3o_jyB@eichest-laptop>
Date: Mon, 5 Jan 2026 17:42:23 +0100
From: Stefan Eichenberger <eichest@...il.com>
To: Andrew Lunn <andrew@...n.ch>
Cc: Maxime Chevallier <maxime.chevallier@...tlin.com>,
	andrew+netdev@...n.ch, davem@...emloft.net, edumazet@...gle.com,
	kuba@...nel.org, pabeni@...hat.com, shawnguo@...nel.org,
	s.hauer@...gutronix.de, kernel@...gutronix.de, festevam@...il.com,
	mcoquelin.stm32@...il.com, alexandre.torgue@...s.st.com,
	linux-stm32@...md-mailman.stormreply.com, netdev@...r.kernel.org,
	imx@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, francesco.dolcini@...adex.com,
	robh@...nel.org,
	Stefan Eichenberger <stefan.eichenberger@...adex.com>
Subject: Re: [PATCH RESEND net-next v2] net: stmmac: dwmac: Add a fixup for
 the Micrel KSZ9131 PHY

Hi Andrew,

On Mon, Jan 05, 2026 at 04:26:40PM +0100, Andrew Lunn wrote:
> > 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.
> 
> What is the exact wording of the standard? I'm assuming this is IEEE
> 802.3?  Please could you quote the relevant part.

Yes this is correct. ERR050694 from NXP states:
The IEEE 802.3 standard states that, in MII/GMII modes, the byte
preceding the SFD (0xD5), SMD-S (0xE6,0x4C, 0x7F, or 0xB3), or SMD-C
(0x61, 0x52, 0x9E, or 0x2A) byte can be a non-PREAMBLE byte or there can
be no preceding preamble byte. The MAC receiver must successfully
receive a packet without any preamble(0x55) byte preceding the SFD,
SMD-S, or SMD-C byte.
However due to the defect, in configurations where frame preemption is
enabled, when preamble byte does not precede the SFD, SMD-S, or SMD-C
byte, the received packet is discarded by the MAC receiver. This is
because, the start-of-packet detection logic of the MAC receiver
incorrectly checks for a preamble byte.

NXP refers to IEEE 802.3 where in clause 35.2.3.2.2 Receive case (GMII)
they show two tables one where the preamble is preceding the SFD and one
where it is not. The text says:
The operation of 1000 Mb/s PHYs can result in shrinkage of the preamble
between transmission at the source GMII and reception at the destination
GMII. Table 35–3 depicts the case where no preamble bytes are conveyed
across the GMII. This case may not be possible with a specific PHY, but
illustrates the minimum preamble with which MAC shall be able to
operate. Table 35–4 depicts the case where the entire preamble is
conveyed across the GMII.

We would change the behavior from "no preamble is preceding SFD" to "the
enitre preamble is preceding SFD". Both are listed in the standard and
shall be supported by the MAC.

I hope this helps.

Regards,
Stefan