Looked over the existing code and what this patch does, and realized you could get almost the same effect by adding, in the DTS,
&gsw {
mediatek,port4="gmac";
};
which will skip the port4 init when the ethernet device also has mediatek,mdio-mode set.
The important difference between your patch and just doing the above is that you skip the setup of global page 0 and 2, so it doesn't execute
/* global page 0 */
_mt7620_mii_write(gsw, gsw->ephy_base + 1, 31, 0x8000);
_mt7620_mii_write(gsw, gsw->ephy_base + 0, 30, 0xa000);
_mt7620_mii_write(gsw, gsw->ephy_base + 1, 30, 0xa000);
_mt7620_mii_write(gsw, gsw->ephy_base + 2, 30, 0xa000);
_mt7620_mii_write(gsw, gsw->ephy_base + 3, 30, 0xa000);
_mt7620_mii_write(gsw, gsw->ephy_base + 0, 4, 0x05e1);
_mt7620_mii_write(gsw, gsw->ephy_base + 1, 4, 0x05e1);
_mt7620_mii_write(gsw, gsw->ephy_base + 2, 4, 0x05e1);
_mt7620_mii_write(gsw, gsw->ephy_base + 3, 4, 0x05e1);
/* global page 2 */
_mt7620_mii_write(gsw, gsw->ephy_base + 1, 31, 0xa000);
_mt7620_mii_write(gsw, gsw->ephy_base + 0, 16, 0x1111);
_mt7620_mii_write(gsw, gsw->ephy_base + 1, 16, 0x1010);
_mt7620_mii_write(gsw, gsw->ephy_base + 2, 16, 0x1515);
_mt7620_mii_write(gsw, gsw->ephy_base + 3, 16, 0x0f0f);
so it would be interesting to know why those writes stop things from working. I tried moving them up into the tail of the preceding if/else so they're skipped if the ethernet is in mdio-mode, and that worked fine. But it leaves me wondering if we're just benefiting from not touching the PHYs after the stock U-Boot has set them up...