So I'm not completely convinced about CONFIG_ARM64_ERRATUM_858921, but

when doing my initial changes [enabled a bunch of PHYs, made NFS/SMB
etc modules... random stuff], I somehow enabled some errata fixes, and
this one showed up in dmesg.

The real star of the show I'm pretty sure is CONFIG_PCS_MTK_USXGMII.
Note, it cannot be a module, but on this platform we probably wouldn't
want it to be anyway. When I say "it can't", I'm saying it breaks the
build during the linker phase for vmlinux. Probably worth fixing, but
I'm unclear where to start, other than it's probably a Kconfig
dependency problem.

CONFIG_I2C_SLAVE_EEPROM came along for the ride b/c it possibly made
sense with SFPs. But I haven't tried a test build without it yet.

For the record: this works with both my AQS-107-B0C2-CX [an 802.3bz
SFP+] and my FS.com SFP-10GSR-85 "generic" SFP+.

config/kernel/linux-filogic-current.config

@@ -339,6 +339,7 @@ CONFIG_MAXLINEAR_GPHY=y
 CONFIG_MEDIATEK_2P5GE_PHY=y
 CONFIG_REALTEK_PHY=y
 CONFIG_MDIO_AN8855=y
+CONFIG_PCS_MTK_USXGMII=y
 CONFIG_PPP=m
 CONFIG_PPP_FILTER=y
 CONFIG_PPP_MPPE=m
@@ -413,6 +414,9 @@ CONFIG_I2C_MUX=m
 CONFIG_I2C_MUX_PCA954x=m
 # CONFIG_I2C_HELPER_AUTO is not set
 CONFIG_I2C_MT65XX=y
+CONFIG_I2C_SLAVE=y
+CONFIG_I2C_SLAVE_EEPROM=m
+CONFIG_I2C_SLAVE_TESTUNIT=m
 CONFIG_SPI=y
 CONFIG_SPI_MT65XX=y
 CONFIG_SPI_MTK_SNFI=y
@@ -515,7 +519,6 @@ CONFIG_STAGING=y
 # CONFIG_COMMON_CLK_MT8516 is not set
 # CONFIG_FSL_ERRATUM_A008585 is not set
 # CONFIG_HISILICON_ERRATUM_161010101 is not set
-# CONFIG_ARM64_ERRATUM_858921 is not set
 # CONFIG_IOMMU_SUPPORT is not set
 CONFIG_MTK_PMIC_WRAP=y
 # CONFIG_MTK_MMSYS is not set