net: phy: dp83867: add w/a for packet errors seen with short cables

Introduce the W/A for packet errors seen with short cables (<1m) between
two DP83867 PHYs.

The W/A recommended by DM requires FFE Equalizer Configuration tuning by
writing value 0x0E81 to DSP_FFE_CFG register (0x012C), surrounded by hard
and soft resets as follows:

write_reg(0x001F, 0x8000); //hard reset
write_reg(DSP_FFE_CFG, 0x0E81);
write_reg(0x001F, 0x4000); //soft reset

Since  DP83867 PHY DM says "Changing this register to 0x0E81, will not
affect Long Cable performance.", enable the W/A by default.

Fixes: 2a10154abc ("net: phy: dp83867: Add TI dp83867 phy")
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/dp83867.c

@@ -44,6 +44,7 @@
 #define DP83867_STRAP_STS1	0x006E
 #define DP83867_STRAP_STS2	0x006f
 #define DP83867_RGMIIDCTL	0x0086
+#define DP83867_DSP_FFE_CFG	0x012c
 #define DP83867_RXFCFG		0x0134
 #define DP83867_RXFPMD1	0x0136
 #define DP83867_RXFPMD2	0x0137
@@ -941,8 +942,27 @@ static int dp83867_phy_reset(struct phy_device *phydev)
 
 	usleep_range(10, 20);
 
-	return phy_modify(phydev, MII_DP83867_PHYCTRL,
+	err = phy_modify(phydev, MII_DP83867_PHYCTRL,
 			 DP83867_PHYCR_FORCE_LINK_GOOD, 0);
+	if (err < 0)
+		return err;
+
+	/* Configure the DSP Feedforward Equalizer Configuration register to
+	 * improve short cable (< 1 meter) performance. This will not affect
+	 * long cable performance.
+	 */
+	err = phy_write_mmd(phydev, DP83867_DEVADDR, DP83867_DSP_FFE_CFG,
+			    0x0e81);
+	if (err < 0)
+		return err;
+
+	err = phy_write(phydev, DP83867_CTRL, DP83867_SW_RESTART);
+	if (err < 0)
+		return err;
+
+	usleep_range(10, 20);
+
+	return 0;
 }
 
 static void dp83867_link_change_notify(struct phy_device *phydev)