HID: fix unit exponent parsing

HID spec details special values for the HID field unit exponent.
Basically, the range [0x8..0xf] correspond to [-8..-1], so this is
a standard two's complement on a half-byte.
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/hid-core.c

@@ -315,6 +315,7 @@ static s32 item_sdata(struct hid_item *item)
 
 static int hid_parser_global(struct hid_parser *parser, struct hid_item *item)
 {
+	__u32 raw_value;
 	switch (item->tag) {
 	case HID_GLOBAL_ITEM_TAG_PUSH:
 
@@ -365,7 +366,14 @@ static int hid_parser_global(struct hid_parser *parser, struct hid_item *item)
 		return 0;
 
 	case HID_GLOBAL_ITEM_TAG_UNIT_EXPONENT:
-		parser->global.unit_exponent = item_sdata(item);
+		/* Units exponent negative numbers are given through a
+		 * two's complement.
+		 * See "6.2.2.7 Global Items" for more information. */
+		raw_value = item_udata(item);
+		if (!(raw_value & 0xfffffff0))
+			parser->global.unit_exponent = hid_snto32(raw_value, 4);
+		else
+			parser->global.unit_exponent = raw_value;
 		return 0;
 
 	case HID_GLOBAL_ITEM_TAG_UNIT:
@@ -865,6 +873,12 @@ static s32 snto32(__u32 value, unsigned n)
 	return value & (1 << (n - 1)) ? value | (-1 << n) : value;
 }
 
+s32 hid_snto32(__u32 value, unsigned n)
+{
+	return snto32(value, n);
+}
+EXPORT_SYMBOL_GPL(hid_snto32);
+
 /*
  * Convert a signed 32-bit integer to a signed n-bit integer.
  */

drivers/hid/hid-input.c

@@ -192,7 +192,6 @@ static int hidinput_setkeycode(struct input_dev *dev,
 	return -EINVAL;
 }
 
-
 /**
  * hidinput_calc_abs_res - calculate an absolute axis resolution
  * @field: the HID report field to calculate resolution for
@@ -235,17 +234,23 @@ __s32 hidinput_calc_abs_res(const struct hid_field *field, __u16 code)
 	case ABS_MT_TOOL_Y:
 	case ABS_MT_TOUCH_MAJOR:
 	case ABS_MT_TOUCH_MINOR:
-		if (field->unit == 0x11) {		/* If centimeters */
+		if (field->unit & 0xffffff00)		/* Not a length */
+			return 0;
+		unit_exponent += hid_snto32(field->unit >> 4, 4) - 1;
+		switch (field->unit & 0xf) {
+		case 0x1:				/* If centimeters */
 			/* Convert to millimeters */
 			unit_exponent += 1;
-		} else if (field->unit == 0x13) {	/* If inches */
+			break;
+		case 0x3:				/* If inches */
 			/* Convert to millimeters */
 			prev = physical_extents;
 			physical_extents *= 254;
 			if (physical_extents < prev)
 				return 0;
 			unit_exponent -= 1;
-		} else {
+			break;
+		default:
 			return 0;
 		}
 		break;

include/linux/hid.h

@@ -717,6 +717,7 @@ int hid_connect(struct hid_device *hid, unsigned int connect_mask);
 void hid_disconnect(struct hid_device *hid);
 const struct hid_device_id *hid_match_id(struct hid_device *hdev,
 					 const struct hid_device_id *id);
+s32 hid_snto32(__u32 value, unsigned n);
 
 /**
  * hid_map_usage - map usage input bits