hwmon: (max34440) Add support for 'lowest' output voltage attribute

MAX34440 and compatibles support reporting the lowest measured output voltage.
Add support for it.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Jean Delvare <khali@linux-fr.org>

Documentation/hwmon/max34440

@@ -56,6 +56,7 @@ in[1-6]_min_alarm	Voltage low alarm. From VOLTAGE_UV_WARNING status.
 in[1-6]_max_alarm	Voltage high alarm. From VOLTAGE_OV_WARNING status.
 in[1-6]_lcrit_alarm	Voltage critical low alarm. From VOLTAGE_UV_FAULT status.
 in[1-6]_crit_alarm	Voltage critical high alarm. From VOLTAGE_OV_FAULT status.
+in[1-6]_lowest		Historical minimum voltage.
 in[1-6]_highest		Historical maximum voltage.
 in[1-6]_reset_history	Write any value to reset history.
 

drivers/hwmon/pmbus/max34440.c

@@ -30,6 +30,7 @@ enum chips { max34440, max34441 };
 #define MAX34440_MFR_VOUT_PEAK		0xd4
 #define MAX34440_MFR_IOUT_PEAK		0xd5
 #define MAX34440_MFR_TEMPERATURE_PEAK	0xd6
+#define MAX34440_MFR_VOUT_MIN		0xd7
 
 #define MAX34440_STATUS_OC_WARN		(1 << 0)
 #define MAX34440_STATUS_OC_FAULT	(1 << 1)
@@ -41,6 +42,10 @@ static int max34440_read_word_data(struct i2c_client *client, int page, int reg)
 	int ret;
 
 	switch (reg) {
+	case PMBUS_VIRT_READ_VOUT_MIN:
+		ret = pmbus_read_word_data(client, page,
+					   MAX34440_MFR_VOUT_MIN);
+		break;
 	case PMBUS_VIRT_READ_VOUT_MAX:
 		ret = pmbus_read_word_data(client, page,
 					   MAX34440_MFR_VOUT_PEAK);
@@ -72,6 +77,10 @@ static int max34440_write_word_data(struct i2c_client *client, int page,
 
 	switch (reg) {
 	case PMBUS_VIRT_RESET_VOUT_HISTORY:
+		ret = pmbus_write_word_data(client, page,
+					    MAX34440_MFR_VOUT_MIN, 0x7fff);
+		if (ret)
+			break;
 		ret = pmbus_write_word_data(client, page,
 					    MAX34440_MFR_VOUT_PEAK, 0);
 		break;