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>

1 files changed, 15 insertions, 1 deletions

diff --git a/drivers/hid/hid-core.c b/drivers/hid/hid-core.c
index f4109fd657ff..f5004e2ca0c5 100644
--- a/drivers/hid/hid-core.c
+++ b/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.
  */