RE: [PATCH net] net: enetc: fix fallback PHY address handling and do not skip setting for addr 0

The current netc_get_phy_addr() implementation falls back to PHY address
0 when the "mdio" node or any PHY child node is missing. On i.MX95, this
causes failures when a real PHY is actually assigned address 0 and is
managed through the EMDIO interface, the bit 0 of phy_mask becomes set,
leading imx95_enetc_mdio_phyaddr_config() to return an error, and the
netc_blk_ctrl driver probe subsequently fails. Fix this by returning
-ENODEV when neither an "mdio" node nor any PHY node is present.

Given that some platforms may use PHY address 0 (I suppose the PHY may
not treat address 0 as a broadcast address or default response address).
It is possible for some boards to connect multiple PHYs to a single
ENETC MAC, for example:

  - a SGMII PHY with a non-zero address (selected via DTS_A)
  - a RGMII PHY with address 0 (selected via DTS_B)

For the case where the ENETC port MDIO is used to manage the PHY, when
switching from DTS_A to DTS_B via soft reboot,
LaBCR[MDIO_PHYAD_PRTAD]
must be updated to 0 because the NETCMIX block is not reset during soft
reboot. However, the current driver explicitly skips configuring address
0, which leaves the hardware in an inconsistent state.

Therefore, remove the special-case skip of PHY address 0 so that valid
configurations using address 0 are properly supported.

Reported-by: Alexander Stein <redacted>
Closes: https://lore.kernel.org/all/7825188.GXAFRqVoOG@steina-w (local)
Fixes: 6633df05f3ad ("net: enetc: set the external PHY address in IERB for
port MDIO usage")
Fixes: 50bfd9c06f0f ("net: enetc: set external PHY address in IERB for i.MX94
ENETC")
Signed-off-by: Wei Fang <wei.fang@nxp.com>

Reviewed-by: Clark Wang <xiaoning.wang@nxp.com>

Thanks!
Clark Wang