This change does not fix any functional issue or address any real life
use case that wasn't possible before. It is just a small step in the
process of standardizing the way in which Ethernet MAC drivers may apply
RGMII delays (traditionally these have been applied by PHYs, with no
clear definition of what to do in the case of a fixed-link).

The sja1105 driver used to apply MAC-level RGMII delays on the RX data
lines when in fixed-link mode and using a phy-mode of "rgmii-rxid" or
"rgmii-id" and on the TX data lines when using "rgmii-txid" or "rgmii-id".
But the standard definitions don't say anything about behaving
differently when the port is in fixed-link vs when it isn't, and the new
device tree bindings are about having a way of applying the delays in a
way that is independent of the phy-mode and of the fixed-link property.

When the {rx,tx}-internal-delay-ps properties are present, use them,
otherwise fall back to the old behavior and warn.

One other thing to note is that the SJA1105 hardware applies a delay
value in degrees rather than in picoseconds (the delay in ps changes
depending on the frequency of the RGMII clock - 125 MHz at 1G, 25 MHz at
100M, 2.5MHz at 10M). I assume that is fine, we calculate the phase
shift of the internal delay lines assuming that the device tree meant
gigabit, and we let the hardware scale those according to the link speed.

Link: https://patchwork.kernel.org/project/netdevbpf/patch/20210723173108.459770-6-prasanna.vengateshan@microchip.com/
Link: https://patchwork.ozlabs.org/project/netdev/patch/20200616074955.GA9092@laureti-dev/#2461123Signed-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>