As noted in commit 28c2d1a7 ("net: bcmgenet: enable loopback
during UniMAC sw_reset") the UniMAC must be clocked while sw_reset
is asserted for its state machines to reset cleanly.

The transmit and receive clocks used by the UniMAC are derived from
the signals used on its PHY interface. The bcmgenet MAC can be
configured to work with different PHY interfaces including MII,
GMII, RGMII, and Reverse MII on internal and external interfaces.
Unfortunately for the UniMAC, when configured for MII the Tx clock
is always driven from the PHY which places it outside of the direct
control of the MAC.

The earlier commit enabled a local loopback mode within the UniMAC
so that the receive clock would be derived from the transmit clock
which addressed the observed issue with an external GPHY disabling
it's Rx clock. However, when a Tx clock is not available this
loopback is insufficient.

This commit implements a workaround that leverages the fact that
the MAC can reliably generate all of its necessary clocking by
enterring the external GPHY RGMII interface mode with the UniMAC in
local loopback during the sw_reset interval. Unfortunately, this
has the undesirable side efect of the RGMII GTXCLK signal being
driven during the same window.

In most configurations this is a benign side effect as the signal
is either not routed to a pin or is already expected to drive the
pin. The one exception is when an external MII PHY is expected to
drive the same pin with its TX_CLK output creating output driver
contention.

This commit exploits the IEEE 802.3 clause 22 standard defined
isolate mode to force an external MII PHY to present a high
impedance on its TX_CLK output during the window to prevent any
contention at the pin.

The MII interface is used internally with the 40nm internal EPHY
which agressively disables its clocks for power savings leading to
incomplete resets of the UniMAC and many instabilities observed
over the years. The workaround of this commit is expected to put
an end to those problems.

Fixes: 1c1008c7 ("net: bcmgenet: add main driver file")
Signed-off-by: Doug Berger <opendmb@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>