linux-kernel - [PATCH] hwmon: (tmp513) Fix division of negative numbers

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Message-Id: <20250114-fix-si-prefix-macro-sign-bugs-v1-1-696fd8d10f00@baylibre.com>
Date: Tue, 14 Jan 2025 15:45:52 -0600
From: David Lechner <dlechner@...libre.com>
To: Eric Tremblay <etremblay@...tech-controls.com>, 
 Jean Delvare <jdelvare@...e.com>, Guenter Roeck <linux@...ck-us.net>, 
 Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org, 
 David Lechner <dlechner@...libre.com>
Subject: [PATCH] hwmon: (tmp513) Fix division of negative numbers

Fix several issues with division of negative numbers in the tmp513
driver.

The docs on the DIV_ROUND_CLOSEST macro explain that dividing a negative
value by an unsigned type is undefined behavior. The driver was doing
this in several places, i.e. data->shunt_uohms has type of u32. The
actual "undefined" behavior is that it converts both values to unsigned
before doing the division, for example:

    int ret = DIV_ROUND_CLOSEST(-100, 3U);

results in ret == 1431655732 instead of -33.

Furthermore the MILLI macro has a type of unsigned long. Multiplying a
signed long by an unsigned long results in an unsigned long.

So, we need to cast both MILLI and data data->shunt_uohms to long when
using the DIV_ROUND_CLOSEST macro.

Fixes: f07f9d2467f4 ("hwmon: (tmp513) Use SI constants from units.h")
Fixes: 59dfa75e5d82 ("hwmon: Add driver for Texas Instruments TMP512/513 sensor chips.")
Signed-off-by: David Lechner <dlechner@...libre.com>
---
 drivers/hwmon/tmp513.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/hwmon/tmp513.c b/drivers/hwmon/tmp513.c
index 1c2cb12071b80866b751b71bf39292580cd47929..1c7601de47d0720352d729e35a5b7eeaf109355f 100644
--- a/drivers/hwmon/tmp513.c
+++ b/drivers/hwmon/tmp513.c
@@ -207,7 +207,9 @@ static int tmp51x_get_value(struct tmp51x_data *data, u8 reg, u8 pos,
 		*val = sign_extend32(regval,
 				     reg == TMP51X_SHUNT_CURRENT_RESULT ?
 				     16 - tmp51x_get_pga_shift(data) : 15);
-		*val = DIV_ROUND_CLOSEST(*val * 10 * MILLI, data->shunt_uohms);
+		*val = DIV_ROUND_CLOSEST(*val * 10 * (long)MILLI,
+					 (long)data->shunt_uohms);
+
 		break;
 	case TMP51X_BUS_VOLTAGE_RESULT:
 	case TMP51X_BUS_VOLTAGE_H_LIMIT:
@@ -223,7 +225,7 @@ static int tmp51x_get_value(struct tmp51x_data *data, u8 reg, u8 pos,
 	case TMP51X_BUS_CURRENT_RESULT:
 		// Current = (ShuntVoltage * CalibrationRegister) / 4096
 		*val = sign_extend32(regval, 15) * (long)data->curr_lsb_ua;
-		*val = DIV_ROUND_CLOSEST(*val, MILLI);
+		*val = DIV_ROUND_CLOSEST(*val, (long)MILLI);
 		break;
 	case TMP51X_LOCAL_TEMP_RESULT:
 	case TMP51X_REMOTE_TEMP_RESULT_1:
@@ -263,7 +265,8 @@ static int tmp51x_set_value(struct tmp51x_data *data, u8 reg, long val)
 		 * The user enter current value and we convert it to
 		 * voltage. 1lsb = 10uV
 		 */
-		val = DIV_ROUND_CLOSEST(val * data->shunt_uohms, 10 * MILLI);
+		val = DIV_ROUND_CLOSEST(val * (long)data->shunt_uohms,
+					10 * (long)MILLI);
 		max_val = U16_MAX >> tmp51x_get_pga_shift(data);
 		regval = clamp_val(val, -max_val, max_val);
 		break;

---
base-commit: dab2734f8e9ecba609d66d1dd087a392a7774c04
change-id: 20250114-fix-si-prefix-macro-sign-bugs-307673e6ea45

Best regards,
-- 
David Lechner <dlechner@...libre.com>