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How do you direct a frame from the
CPU out a specific user port? Via the DMA ring you place it into, or
do you need a tag on the frame to indicate its egress port?

Via the DMA ring.

Are there bits in the ring descriptor which indicate the user port?
Can you set these bits to some other value which causes the switch to
use its MAC table to determine the egress interface?

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The PHY is 88E2110 on my environment, so Linux has a driver in
drivers/net/phy/marvell10g.c. However, I guess this is related to
configuration of the PHY chip on the board, it needs to change
the host 7interface mode, but the driver doesn't support it for now.

Please give us more details. The marvell10g driver will change its
host side depending on the result of the line side negotiation. It
changes the value of phydev->interface to indicate what is it doing on
its host side, and you have some control over what modes it will use
on the host side. You can probably define its initial host side mode
via phy-mode in DT.

I'm sorry, my explanation was completely wrong.
My environment needs to change default MAC speed from 2.5G/5G to 1000M.
The register of 88E2110 is 31.F000.7:6. And sets the register to "10" (1000 Mbps).
(Default value of the register is "11" (Speed controlled by other register).)

Is this the host side speed? The speed of the SERDES between the
switch and the PHY? Normally, the PHY determines this from the line
side. If the line side is using 2.5G, it will set the host side to
2500BaseX. If the line side is 1G, the host side is likely to be
SGMII.

You have already removed speeds you don't support. So the PHY will not
negotiate 2.5G or 5G. It is limited to 1G. So it should always have
the host side as SGMII. This should be enough to make it work.

    Andrew