net: dsa: mv88e6xxx: Improve indirect addressing performance

Before this change, both the read and write callback would start out
by asserting that the chip's busy flag was cleared. However, both
callbacks also made sure to wait for the clearing of the busy bit
before returning - making the initial check superfluous. The only
time that would ever have an effect was if the busy bit was initially
set for some reason.

With that in mind, make sure to perform an initial check of the busy
bit, after which both read and write can rely the previous operation
to have waited for the bit to clear.

This cuts the number of operations on the underlying MDIO bus by 25%
Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/mv88e6xxx/chip.h

@@ -392,6 +392,7 @@ struct mv88e6xxx_chip {
 struct mv88e6xxx_bus_ops {
 	int (*read)(struct mv88e6xxx_chip *chip, int addr, int reg, u16 *val);
 	int (*write)(struct mv88e6xxx_chip *chip, int addr, int reg, u16 val);
+	int (*init)(struct mv88e6xxx_chip *chip);
 };
 
 struct mv88e6xxx_mdio_bus {

drivers/net/dsa/mv88e6xxx/smi.c

@@ -111,11 +111,6 @@ static int mv88e6xxx_smi_indirect_read(struct mv88e6xxx_chip *chip,
 {
 	int err;
 
-	err = mv88e6xxx_smi_direct_wait(chip, chip->sw_addr,
-					MV88E6XXX_SMI_CMD, 15, 0);
-	if (err)
-		return err;
-
 	err = mv88e6xxx_smi_direct_write(chip, chip->sw_addr,
 					 MV88E6XXX_SMI_CMD,
 					 MV88E6XXX_SMI_CMD_BUSY |
@@ -139,11 +134,6 @@ static int mv88e6xxx_smi_indirect_write(struct mv88e6xxx_chip *chip,
 {
 	int err;
 
-	err = mv88e6xxx_smi_direct_wait(chip, chip->sw_addr,
-					MV88E6XXX_SMI_CMD, 15, 0);
-	if (err)
-		return err;
-
 	err = mv88e6xxx_smi_direct_write(chip, chip->sw_addr,
 					 MV88E6XXX_SMI_DATA, data);
 	if (err)
@@ -162,9 +152,20 @@ static int mv88e6xxx_smi_indirect_write(struct mv88e6xxx_chip *chip,
 					 MV88E6XXX_SMI_CMD, 15, 0);
 }
 
+static int mv88e6xxx_smi_indirect_init(struct mv88e6xxx_chip *chip)
+{
+	/* Ensure that the chip starts out in the ready state. As both
+	 * reads and writes always ensure this on return, they can
+	 * safely depend on the chip not being busy on entry.
+	 */
+	return mv88e6xxx_smi_direct_wait(chip, chip->sw_addr,
+					 MV88E6XXX_SMI_CMD, 15, 0);
+}
+
 static const struct mv88e6xxx_bus_ops mv88e6xxx_smi_indirect_ops = {
 	.read = mv88e6xxx_smi_indirect_read,
 	.write = mv88e6xxx_smi_indirect_write,
+	.init = mv88e6xxx_smi_indirect_init,
 };
 
 int mv88e6xxx_smi_init(struct mv88e6xxx_chip *chip,
@@ -182,5 +183,8 @@ int mv88e6xxx_smi_init(struct mv88e6xxx_chip *chip,
 	chip->bus = bus;
 	chip->sw_addr = sw_addr;
 
+	if (chip->smi_ops->init)
+		return chip->smi_ops->init(chip);
+
 	return 0;
 }