Re: mv88e6xxx: 2500base-x inband AN is broken on Amethyst? what to do?

["Marek Behún" <kabel@kernel.org>]

OK, so I did some tests with Peridot and 88X3310:

On 88E6390 switch, when CMODE in the port register for a port capable of
SerDes is set to 1000base-x, the switch self-configures the SerDes PHY
to inband AN:
  register 4.2000 has value 0x1940

but when CMODE is set to 2500base-x, the switch self-configure the PHY
without inband AN:
  register 4.2000 has value 0x0940

Also the 88X3310 PHY, when configured with MACTYPE=4
 (that is the mode that switches host interface between
  10gbase-r/5gbase-r/2500base-x/sgmii, depending on copper speed)
and when copper speed is 2500, the PHY self-configures without inband
AN:
  register 4.2000 has value 0x0140

It seems to me that on these Marvell devices, they consider 2500base-x
not capable of inband AN and disable it by default.

Moreover when the PHY has disabled inband AN and the Peridot switch has
it enabled (by software), they won't link. I tried enabling the inband
AN on the PHY, but it does not seem to work. Peridot can only
communicate with the PHY in 2500base-x with inband AN disabled.

This means that the commit
  a5a6858b793ff ("net: dsa: mv88e6xxx: extend phylink to Serdes PHYs")
causes a regression, since the code started enabling inband AN on
2500base-x mode on the mv88e6390 family, and they stopped working with
the PHY.

Russell, could we, for now, just edit the code so that when
  mv88e6390_serdes_pcs_config
is being configured with inband mode in 2500base-x, the inband mode
won't be enabled and the function will print a warning?
This could come with a Fixes tag so that it is backported to stable.

Afterwards we can work on refactoring the phylink code so that either
the driver can inform phylink whether 2500base-x inband AN is supported,
or maybe we can determine from some documentation or whatnot whether
inband AN is supported on 2500base-x at all.

Marek