If the network port is linked, but the link fails between the SGMII and PHY,
is there any method to find out the cause?

I've had a problem with phy link but SGMII no link due to poor contact.

So a hardware design issue? Has it been solved now, or is there
shipped hardware with this problem?

I would say there is a difference between the first prototype board,
and real products. If the real product has issues, then we should try
to address it. If it is just a prototype board, i would ignore it.

One option might be to expose your PCS as a phylib PCS. Do you get
interrupts from it when it has link up? Link down? phylink will then
combine media side status with PCS status.

	Andrew