[PATCH v3 2/4] hwmon: (pmbus/max20830): add VOUT feedback resistor scaling support

[Alexis Czezar Torreno <alexisczezar.torreno@analog.com>]

Implement support for external voltage divider scaling using the
adi,vout-rfb1-ohms and adi,vout-rfb2-ohms device tree properties.

When the desired output voltage exceeds VREF, a resistor divider
(RFB1 and RFB2) is used to scale down the feedback voltage. The
driver reads these resistor values from device tree and applies
the scaling formula: VOUT_actual = VOUT_measured × (1 + RFB1/RFB2)

The properties are optional. If not specified, the driver assumes
no voltage divider is present and reports the raw VOUT reading.

Signed-off-by: Alexis Czezar Torreno <alexisczezar.torreno@analog.com>
---
 drivers/hwmon/pmbus/max20830.c | 59 +++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 58 insertions(+), 1 deletion(-)

diff --git a/drivers/hwmon/pmbus/max20830.c b/drivers/hwmon/pmbus/max20830.c
index cb2c23672166d641852199ca07eb716924f4f286..7a5e67bd33c0a8b84511d138ac5f5f48334a0a45 100644
--- a/drivers/hwmon/pmbus/max20830.c
+++ b/drivers/hwmon/pmbus/max20830.c
@@ -7,6 +7,7 @@
 
 #include <linux/errno.h>
 #include <linux/i2c.h>
+#include <linux/math64.h>
 #include <linux/mod_devicetable.h>
 #include <linux/module.h>
 #include <linux/string.h>
@@ -14,6 +15,50 @@
 
 #define MAX20830_IC_DEVICE_ID_LENGTH	9
 
+struct max20830_data {
+	struct pmbus_driver_info info;
+	u32 vout_rfb1;
+	u32 vout_rfb2;
+};
+
+/*
+ * MAX20830 only supports READ_VOUT for VOUT monitoring.
+ *
+ * MAX20830 uses an external resistor divider for voltage sensing:
+ * - VOUT_COMMAND and VOUT_MAX set the reference voltage at the feedback pin
+ * - READ_VOUT reports the feedback voltage, which needs to be scaled for actual
+ *   output voltage
+ *
+ * Scaling formula: vout_actual = vout_fb × (1 + RFB1 / RFB2)
+ *
+ * If regulator support is added in the future, some adjustments are needed to
+ * ensure correct feedback voltages are set.
+ */
+static int max20830_read_word_data(struct i2c_client *client, int page,
+				   int phase, int reg)
+{
+	const struct pmbus_driver_info *info = pmbus_get_driver_info(client);
+	const struct max20830_data *data = container_of(info, struct max20830_data, info);
+	int ret;
+
+	switch (reg) {
+	case PMBUS_READ_VOUT:
+		ret = pmbus_read_word_data(client, page, phase, reg);
+		if (ret < 0)
+			return ret;
+
+		/* Apply voltage divider scaling if resistors are non-zero */
+		if (data->vout_rfb1 && data->vout_rfb2) {
+			ret = DIV_ROUND_CLOSEST_ULL((u64)ret * (data->vout_rfb1 +
+						    data->vout_rfb2), data->vout_rfb2);
+			ret = clamp_val(ret, 0, 0xFFFF);
+		}
+		return ret;
+	default:
+		return -ENODATA;
+	}
+}
+
 static struct pmbus_driver_info max20830_info = {
 	.pages = 1,
 	.format[PSC_VOLTAGE_IN] = linear,
@@ -24,13 +69,25 @@ static struct pmbus_driver_info max20830_info = {
 		PMBUS_HAVE_TEMP |
 		PMBUS_HAVE_STATUS_VOUT | PMBUS_HAVE_STATUS_IOUT |
 		PMBUS_HAVE_STATUS_INPUT | PMBUS_HAVE_STATUS_TEMP,
+	.read_word_data = max20830_read_word_data,
 };
 
 static int max20830_probe(struct i2c_client *client)
 {
 	u8 buf[I2C_SMBUS_BLOCK_MAX + 1] = {};
+	struct max20830_data *data;
 	int ret;
 
+	data = devm_kzalloc(&client->dev, sizeof(*data), GFP_KERNEL);
+	if (!data)
+		return -ENOMEM;
+
+	data->info = max20830_info;
+
+	/* Read optional voltage divider resistor values */
+	device_property_read_u32(&client->dev, "adi,vout-rfb1-ohms", &data->vout_rfb1);
+	device_property_read_u32(&client->dev, "adi,vout-rfb2-ohms", &data->vout_rfb2);
+
 	if (!i2c_check_functionality(client->adapter, I2C_FUNC_SMBUS_READ_BLOCK_DATA) &&
 	    !i2c_check_functionality(client->adapter, I2C_FUNC_SMBUS_READ_I2C_BLOCK))
 		return -ENODEV;
@@ -78,7 +135,7 @@ static int max20830_probe(struct i2c_client *client)
 		return dev_err_probe(&client->dev, -ENODEV,
 				     "Unsupported device: '%s'\n", buf);
 
-	return pmbus_do_probe(client, &max20830_info);
+	return pmbus_do_probe(client, &data->info);
 }
 
 static const struct i2c_device_id max20830_id[] = {

-- 
2.34.1