Until recently, the Micrel KSZ9031 PHY driver ignored any PHY mode
("RGMII-*ID") settings, but used the hardware defaults, augmented by
explicit configuration of individual skew values using the "*-skew-ps"
DT properties.  The lack of PHY mode support was compensated by the
EtherAVB MAC driver, which configures TX and/or RX internal delay
itself, based on the PHY mode.

However, now the KSZ9031 driver has gained PHY mode support, delays may
be configured twice, causing regressions.  E.g. on the Renesas
Salvator-X board with R-Car M3-W ES1.0, TX performance dropped from ca.
400 Mbps to 0.1-0.3 Mbps, as measured by nuttcp.

As internal delay configuration supported by the KSZ9031 PHY is too
limited for some use cases, the ability to configure MAC internal delay
is deemed useful and necessary.  Hence a proper fix would involve
splitting internal delay configuration in two parts, one for the PHY,
and one for the MAC.  However, this would require adding new DT
properties, thus breaking DTB backwards-compatibility.

Hence fix the regression in a backwards-compatibility way, by letting
the EtherAVB driver mask the PHY mode when it has inserted a delay, to
avoid the PHY driver adding a second delay.  This also fixes messages
like:

    Micrel KSZ9031 Gigabit PHY e6800000.ethernet-ffffffff:00: *-skew-ps values should be used only with phy-mode = "rgmii"

as the PHY no longer sees the original RGMII-*ID mode.

Solving the issue by splitting configuration in two parts can be handled
in future patches, and would require retaining a backwards-compatibility
mode anyway.

Fixes: bcf3440c ("net: phy: micrel: add phy-mode support for the KSZ9031 PHY")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>