During the refactoring in the commit below, vsc9953_mdio_read() was
replaced with mscc_miim_read(), which has one extra step: it checks for
the MSCC_MIIM_DATA_ERROR bits before returning the result.

On T1040RDB, there are 8 QSGMII PCSes belonging to the switch, and they
are organized in 2 groups. First group responds to MDIO addresses 4-7
because QSGMIIACR1[MDEV_PORT] is 1, and the second group responds to
MDIO addresses 8-11 because QSGMIIBCR1[MDEV_PORT] is 2. I have double
checked that these values are correctly set in the SERDES, as well as
PCCR1[QSGMA_CFG] and PCCR1[QSGMB_CFG] are both 0b01.

mscc_miim_read: phyad 8 reg 0x1 MIIM_DATA 0x2d
mscc_miim_read: phyad 8 reg 0x5 MIIM_DATA 0x5801
mscc_miim_read: phyad 8 reg 0x1 MIIM_DATA 0x2d
mscc_miim_read: phyad 8 reg 0x5 MIIM_DATA 0x5801
mscc_miim_read: phyad 9 reg 0x1 MIIM_DATA 0x2d
mscc_miim_read: phyad 9 reg 0x5 MIIM_DATA 0x5801
mscc_miim_read: phyad 9 reg 0x1 MIIM_DATA 0x2d
mscc_miim_read: phyad 9 reg 0x5 MIIM_DATA 0x5801
mscc_miim_read: phyad 10 reg 0x1 MIIM_DATA 0x2d
mscc_miim_read: phyad 10 reg 0x5 MIIM_DATA 0x5801
mscc_miim_read: phyad 10 reg 0x1 MIIM_DATA 0x2d
mscc_miim_read: phyad 10 reg 0x5 MIIM_DATA 0x5801
mscc_miim_read: phyad 11 reg 0x1 MIIM_DATA 0x2d
mscc_miim_read: phyad 11 reg 0x5 MIIM_DATA 0x5801
mscc_miim_read: phyad 11 reg 0x1 MIIM_DATA 0x2d
mscc_miim_read: phyad 11 reg 0x5 MIIM_DATA 0x5801
mscc_miim_read: phyad 4 reg 0x1 MIIM_DATA 0x3002d, ERROR
mscc_miim_read: phyad 4 reg 0x5 MIIM_DATA 0x3da01, ERROR
mscc_miim_read: phyad 5 reg 0x1 MIIM_DATA 0x3002d, ERROR
mscc_miim_read: phyad 5 reg 0x5 MIIM_DATA 0x35801, ERROR
mscc_miim_read: phyad 5 reg 0x1 MIIM_DATA 0x3002d, ERROR
mscc_miim_read: phyad 5 reg 0x5 MIIM_DATA 0x35801, ERROR
mscc_miim_read: phyad 6 reg 0x1 MIIM_DATA 0x3002d, ERROR
mscc_miim_read: phyad 6 reg 0x5 MIIM_DATA 0x35801, ERROR
mscc_miim_read: phyad 6 reg 0x1 MIIM_DATA 0x3002d, ERROR
mscc_miim_read: phyad 6 reg 0x5 MIIM_DATA 0x35801, ERROR
mscc_miim_read: phyad 7 reg 0x1 MIIM_DATA 0x3002d, ERROR
mscc_miim_read: phyad 7 reg 0x5 MIIM_DATA 0x35801, ERROR
mscc_miim_read: phyad 7 reg 0x1 MIIM_DATA 0x3002d, ERROR
mscc_miim_read: phyad 7 reg 0x5 MIIM_DATA 0x35801, ERROR

As can be seen, the data in MIIM_DATA is still valid despite having the
MSCC_MIIM_DATA_ERROR bits set. The driver as introduced in commit
84705fc1 ("net: dsa: felix: introduce support for Seville VSC9953
switch") was ignoring these bits, perhaps deliberately (although
unbeknownst to me).

This is an old IP and the hardware team cannot seem to be able to help
me track down a plausible reason for these failures. I'll keep
investigating, but in the meantime, this is a direct regression which
must be restored to a working state.

The only thing I can do is keep ignoring the errors as before.

Fixes: b9965845 ("net: dsa: ocelot: felix: utilize shared mscc-miim driver for indirect MDIO access")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>