[RFC net-next v2 3/3] net: dsa: microchip: implement PHY loopback configuration for KSZ8794 and KSZ8873

[Oleksij Rempel]

Correct the PHY loopback bit handling in the ksz8_w_phy_bmcr and
ksz8_r_phy_bmcr functions for KSZ8794 and KSZ8873 variants in the ksz8795
driver. Previously, the code erroneously used Bit 7 of port register 0xD
for both chip variants, which is actually for LED configuration. This
update ensures the correct registers and bits are used for the PHY
loopback feature:

- For KSZ8794: Use 0xF / Bit 7.
- For KSZ8873: Use 0xD / Bit 0.

The lack of loopback support was seen on KSZ8873 system by using
"ethtool -t lanX". After this patch, the ethtool selftest will work,
but only if port is not part of a bridge.

Signed-off-by: Oleksij Rempel <o.rempel@xxxxxxxxxxxxxx>
---
 drivers/net/dsa/microchip/ksz8795.c     | 36 ++++++++++++++++++++-----
 drivers/net/dsa/microchip/ksz8795_reg.h |  1 +
 2 files changed, 30 insertions(+), 7 deletions(-)

diff --git a/drivers/net/dsa/microchip/ksz8795.c b/drivers/net/dsa/microchip/ksz8795.c
index 51e0194453df..1d8377640a3d 100644
--- a/drivers/net/dsa/microchip/ksz8795.c
+++ b/drivers/net/dsa/microchip/ksz8795.c
@@ -731,16 +731,25 @@ static int ksz8_r_phy_bmcr(struct ksz_device *dev, u16 port, u16 *val)
 	if (ret)
 		return ret;
 
-	if (restart & PORT_PHY_LOOPBACK)
-		*val |= BMCR_LOOPBACK;
-
 	if (ctrl & PORT_FORCE_100_MBIT)
 		*val |= BMCR_SPEED100;
 
 	if (ksz_is_ksz88x3(dev)) {
+		if (restart & KSZ8873_PORT_PHY_LOOPBACK)
+			*val |= BMCR_LOOPBACK;
+
 		if ((ctrl & PORT_AUTO_NEG_ENABLE))
 			*val |= BMCR_ANENABLE;
 	} else {
+		u8 stat3;
+
+		ret = ksz_pread8(dev, port, REG_PORT_STATUS_3, &stat3);
+		if (ret)
+			return ret;
+
+		if (stat3 & PORT_PHY_LOOPBACK)
+			*val |= BMCR_LOOPBACK;
+
 		if (!(ctrl & PORT_AUTO_NEG_DISABLE))
 			*val |= BMCR_ANENABLE;
 	}
@@ -1001,8 +1010,7 @@ static int ksz8_w_phy_bmcr(struct ksz_device *dev, u16 port, u16 val)
 
 	restart = 0;
 	restart_mask = PORT_LED_OFF | PORT_TX_DISABLE | PORT_AUTO_NEG_RESTART |
-		PORT_POWER_DOWN | PORT_AUTO_MDIX_DISABLE | PORT_FORCE_MDIX |
-		PORT_PHY_LOOPBACK;
+		PORT_POWER_DOWN | PORT_AUTO_MDIX_DISABLE | PORT_FORCE_MDIX;
 
 	if (val & KSZ886X_BMCR_DISABLE_LED)
 		restart |= PORT_LED_OFF;
@@ -1022,8 +1030,22 @@ static int ksz8_w_phy_bmcr(struct ksz_device *dev, u16 port, u16 val)
 	if (val & KSZ886X_BMCR_FORCE_MDI)
 		restart |= PORT_FORCE_MDIX;
 
-	if (val & BMCR_LOOPBACK)
-		restart |= PORT_PHY_LOOPBACK;
+	if (ksz_is_ksz88x3(dev)) {
+		restart_mask |= KSZ8873_PORT_PHY_LOOPBACK;
+
+		if (val & BMCR_LOOPBACK)
+			restart |= KSZ8873_PORT_PHY_LOOPBACK;
+	} else {
+		u8 stat3 = 0;
+
+		if (val & BMCR_LOOPBACK)
+			stat3 |= PORT_PHY_LOOPBACK;
+
+		ret = ksz_prmw8(dev, port, REG_PORT_STATUS_3, PORT_PHY_LOOPBACK,
+				stat3);
+		if (ret)
+			return ret;
+	}
 
 	return ksz_prmw8(dev, port, regs[P_NEG_RESTART_CTRL], restart_mask,
 			 restart);
diff --git a/drivers/net/dsa/microchip/ksz8795_reg.h b/drivers/net/dsa/microchip/ksz8795_reg.h
index beca974e0171..7c9341ef73b0 100644
--- a/drivers/net/dsa/microchip/ksz8795_reg.h
+++ b/drivers/net/dsa/microchip/ksz8795_reg.h
@@ -265,6 +265,7 @@
 #define PORT_AUTO_MDIX_DISABLE		BIT(2)
 #define PORT_FORCE_MDIX			BIT(1)
 #define PORT_MAC_LOOPBACK		BIT(0)
+#define KSZ8873_PORT_PHY_LOOPBACK	BIT(0)
 
 #define REG_PORT_1_STATUS_2		0x1E
 #define REG_PORT_2_STATUS_2		0x2E
-- 
2.39.2