[RFC PATCH net-next 15/22] net: dsa: mt7530: set TRGMII RD TAP if trgmii is being used

[arinc9.unal at gmail.com]

From: Arınç ÜNAL <arinc.unal at arinc9.com>

This code sets the Read Data (RD) TAP value to 16 for all TRGMII control
registers.

The for loop iterates over all the TRGMII control registers, and
mt7530_rmw() function is used to perform a read-modify-write operation on
each register's RD_TAP field to set its value to 16.

This operation is used to tune the timing of the read data signal in
TRGMII to match the TX signal of the link partner.

Run this if trgmii is being used. Since this code doesn't lower the
driving, there's no apparent benefit to run this if trgmii is not being
used.

Add a comment to explain the code.

Thanks to 趙皎宏 (Landen Chao) for pointing out what the code does.

Tested-by: Arınç ÜNAL <arinc.unal at arinc9.com>
Signed-off-by: Arınç ÜNAL <arinc.unal at arinc9.com>
---
 drivers/net/dsa/mt7530.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/drivers/net/dsa/mt7530.c b/drivers/net/dsa/mt7530.c
index 610828b56eac..029d3129bb8b 100644
--- a/drivers/net/dsa/mt7530.c
+++ b/drivers/net/dsa/mt7530.c
@@ -404,7 +404,7 @@ static void
 mt7530_setup_port6(struct dsa_switch *ds, phy_interface_t interface)
 {
 	struct mt7530_priv *priv = ds->priv;
-	u32 ncpo1, ssc_delta, xtal, val;
+	u32 ncpo1, ssc_delta, i, xtal, val;
 
 	val = mt7530_read(priv, MT7530_MHWTRAP);
 	val &= ~MHWTRAP_P6_DIS;
@@ -457,6 +457,11 @@ mt7530_setup_port6(struct dsa_switch *ds, phy_interface_t interface)
 
 		/* Enable the MT7530 TRGMII clocks */
 		core_set(priv, CORE_TRGMII_GSW_CLK_CG, REG_TRGMIICK_EN);
+
+		/* Set the Read Data TAP value of the MT7530 TRGMII */
+		for (i = 0; i < NUM_TRGMII_CTRL; i++)
+			mt7530_rmw(priv, MT7530_TRGMII_RD(i),
+				   RD_TAP_MASK, RD_TAP(16));
 	}
 }
 
@@ -2214,10 +2219,6 @@ mt7530_setup(struct dsa_switch *ds)
 		mt7530_write(priv, MT7530_TRGMII_TD_ODT(i),
 			     TD_DM_DRVP(8) | TD_DM_DRVN(8));
 
-	for (i = 0; i < NUM_TRGMII_CTRL; i++)
-		mt7530_rmw(priv, MT7530_TRGMII_RD(i),
-			   RD_TAP_MASK, RD_TAP(16));
-
 	/* Enable PHY access and operate in manual mode */
 	val = mt7530_read(priv, MT7530_MHWTRAP);
 	val &= ~MHWTRAP_PHY_ACCESS;
-- 
2.37.2