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From:   Ivan Zaentsev <ivan.zaentsev@wirenboard.ru>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Evgeniy Polyakov <zbr@ioremap.net>,
        Akira Shimahara <akira215corp@gmail.com>,
        Ivan Zaentsev <ivan.zaentsev@wirenboard.ru>,
        Dan Carpenter <dan.carpenter@oracle.com>,
        Evgeny Boger <boger@wirenboard.com>,
        Rikard Falkeborn <rikard.falkeborn@gmail.com>,
        linux-kernel@vger.kernel.org,
        =?UTF-8?q?Pawe=C5=82=20Marciniak?= <sunwire@gmail.com>
Subject: [PATCH] w1: w1_therm: Fix conversion result for negative temperatures
Date:   Thu, 21 Jan 2021 12:30:21 +0300
Message-Id: <20210121093021.224764-1-ivan.zaentsev@wirenboard.ru>
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DS18B20 device driver returns an incorrect value for negative temperatures
due to a missing sign-extension in w1_DS18B20_convert_temp().

Fix by using s16 temperature value when converting to int.

Fixes: 9ace0b4dab1c (w1: w1_therm: Add support for GXCAS GX20MH01 device.)
Reported-by: Paweł Marciniak <sunwire@gmail.com>
Signed-off-by: Ivan Zaentsev <ivan.zaentsev@wirenboard.ru>
---
 drivers/w1/slaves/w1_therm.c | 22 +++++++++-------------
 1 file changed, 9 insertions(+), 13 deletions(-)

diff --git a/drivers/w1/slaves/w1_therm.c b/drivers/w1/slaves/w1_therm.c
index 3712b1e6dc71..976eea28f268 100644
--- a/drivers/w1/slaves/w1_therm.c
+++ b/drivers/w1/slaves/w1_therm.c
@@ -667,28 +667,24 @@ static inline int w1_DS18B20_get_resolution(struct w1_slave *sl)
  */
 static inline int w1_DS18B20_convert_temp(u8 rom[9])
 {
-	int t;
-	u32 bv;
+	u16 bv;
+	s16 t;
+
+	/* Signed 16-bit value to unsigned, cpu order */
+	bv = le16_to_cpup((__le16 *)rom);
 
 	/* Config register bit R2 = 1 - GX20MH01 in 13 or 14 bit resolution mode */
 	if (rom[4] & 0x80) {
-		/* Signed 16-bit value to unsigned, cpu order */
-		bv = le16_to_cpup((__le16 *)rom);
-
 		/* Insert two temperature bits from config register */
 		/* Avoid arithmetic shift of signed value */
 		bv = (bv << 2) | (rom[4] & 3);
-
-		t = (int) sign_extend32(bv, 17); /* Degrees, lowest bit is 2^-6 */
-		return (t*1000)/64;  /* Millidegrees */
+		t = (s16) bv;	/* Degrees, lowest bit is 2^-6 */
+		return (int)t * 1000 / 64;	/* Sign-extend to int; millidegrees */
 	}
-
-	t = (int)le16_to_cpup((__le16 *)rom);
-	return t*1000/16;
+	t = (s16)bv;	/* Degrees, lowest bit is 2^-4 */
+	return (int)t * 1000 / 16;	/* Sign-extend to int; millidegrees */
 }
 
-
-
 /**
  * w1_DS18S20_convert_temp() - temperature computation for DS18S20
  * @rom: data read from device RAM (8 data bytes + 1 CRC byte)
-- 
2.25.1