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RGMII requires a delay of 2ns between the data and the clock signal.
There are at least three ways this can happen. One possibility is by
having the PHY generate this delay.
This is a common source for problems (for example with slow TX speeds or
packet loss when sending data). The TX delay configuration of the
RTL8211F PHY can be set either by pin-strappping the RXD1 pin (HIGH
means enabled, LOW means disabled) or through configuring a paged
register. The setting from the RXD1 pin is also reflected in the
register.

Add debug logging to the TX delay configuration on RTL8211F so it's
easier to spot these issues (for example if the TX delay is enabled for
both, the RTL8211F PHY and the MAC).
This is especially helpful because there is no public datasheet for the
RTL8211F PHY available with all the RX/TX delay specifics.
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>