There are two chip pins named TXDLY and RXDLY which actually adds the 2ns
delays to TXC and RXC for TXD/RXD latching. Alas this is the only
documented info regarding the RGMII timing control configurations the PHY
provides. It turns out the same settings can be setup via MDIO registers
hidden in the extension pages layout. Particularly the extension page 0xa4
provides a register 0x1c, which bits 1 and 2 control the described delays.
They are used to implement the "rgmii-{id,rxid,txid}" phy-mode.

The hidden RGMII configs register utilization was found in the rtl8211e
U-boot driver:
https://elixir.bootlin.com/u-boot/v2019.01/source/drivers/net/phy/realtek.c#L99

There is also a freebsd-folks discussion regarding this register:
https://reviews.freebsd.org/D13591

It confirms that the register bits field must control the so called
configuration pins described in the table 12-13 of the official PHY
datasheet:
8:6 = PHY Address
5:4 = Auto-Negotiation
3 = Interface Mode Select
2 = RX Delay
1 = TX Delay
0 = SELRGV
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>