The Tegra MGBE driver sometimes fails to initialize, reporting the
following error, and as a result, it is unable to acquire an IP
address with DHCP:

 tegra-mgbe 6800000.ethernet: timeout waiting for link to become ready

As per the recommendation from the Tegra hardware design team, fix this
issue by:
- clearing the PHY_RDY bit before setting the CDR_RESET bit and then
setting PHY_RDY bit before clearing CDR_RESET bit. This ensures valid
data is present at UPHY RX inputs before starting the CDR lock.
- adding the required delays when bringing up the UPHY lane. Note we
need to use delays here because there is no alternative, such as
polling, for these cases. Using the usleep_range() instead of ndelay()
as sleeping is preferred over busy wait loop.

Without this change we would see link failures on boot sometimes as
often as 1 in 5 boots. With this fix we have not observed any failures
in over 1000 boots.

Fixes: d8ca1137 ("net: stmmac: tegra: Add MGBE support")
Signed-off-by: Paritosh Dixit <paritoshd@nvidia.com>
Link: https://patch.msgid.link/20241010142908.602712-1-paritoshd@nvidia.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>