w1: w1_therm: Fix conversion result for negative temperatures

commit 2f6055c upstream.

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.

See: raspberrypi/linux#4124

Fixes: 9ace0b4 (w1: w1_therm: Add support for GXCAS GX20MH01 device.)
Cc: stable <stable@vger.kernel.org>
Reported-by: Paweł Marciniak <sunwire@gmail.com>
Signed-off-by: Ivan Zaentsev <ivan.zaentsev@wirenboard.ru>
Link: https://lore.kernel.org/r/20210121093021.224764-1-ivan.zaentsev@wirenboard.ru
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Fang Yafen <yafen@iscas.ac.cn>

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)