[PATCH v2 1/2] hwmon: (max6621) fix temperature clamp range

[PATCH v2 1/2] hwmon: (max6621) fix temperature clamp range

[Cong Nguyen]

MAX6621_TEMP_INPUT_MIN and MAX6621_TEMP_INPUT_MAX are used to clamp the
writable offset and critical thresholds. They are defined as -127000 and
128000.

The driver decodes the temperature through an s8 and its own comment in
max6621_read() documents an 8-bit two's complement value, whose range is
-128 to +127 degrees C. The current limits therefore reject the valid
-128 degrees C and accept +128 degrees C, which does not fit the 8-bit
range.

Correct the limits to -128000 and 127000.

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>
---
Changes in v2:
 - Drop the temperature input change; it was not a bug (temp_input already
   sign-extends correctly via an s8 intermediate).
 - Drop the incorrect changelog reasoning (no "+128 degC", no PECI/16-bit).
 - Split into two patches per review: this one fixes the clamp range; 2/2
   fixes the negative offset/crit reads.
 - No 1/64 degC precision change (not documented in the datasheet).

 drivers/hwmon/max6621.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/hwmon/max6621.c b/drivers/hwmon/max6621.c
index a7066f3a0bb4..ee5898fbe110 100644
--- a/drivers/hwmon/max6621.c
+++ b/drivers/hwmon/max6621.c
@@ -17,8 +17,8 @@
 
 #define MAX6621_DRV_NAME		"max6621"
 #define MAX6621_TEMP_INPUT_REG_NUM	9
-#define MAX6621_TEMP_INPUT_MIN		-127000
-#define MAX6621_TEMP_INPUT_MAX		128000
+#define MAX6621_TEMP_INPUT_MIN		-128000
+#define MAX6621_TEMP_INPUT_MAX		127000
 #define MAX6621_TEMP_ALERT_CHAN_SHIFT	1
 
 #define MAX6621_TEMP_S0D0_REG		0x00
-- 
2.25.1

[PATCH v2 2/2] hwmon: (max6621) fix negative temperature offset and crit readings

[Cong Nguyen]

max6621_read() reads the CONFIG2 offset and the critical alert threshold
registers into a u32 and scales them without sign extension:

	/* offset */ *val = (regval >> MAX6621_REG_TEMP_SHIFT) * 1000L;
	/* crit   */ *val = regval * 1000L;

Both attributes are writable and their write paths clamp to a negative
minimum and encode negative values, so a value written as negative is read
back as a large positive number. For example, writing a -10 degrees C
offset stores max6621_temp_mc2reg(-10000) = (-10 << 6) = 0xfd80; the read
then computes 0xfd80 >> 6 = 1014 -> 1014000 instead of -10000.

Cast the register value to s16 before scaling so the read preserves the
sign the write path encodes. The temperature input path already uses an s8
intermediate and is left unchanged.

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 | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/hwmon/max6621.c b/drivers/hwmon/max6621.c
index ee5898fbe110..3690f2ffe5fc 100644
--- a/drivers/hwmon/max6621.c
+++ b/drivers/hwmon/max6621.c
@@ -239,7 +239,7 @@ max6621_read(struct device *dev, enum hwmon_sensor_types type, u32 attr,
 			if (ret)
 				return ret;
 
-			*val = (regval >> MAX6621_REG_TEMP_SHIFT) *
+			*val = ((s16)regval >> MAX6621_REG_TEMP_SHIFT) *
 			       1000L;
 
 			break;
@@ -254,7 +254,7 @@ max6621_read(struct device *dev, enum hwmon_sensor_types type, u32 attr,
 			if (ret)
 				return ret;
 
-			*val = regval * 1000L;
+			*val = (s16)regval * 1000L;
 
 			break;
 		case hwmon_temp_crit_alarm:
-- 
2.25.1

Re: [PATCH v2 1/2] hwmon: (max6621) fix temperature clamp range

[Guenter Roeck]

On Mon, Aug 10, 2026 at 11:27:54AM +0700, Cong Nguyen wrote:
> MAX6621_TEMP_INPUT_MIN and MAX6621_TEMP_INPUT_MAX are used to clamp the
> writable offset and critical thresholds. They are defined as -127000 and
> 128000.
> 
> The driver decodes the temperature through an s8 and its own comment in
> max6621_read() documents an 8-bit two's complement value, whose range is
> -128 to +127 degrees C. The current limits therefore reject the valid
> -128 degrees C and accept +128 degrees C, which does not fit the 8-bit
> range.
> 
> Correct the limits to -128000 and 127000.
> 
> 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>

Applied.

Thanks,
Guenter

Re: [PATCH v2 2/2] hwmon: (max6621) fix negative temperature offset and crit readings

[Guenter Roeck]

On Mon, Aug 10, 2026 at 11:28:39AM +0700, Cong Nguyen wrote:
> max6621_read() reads the CONFIG2 offset and the critical alert threshold
> registers into a u32 and scales them without sign extension:
> 
> 	/* offset */ *val = (regval >> MAX6621_REG_TEMP_SHIFT) * 1000L;
> 	/* crit   */ *val = regval * 1000L;
> 
> Both attributes are writable and their write paths clamp to a negative
> minimum and encode negative values, so a value written as negative is read
> back as a large positive number. For example, writing a -10 degrees C
> offset stores max6621_temp_mc2reg(-10000) = (-10 << 6) = 0xfd80; the read
> then computes 0xfd80 >> 6 = 1014 -> 1014000 instead of -10000.
> 
> Cast the register value to s16 before scaling so the read preserves the
> sign the write path encodes. The temperature input path already uses an s8
> intermediate and is left unchanged.
> 
> 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>

Applied.

Thanks,
Guenter