hwmon: (max6639) Allow setting target RPM

Currently, during startup, the fan is set to its maximum RPM by default,
which may not be suitable for all use cases.
This patch introduces support for specifying a target RPM via the Device
Tree property "target-rpm".

Changes:
- Added `target_rpm` field to `max6639_data` structure to store the
  target RPM for each fan channel.
- Modified `max6639_probe_child_from_dt()` to read the `"target-rpm"`
  property from the Device Tree and set `target_rpm` accordingly.
- Updated `max6639_init_client()` to use `target_rpm` to compute the
  initial PWM duty cycle instead of defaulting to full speed (120/120).

Behavior:
- If `"target-rpm"` is specified, the fan speed is set accordingly.
- If `"target-rpm"` is not specified, the previous behavior (full speed
  at startup) is retained.

This allows better control over fan speed during system initialization.

Signed-off-by: Naresh Solanki <naresh.solanki@9elements.com>
Link: https://lore.kernel.org/r/20250404115646.2000563-1-you@example.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Author: nsolanki22

drivers/hwmon/max6639.c

@@ -80,6 +80,7 @@ struct max6639_data {
 	/* Register values initialized only once */
 	u8 ppr[MAX6639_NUM_CHANNELS];	/* Pulses per rotation 0..3 for 1..4 ppr */
 	u8 rpm_range[MAX6639_NUM_CHANNELS]; /* Index in above rpm_ranges table */
+	u32 target_rpm[MAX6639_NUM_CHANNELS];
 
 	/* Optional regulator for FAN supply */
 	struct regulator *reg;
@@ -563,6 +564,10 @@ static int max6639_probe_child_from_dt(struct i2c_client *client,
 	if (!err)
 		data->rpm_range[i] = rpm_range_to_reg(val);
 
+	err = of_property_read_u32(child, "target-rpm", &val);
+	if (!err)
+		data->target_rpm[i] = val;
+
 	return 0;
 }
 
@@ -573,6 +578,7 @@ static int max6639_init_client(struct i2c_client *client,
 	const struct device_node *np = dev->of_node;
 	struct device_node *child;
 	int i, err;
+	u8 target_duty;
 
 	/* Reset chip to default values, see below for GCONFIG setup */
 	err = regmap_write(data->regmap, MAX6639_REG_GCONFIG, MAX6639_GCONFIG_POR);
@@ -586,6 +592,8 @@ static int max6639_init_client(struct i2c_client *client,
 	/* default: 4000 RPM */
 	data->rpm_range[0] = 1;
 	data->rpm_range[1] = 1;
+	data->target_rpm[0] = 4000;
+	data->target_rpm[1] = 4000;
 
 	for_each_child_of_node(np, child) {
 		if (strcmp(child->name, "fan"))
@@ -639,8 +647,12 @@ static int max6639_init_client(struct i2c_client *client,
 		if (err)
 			return err;
 
-		/* PWM 120/120 (i.e. 100%) */
-		err = regmap_write(data->regmap, MAX6639_REG_TARGTDUTY(i), 120);
+		/* Set PWM based on target RPM if specified */
+		if (data->target_rpm[i] >  rpm_ranges[data->rpm_range[i]])
+			data->target_rpm[i] = rpm_ranges[data->rpm_range[i]];
+
+		target_duty = 120 * data->target_rpm[i] / rpm_ranges[data->rpm_range[i]];
+		err = regmap_write(data->regmap, MAX6639_REG_TARGTDUTY(i), target_duty);
 		if (err)
 			return err;
 	}