quoted

+/* Set Management Data Clock, must be call after device reset */
+static void sun8i_emac_set_mdc(struct net_device *ndev)
+{
+	struct sun8i_emac_priv *priv = netdev_priv(ndev);
+	unsigned long rate;
+	u32 reg;
+
+	rate = clk_get_rate(priv->ahb_clk);
+	if (rate > 160000000)
+		reg = 0x3 << 20; /* AHB / 128 */
+	else if (rate > 80000000)
+		reg = 0x2 << 20; /* AHB / 64 */
+	else if (rate > 40000000)
+		reg = 0x1 << 20; /* AHB / 32 */
+	else
+		reg = 0x0 << 20; /* AHB / 16 */
+	netif_dbg(priv, link, ndev, "MDC auto : %x\n", reg);
+	writel(reg, priv->base + SUN8I_EMAC_MDIO_CMD);

You could also expose that as a clock.

For which purpose ?
No ethernet driver expose the MDC as clock and I dont see any interest:
- I dont think that tuning it give any gain
- Knowing it's value is of little interest

Regards