[PATCH v3 02/11] hwmon: (amc6821) Make reading and writing fan speed limits consistent

[Guenter Roeck <linux@roeck-us.net>]

The default value of the maximum fan speed limit register is 0,
essentially translating to an unlimited fan speed. When reading
the limit, a value of 0 is reported in this case. However, writing
a value of 0 results in writing a value of 0xffff into the register,
which is inconsistent.

To solve the problem, permit writing a limit of 0 for the maximim fan
speed, effectively translating to "no limit". Write 0 into the register
if a limit value of 0 is written. Otherwise limit the range to
<1..6000000> and write 1..0xffff into the register. This ensures that
reading and writing from and to a limit register return the same value
while at the same time not changing reported values when reading the
speed or limits.

While at it, restrict fan limit writes to non-negative numbers; writing
a negative limit does not make sense and should be reported instead of
being corrected.

Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
---
v3: Add Quentin's Reviewed-by: tag

v2: Do not accept negative fan speed values
    Display fan speed and speed limit as 0 if register value is 0
    (instead of 6000000), as in original code.
    Only permit writing 0 (unlimited) for the maximum fan speed.

 drivers/hwmon/amc6821.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/drivers/hwmon/amc6821.c b/drivers/hwmon/amc6821.c
index dc35e9b21f91..8e3cc33d8073 100644
--- a/drivers/hwmon/amc6821.c
+++ b/drivers/hwmon/amc6821.c
@@ -616,15 +616,20 @@ static ssize_t fan_store(struct device *dev, struct device_attribute *attr,
 {
 	struct amc6821_data *data = dev_get_drvdata(dev);
 	struct i2c_client *client = data->client;
-	long val;
+	unsigned long val;
 	int ix = to_sensor_dev_attr(attr)->index;
-	int ret = kstrtol(buf, 10, &val);
+	int ret = kstrtoul(buf, 10, &val);
 	if (ret)
 		return ret;
-	val = 1 > val ? 0xFFFF : 6000000/val;
+
+	/* The minimum fan speed must not be unlimited (0) */
+	if (ix == IDX_FAN1_MIN && !val)
+		return -EINVAL;
+
+	val = val > 0 ? 6000000 / clamp_val(val, 1, 6000000) : 0;
 
 	mutex_lock(&data->update_lock);
-	data->fan[ix] = (u16) clamp_val(val, 1, 0xFFFF);
+	data->fan[ix] = clamp_val(val, 0, 0xFFFF);
 	if (i2c_smbus_write_byte_data(client, fan_reg_low[ix],
 			data->fan[ix] & 0xFF)) {
 		dev_err(&client->dev, "Register write error, aborting.\n");
-- 
2.39.2