hwmon: pmbus: Fix -EIO seen on pli1209

After doing performance optimizations the pli1209 driver failed to
probe with a probabilty of 2%. It wasn't able to read the PMBUS_OPERATION
register due to an -EIO error.

An investigation showed that the PLI1209 takes 230 usec to execute the
CLEAR_FAULTS command. During that time it's busy and NACKs all requests
on the SMBUS interface. It also NACKs reads on PMBUS_STATUS_BYTE
making it impossible to poll the BUSY flag.

Add a custom write_data function to just wait for not BUSY unconditionally
after sending a CLEAR_FAULTS command.

TEST: Verified using an I2C bus analyser that no more NACKs are seen after
      sending a CLEAR_FAULTS command.

Signed-off-by: Patrick Rudolph <[email protected]>
Signed-off-by: Naresh Solanki <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Guenter Roeck <[email protected]>

Author: PatrickRudolph

drivers/hwmon/pmbus/pli1209bc.c

@@ -5,6 +5,7 @@
  * Copyright (c) 2022 9elements GmbH
  */
 
+#include <linux/delay.h>
 #include <linux/i2c.h>
 #include <linux/module.h>
 #include <linux/pmbus.h>
@@ -53,6 +54,30 @@ static int pli1209bc_read_word_data(struct i2c_client *client, int page,
 	}
 }
 
+static int pli1209bc_write_byte(struct i2c_client *client, int page, u8 reg)
+{
+	int ret;
+
+	switch (reg) {
+	case PMBUS_CLEAR_FAULTS:
+		ret = pmbus_write_byte(client, page, reg);
+		/*
+		 * PLI1209 takes 230 usec to execute the CLEAR_FAULTS command.
+		 * During that time it's busy and NACKs all requests on the
+		 * SMBUS interface. It also NACKs reads on PMBUS_STATUS_BYTE
+		 * making it impossible to poll the BUSY flag.
+		 *
+		 * Just wait for not BUSY unconditionally.
+		 */
+		usleep_range(250, 300);
+		break;
+	default:
+		ret = -ENODATA;
+		break;
+	}
+	return ret;
+}
+
 #if IS_ENABLED(CONFIG_SENSORS_PLI1209BC_REGULATOR)
 static const struct regulator_desc pli1209bc_reg_desc = {
 	.name = "vout2",
@@ -102,6 +127,7 @@ static struct pmbus_driver_info pli1209bc_info = {
 	    | PMBUS_HAVE_TEMP | PMBUS_HAVE_STATUS_TEMP
 	    | PMBUS_HAVE_STATUS_IOUT | PMBUS_HAVE_STATUS_INPUT,
 	.read_word_data = pli1209bc_read_word_data,
+	.write_byte = pli1209bc_write_byte,
 #if IS_ENABLED(CONFIG_SENSORS_PLI1209BC_REGULATOR)
 	.num_regulators = 1,
 	.reg_desc = &pli1209bc_reg_desc,