net: stmmac: dwmac-imx: keep preamble before sfd on i.MX8MP

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@toradex.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20260120203905.23805-4-eichest@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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