[PATCH net v3] net: phy: dp83867: add w/a for packet errors seen with short cables

[Siddharth Vadapalli]

From: Grygorii Strashko <grygorii.strashko at ti.com>

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: 2a10154abcb7 ("net: phy: dp83867: Add TI dp83867 phy")
Signed-off-by: Grygorii Strashko <grygorii.strashko at ti.com>
Signed-off-by: Siddharth Vadapalli <s-vadapalli at ti.com>
---

V2 patch at:
https://lore.kernel.org/r/20230509052124.611875-1-s-vadapalli@ti.com/

Changes since v2 patch:
- Add comment describing the short cable w/a register configuration.

V1 patch at:
https://lore.kernel.org/r/20230508070019.356548-1-s-vadapalli@ti.com

Changes since v1 patch:
- Wrap the line invoking phy_write_mmd(), limiting it to 80 characters.
- Replace 0X0E81 with 0x0e81 in the call to phy_write_mmd().
- Replace 0X012C with 0x012c in the new define for DP83867_DSP_FFE_CFG.

RFC patch at:
https://lore.kernel.org/r/20230425054429.3956535-2-s-vadapalli@ti.com/

Changes since RFC patch:
- Change patch subject to PATCH net.
- Add Fixes tag.
- Check return value of phy_write_mmd().

 drivers/net/phy/dp83867.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/drivers/net/phy/dp83867.c b/drivers/net/phy/dp83867.c
index d75f526a20a4..76f5a2402fb0 100644
--- a/drivers/net/phy/dp83867.c
+++ b/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)
-- 
2.25.1