hwmon: (pmbus/max20730) adjust the vout reading given voltage divider

Problem:
We use voltage dividers so that the voltage presented at the voltage
sense pins is confusing. We might need to convert these readings to more
meaningful readings given the voltage divider.

Solution:
Read the voltage divider resistance from dts and convert the voltage
reading to a more meaningful reading.

Testing:
max20730 with voltage divider

Signed-off-by: Chu Lin <linchuyuan@google.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20201004031445.2321090-3-linchuyuan@google.com
[groeck: Return -EINVAL instead of -ENODEV on bad deevicetree data]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/pmbus/max20730.c

@@ -53,6 +53,7 @@ struct max20730_data {
 	u16 mfr_devset1;
 	u16 mfr_devset2;
 	u16 mfr_voutmin;
+	u32 vout_voltage_divider[2];
 };
 
 #define to_max20730_data(x)  container_of(x, struct max20730_data, info)
@@ -468,6 +469,14 @@ static int max20730_read_word_data(struct i2c_client *client, int page,
 		max_c = max_current[data->id][(data->mfr_devset1 >> 5) & 0x3];
 		ret = val_to_direct(max_c, PSC_CURRENT_OUT, info);
 		break;
+	case PMBUS_READ_VOUT:
+		ret = pmbus_read_word_data(client, page, phase, reg);
+		if (ret > 0 && data->vout_voltage_divider[0] && data->vout_voltage_divider[1]) {
+			u64 temp = DIV_ROUND_CLOSEST_ULL((u64)ret * data->vout_voltage_divider[1],
+							 data->vout_voltage_divider[0]);
+			ret = clamp_val(temp, 0, 0xffff);
+		}
+		break;
 	default:
 		ret = -ENODATA;
 		break;
@@ -717,6 +726,15 @@ static int max20730_probe(struct i2c_client *client)
 	data->id = chip_id;
 	mutex_init(&data->lock);
 	memcpy(&data->info, &max20730_info[chip_id], sizeof(data->info));
+	if (of_property_read_u32_array(client->dev.of_node, "vout-voltage-divider",
+				       data->vout_voltage_divider,
+				       ARRAY_SIZE(data->vout_voltage_divider)) != 0)
+		memset(data->vout_voltage_divider, 0, sizeof(data->vout_voltage_divider));
+	if (data->vout_voltage_divider[1] < data->vout_voltage_divider[0]) {
+		dev_err(dev,
+			"The total resistance of voltage divider is less than output resistance\n");
+		return -EINVAL;
+	}
 
 	ret = i2c_smbus_read_word_data(client, MAX20730_MFR_DEVSET1);
 	if (ret < 0)