net: fec_mxc: configure MDIO hold time

If the host clock frequency is higher than 100 MHz, the MDIO hold
time needs to be increased from its current setting of one cycle in
order to meet the specified minium of 10 ns.  Writing an appropriate
value to the HOLDTIME field of the MII_SPEED register achieves this.

Comment copied from Linux kernel.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Reviewed-by: Eric Nelson <eric@nelint.com>

drivers/net/fec_mxc.c

@@ -131,13 +131,25 @@ static void fec_mii_setspeed(struct ethernet_regs *eth)
 	/*
 	 * Set MII_SPEED = (1/(mii_speed * 2)) * System Clock
 	 * and do not drop the Preamble.
+	 *
+	 * The i.MX28 and i.MX6 types have another field in the MSCR (aka
+	 * MII_SPEED) register that defines the MDIO output hold time. Earlier
+	 * versions are RAZ there, so just ignore the difference and write the
+	 * register always.
+	 * The minimal hold time according to IEE802.3 (clause 22) is 10 ns.
+	 * HOLDTIME + 1 is the number of clk cycles the fec is holding the
+	 * output.
+	 * The HOLDTIME bitfield takes values between 0 and 7 (inclusive).
+	 * Given that ceil(clkrate / 5000000) <= 64, the calculation for
+	 * holdtime cannot result in a value greater than 3.
 	 */
-	register u32 speed = DIV_ROUND_UP(imx_get_fecclk(), 5000000);
+	u32 pclk = imx_get_fecclk();
+	u32 speed = DIV_ROUND_UP(pclk, 5000000);
+	u32 hold = DIV_ROUND_UP(pclk, 100000000) - 1;
 #ifdef FEC_QUIRK_ENET_MAC
 	speed--;
 #endif
-	speed <<= 1;
-	writel(speed, &eth->mii_speed);
+	writel(speed << 1 | hold << 8, &eth->mii_speed);
 	debug("%s: mii_speed %08x\n", __func__, readl(&eth->mii_speed));
 }