linux-kernel - [PATCH] hwmon: (ltc2991) Fix mixed signed/unsigned in DIV_ROUND

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Message-Id: <20250115-hwmon-ltc2991-fix-div-round-closest-v1-1-b4929667e457@baylibre.com>
Date: Wed, 15 Jan 2025 14:48:27 -0600
From: David Lechner <dlechner@...libre.com>
To: Antoniu Miclaus <antoniu.miclaus@...log.com>, 
 Jean Delvare <jdelvare@...e.com>, Guenter Roeck <linux@...ck-us.net>
Cc: linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org, 
 David Lechner <dlechner@...libre.com>
Subject: [PATCH] hwmon: (ltc2991) Fix mixed signed/unsigned in
 DIV_ROUND_CLOSEST

Fix use of DIV_ROUND_CLOSEST where a possibly negative value is divided
by an unsigned type by casting the unsigned type to the signed type of
the same size (st->r_sense_uohm[channel] has type of u32).

The docs on the DIV_ROUND_CLOSEST macro explain that dividing a negative
value by an unsigned type is undefined behavior. The actual 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.

Fixes: 2b9ea4262ae9 ("hwmon: Add driver for ltc2991")
Signed-off-by: David Lechner <dlechner@...libre.com>
---
 drivers/hwmon/ltc2991.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/hwmon/ltc2991.c b/drivers/hwmon/ltc2991.c
index 7ca139e4b6aff0b6c3024183e19970b36f927f6e..6d5d4cb846daf3025e55d43cfe311f38bb353621 100644
--- a/drivers/hwmon/ltc2991.c
+++ b/drivers/hwmon/ltc2991.c
@@ -125,7 +125,7 @@ static int ltc2991_get_curr(struct ltc2991_state *st, u32 reg, int channel,
 
 	/* Vx-Vy, 19.075uV/LSB */
 	*val = DIV_ROUND_CLOSEST(sign_extend32(reg_val, 14) * 19075,
-				 st->r_sense_uohm[channel]);
+				 (s32)st->r_sense_uohm[channel]);
 
 	return 0;
 }

---
base-commit: dab2734f8e9ecba609d66d1dd087a392a7774c04
change-id: 20250115-hwmon-ltc2991-fix-div-round-closest-c65c05afb881

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