hwmon: (pmbus) Add new flag PMBUS_READ_STATUS_AFTER_FAILED_CHECK

Some PMBus chips end up in an undefined state when trying to read an
unsupported register. For such chips, it is necessary to reset the
chip pmbus controller to a known state after a failed register check.
This can be done by reading a known register. By setting this flag the
driver will try to read the STATUS register after each failed
register check. This read may fail, but it will put the chip into a
known state.
Signed-off-by: Erik Rosen <erik.rosen@metormote.com>
Link: https://lore.kernel.org/r/20210507194023.61138-2-erik.rosen@metormote.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/pmbus/pmbus_core.c

@@ -523,6 +523,8 @@ static bool pmbus_check_register(struct i2c_client *client,
 	rv = func(client, page, reg);
 	if (rv >= 0 && !(data->flags & PMBUS_SKIP_STATUS_CHECK))
 		rv = pmbus_check_status_cml(client);
+	if (rv < 0 && (data->flags & PMBUS_READ_STATUS_AFTER_FAILED_CHECK))
+		data->read_status(client, -1);
 	pmbus_clear_fault_page(client, -1);
 	return rv >= 0;
 }

include/linux/pmbus.h

@@ -43,6 +43,19 @@
  */
 #define PMBUS_NO_CAPABILITY			BIT(2)
 
+/*
+ * PMBUS_READ_STATUS_AFTER_FAILED_CHECK
+ *
+ * Some PMBus chips end up in an undefined state when trying to read an
+ * unsupported register. For such chips, it is necessary to reset the
+ * chip pmbus controller to a known state after a failed register check.
+ * This can be done by reading a known register. By setting this flag the
+ * driver will try to read the STATUS register after each failed
+ * register check. This read may fail, but it will put the chip in a
+ * known state.
+ */
+#define PMBUS_READ_STATUS_AFTER_FAILED_CHECK	BIT(3)
+
 struct pmbus_platform_data {
 	u32 flags;		/* Device specific flags */