net: dsa: vsc73xx: speed up MDIO bus to max allowed value

According to the datasheet, the VSC73xx family's maximum internal MDIO bus
speed is 20 MHz. It also allows disabling the preamble.

This commit sets the MDIO clock prescaler to the minimum value and
disables the preamble to speed up MDIO operations.

It doesn't affect the external bus because it's configured in a different
subblock.
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20240731203455.580262-1-paweldembicki@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/vitesse-vsc73xx-core.c

@@ -40,6 +40,9 @@
 #define VSC73XX_BLOCK_ARBITER	0x5 /* Only subblock 0 */
 #define VSC73XX_BLOCK_SYSTEM	0x7 /* Only subblock 0 */
 
+/* MII Block subblock */
+#define VSC73XX_BLOCK_MII_INTERNAL     0x0 /* Internal MDIO subblock */
+
 #define CPU_PORT	6 /* CPU port */
 
 /* MAC Block registers */
@@ -224,6 +227,11 @@
 #define VSC73XX_MII_STAT	0x0
 #define VSC73XX_MII_CMD		0x1
 #define VSC73XX_MII_DATA	0x2
+#define VSC73XX_MII_MPRES	0x3
+
+#define VSC73XX_MII_MPRES_NOPREAMBLE	BIT(6)
+#define VSC73XX_MII_MPRES_PRESCALEVAL	GENMASK(5, 0)
+#define VSC73XX_MII_PRESCALEVAL_MIN	3 /* min allowed mdio clock prescaler */
 
 /* Arbiter block 5 registers */
 #define VSC73XX_ARBEMPTY		0x0c
@@ -748,7 +756,7 @@ static int vsc73xx_configure_rgmii_port_delay(struct dsa_switch *ds)
 static int vsc73xx_setup(struct dsa_switch *ds)
 {
 	struct vsc73xx *vsc = ds->priv;
-	int i, ret;
+	int i, ret, val;
 
 	dev_info(vsc->dev, "set up the switch\n");
 
@@ -821,6 +829,15 @@ static int vsc73xx_setup(struct dsa_switch *ds)
 
 	mdelay(50);
 
+	/* Disable preamble and use maximum allowed clock for the internal
+	 * mdio bus, used for communication with internal PHYs only.
+	 */
+	val = VSC73XX_MII_MPRES_NOPREAMBLE |
+	      FIELD_PREP(VSC73XX_MII_MPRES_PRESCALEVAL,
+			 VSC73XX_MII_PRESCALEVAL_MIN);
+	vsc73xx_write(vsc, VSC73XX_BLOCK_MII, VSC73XX_BLOCK_MII_INTERNAL,
+		      VSC73XX_MII_MPRES, val);
+
 	/* Release reset from the internal PHYs */
 	vsc73xx_write(vsc, VSC73XX_BLOCK_SYSTEM, 0, VSC73XX_GLORESET,
 		      VSC73XX_GLORESET_PHY_RESET);