hwmon: (applesmc) Allow negative temperature values

There are many userland reports of sensors with unreasonably small and
large temperatures. There seem to be several reasons for this:

Firstly, the major sensor type (sp78) is actually a signed number.
This explains why some sensors show very small or large values - they
are in fact all small, but of different sign.

Secondly, the other sensor type (1-hex) is not properly understood; it
may be that it is not a temperature after all.

Thirdly, some sensors are differential in nature, showing changes over
time rather than absolute numbers.  This explains why those values are
small and of varying sign.

This patch interprets the sp78 type as signed short, but keeps the
original scaling. For other types, -EINVAL is returned, since the
nature of those sensors is unknown.

Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/applesmc.c

@@ -80,6 +80,8 @@
 #define FANS_MANUAL		"FS! " /* r-w ui16 */
 #define FAN_ID_FMT		"F%dID" /* r-o char[16] */
 
+#define TEMP_SENSOR_TYPE	"sp78"
+
 /* List of keys used to read/write fan speeds */
 static const char *const fan_speed_fmt[] = {
 	"F%dAc",		/* actual speed */
@@ -720,27 +722,22 @@ static ssize_t applesmc_show_temperature(struct device *dev,
 	int index = smcreg.temp_begin + to_index(devattr);
 	const struct applesmc_entry *entry;
 	int ret;
-	u8 buffer[2];
-	unsigned int temp;
+	s16 value;
+	int temp;
 
 	entry = applesmc_get_entry_by_index(index);
 	if (IS_ERR(entry))
 		return PTR_ERR(entry);
-	if (entry->len > 2)
+	if (strcmp(entry->type, TEMP_SENSOR_TYPE))
 		return -EINVAL;
 
-	ret = applesmc_read_entry(entry, buffer, entry->len);
+	ret = applesmc_read_s16(entry->key, &value);
 	if (ret)
 		return ret;
 
-	if (entry->len == 2) {
-		temp = buffer[0] * 1000;
-		temp += (buffer[1] >> 6) * 250;
-	} else {
-		temp = buffer[0] * 4000;
-	}
+	temp = 250 * (value >> 6);
 
-	return snprintf(sysfsbuf, PAGE_SIZE, "%u\n", temp);
+	return snprintf(sysfsbuf, PAGE_SIZE, "%d\n", temp);
 }
 
 static ssize_t applesmc_show_fan_speed(struct device *dev,