[PATCH] hwmon: (lm70) Fix rounding of negative temperatures

[Ridham Khurana]

temp1_input_show() drops the low bits of the temperature register by
dividing the raw value (raw / 32 on the LM70). These bits are not part
of the temperature: the LM70, LM71, LM74 and TMP122/TMP124 always
return some of them as 1, and the TMP125 fills them with copies of the
temperature LSB. Division rounds toward zero, so a negative temperature
with any of these bits set is reported one LSB too high.

For example, the TMP122 returns 0xffff for -0.0625 degrees C, which the
driver reports as 0. The LM70 returns 0xf39f for -25 degrees C, which
the driver reports as -24750.

Use an arithmetic right shift instead, which drops the low bits and
rounds down. tmp421 had a similar problem with negative values, fixed
by commit 724e8af85854 ("hwmon: (tmp421) fix rounding for negative
values").

Fixes: e1a8e913f97e ("[PATCH] lm70: New hardware monitoring driver")
Fixes: a86e94dc946d ("hwmon: (lm70) Add support for LM71 and LM74")
Fixes: cd929672a9ef ("hwmon: (lm70) Add ti,tmp125 support")
Signed-off-by: Ridham Khurana <khurana.ridham222@xxxxxxxxx>
---
Build-tested on arm64 and arm (ep93xx_defconfig) with CONFIG_SENSORS_LM70=m. Tested in QEMU (arm64 and arm) with a stub SPI controller returning the datasheet register values; not tested on hardware.

 drivers/hwmon/lm70.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/hwmon/lm70.c b/drivers/hwmon/lm70.c
index 8b525725fef4..b29bac7a4cc1 100644
--- a/drivers/hwmon/lm70.c
+++ b/drivers/hwmon/lm70.c
@@ -96,21 +96,21 @@ static ssize_t temp1_input_show(struct device *dev,
 	 */
 	switch (p_lm70->chip) {
 	case LM70_CHIP_LM70:
-		val = ((int)raw / 32) * 250;
+		val = ((int)raw >> 5) * 250;
 		break;
 
 	case LM70_CHIP_TMP121:
 	case LM70_CHIP_TMP122:
 	case LM70_CHIP_LM74:
-		val = ((int)raw / 8) * 625 / 10;
+		val = ((int)raw >> 3) * 625 / 10;
 		break;
 
 	case LM70_CHIP_LM71:
-		val = ((int)raw / 4) * 3125 / 100;
+		val = ((int)raw >> 2) * 3125 / 100;
 		break;
 
 	case LM70_CHIP_TMP125:
-		val = (sign_extend32(raw, 14) / 32) * 250;
+		val = (sign_extend32(raw, 14) >> 5) * 250;
 		break;
 	}
 
-- 
2.47.3