[PATCH] hwmon: (max6621) fix negative temperature offset and crit readings

[Cong Nguyen <congnt264@gmail.com>]

max6621_read() reads the temperature input, offset and critical alert
registers into a u32 and assigns them to the output without sign
extension. The device reports these values in two's complement (the
driver comment and the write path, which clamps to
MAX6621_TEMP_INPUT_MIN == -127000 and encodes negatives, both rely on
this), so negative values are misreported as large positive numbers:

  - temp_offset: *val = (regval >> MAX6621_REG_TEMP_SHIFT) * 1000, an
    unsigned shift, so e.g. a -10 degrees C offset (register 0xfd80)
    reads back as ~+1014000 millidegrees.
  - temp_crit: *val = regval * 1000, so a negative critical threshold
    reads back as a large positive value.
  - temp_input used an s8 intermediate, which is correct for the
    -127..127 range but reports the documented +128 degrees C maximum as
    -128 degrees C.

The registers are 16-bit (the driver's own PECI error codes occupy
0x8000-0x80ff), so sign-extend from bit 15 before scaling. This fixes
all three reads and drops the now-unused s8 intermediate.

Fixes: 92b64580f14b ("hwmon: (max6621) Add support for Maxim MAX6621 temperature sensor")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4
Signed-off-by: Cong Nguyen <congnt264@gmail.com>
---
 drivers/hwmon/max6621.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/drivers/hwmon/max6621.c b/drivers/hwmon/max6621.c
index a7066f3a0bb4..03ac3eb5a594 100644
--- a/drivers/hwmon/max6621.c
+++ b/drivers/hwmon/max6621.c
@@ -204,7 +204,6 @@ max6621_read(struct device *dev, enum hwmon_sensor_types type, u32 attr,
 	struct max6621_data *data = dev_get_drvdata(dev);
 	u32 regval;
 	int reg;
-	s8 temp;
 	int ret;
 
 	switch (type) {
@@ -225,8 +224,8 @@ max6621_read(struct device *dev, enum hwmon_sensor_types type, u32 attr,
 			 * The temperature is given in two's complement and 8
 			 * bits is used for the register conversion.
 			 */
-			temp = (regval >> MAX6621_REG_TEMP_SHIFT);
-			*val = temp * 1000L;
+			*val = (sign_extend32(regval, 15) >>
+				MAX6621_REG_TEMP_SHIFT) * 1000L;
 
 			break;
 		case hwmon_temp_offset:
@@ -239,8 +238,8 @@ max6621_read(struct device *dev, enum hwmon_sensor_types type, u32 attr,
 			if (ret)
 				return ret;
 
-			*val = (regval >> MAX6621_REG_TEMP_SHIFT) *
-			       1000L;
+			*val = (sign_extend32(regval, 15) >>
+				MAX6621_REG_TEMP_SHIFT) * 1000L;
 
 			break;
 		case hwmon_temp_crit:
@@ -254,7 +253,7 @@ max6621_read(struct device *dev, enum hwmon_sensor_types type, u32 attr,
 			if (ret)
 				return ret;
 
-			*val = regval * 1000L;
+			*val = sign_extend32(regval, 15) * 1000L;
 
 			break;
 		case hwmon_temp_crit_alarm:
-- 
2.25.1