In general, PHY I/O voltage must be the same as I/O voltage on MAC side, for T1040 this interface is specified at 2.5V or 1.8V, so PHY I/O voltage must be the same - ether 1.8V or 2.5V, depending on which voltage is selected at T1040 side. This is for DVDDH voltage in terms of KSZ9031RNX PHY documentation. For AVDDH you should use 3.3V or 2.5V, this is analog power supply, used for analog circuits, and not related to digital interface between PHY and MAC. Datasheet says: "2.5V AVDDH is recommended for commercial temperature range (0°C to +70°C) operation only."
Sorry, we do not have any experience with PHY KSZ9031RNX, and my response is based on information from PHY datasheet only, so it will be better to ask these questions to PHY vendor to double-check.
Have a great day,
Alexander
TIC
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