hwmon: (pmbus) Fix vout margin caching

The code currently uses a zero margin to mean not cached, but this
results in the cache being bypassed if the (low) margin is set to zero,
leading to lots of unnecessary SMBus transactions in that case.  Use a
negative value instead.

Fixes: 07fb7627 ("hwmon: (pmbus) Introduce and use cached vout margins")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Link: https://lore.kernel.org/r/20220816144414.2358974-1-vincent.whitchurch@axis.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/pmbus/pmbus_core.c

@@ -2861,7 +2861,7 @@ static int pmbus_regulator_get_low_margin(struct i2c_client *client, int page)
 		.data = -1,
 	};
 
-	if (!data->vout_low[page]) {
+	if (data->vout_low[page] < 0) {
 		if (pmbus_check_word_register(client, page, PMBUS_MFR_VOUT_MIN))
 			s.data = _pmbus_read_word_data(client, page, 0xff,
 						       PMBUS_MFR_VOUT_MIN);
@@ -2887,7 +2887,7 @@ static int pmbus_regulator_get_high_margin(struct i2c_client *client, int page)
 		.data = -1,
 	};
 
-	if (!data->vout_high[page]) {
+	if (data->vout_high[page] < 0) {
 		if (pmbus_check_word_register(client, page, PMBUS_MFR_VOUT_MAX))
 			s.data = _pmbus_read_word_data(client, page, 0xff,
 						       PMBUS_MFR_VOUT_MAX);
@@ -3319,6 +3319,7 @@ int pmbus_do_probe(struct i2c_client *client, struct pmbus_driver_info *info)
 	struct pmbus_data *data;
 	size_t groups_num = 0;
 	int ret;
+	int i;
 	char *name;
 
 	if (!info)
@@ -3352,6 +3353,11 @@ int pmbus_do_probe(struct i2c_client *client, struct pmbus_driver_info *info)
 	data->currpage = -1;
 	data->currphase = -1;
 
+	for (i = 0; i < ARRAY_SIZE(data->vout_low); i++) {
+		data->vout_low[i] = -1;
+		data->vout_high[i] = -1;
+	}
+
 	ret = pmbus_init_common(client, data, info);
 	if (ret < 0)
 		return ret;