[PATCH net-next v3 3/3] net: stmmac: dwmac-imx: keep preamble before sfd on i.MX8MP

[Andrew Lunn]

On Tue, Jan 20, 2026 at 09:30:04PM +0100, Stefan Eichenberger wrote:
> From: Stefan Eichenberger <stefan.eichenberger at toradex.com>
> 
> The stmmac implementation used by NXP for the i.MX8MP SoC is subject to
> errata ERR050694. According to this errata, when no preamble byte is
> transferred before the SFD from the PHY to the MAC, the MAC will discard
> the frame.
> 
> Setting the PHY_F_KEEP_PREAMBLE_BEFORE_SFD flag instructs PHYs that
> support it to keep the preamble byte before the SFD. This ensures that
> the MAC successfully receives frames.
> 
> As this is an issue in the MAC implementation, only enable the flag for
> the i.MX8MP SoC where the errata applies but not for other SoCs using a
> working stmmac implementation.
> 
> The exact wording of the errata ERR050694 from NXP:
> 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.
> 
> This workaround was tested on a Verdin iMX8MP by enforcing 10 MBit/s:
> ethtool -s end0 speed 10
> Without keeping the preamble, no packet were received. With keeping the
> preamble, everything worked as expected.
> 
> Signed-off-by: Stefan Eichenberger <stefan.eichenberger at toradex.com>

Reviewed-by: Andrew Lunn <andrew at lunn.ch>

    Andrew