hwmon: applesmc: fix light sensor readings on newer MacBooks

The light sensors ALV0 and ALV1 on newer MacBooks (early 2008 and later)
changed to report 10 bytes instead the earlier 6, and the sensor encoding
subsequently changed.  As a result, the reported light sensors readings
are much too low.

Via experiments leading up to this patch, it seems only the ALV0 is
reporting data, and the most useful value therein is a 10-bit big-endian
value at offset 6.  This suggests that a new protocol was added as a
backward-compatible replacement on top of the old one.

This patch makes applesmc report the improved light sensor reading for the
new machines, on a scale in conformance with earlier ones.

Signed-off-by: Alex Murray <murray.alex@gmail.com>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Cc: Nicolas Boichat <nicolas@boichat.ch>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/hwmon/applesmc.c

@@ -590,6 +590,11 @@ static ssize_t applesmc_light_show(struct device *dev,
 	}
 
 	ret = applesmc_read_key(LIGHT_SENSOR_LEFT_KEY, buffer, data_length);
+	/* newer macbooks report a single 10-bit bigendian value */
+	if (data_length == 10) {
+		left = be16_to_cpu(*(__be16 *)(buffer + 6)) >> 2;
+		goto out;
+	}
 	left = buffer[2];
 	if (ret)
 		goto out;