3 files changed, 25 insertions, 5 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.
  */
diff --git a/drivers/hid/hid-input.c b/drivers/hid/hid-input.c
index 1b0adc3f7803..007a9433bfa7 100644
--- a/drivers/hid/hid-input.c
+++ b/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;
diff --git a/include/linux/hid.h b/include/linux/hid.h
index c6bef8f54a82..4161bf2c0b5b 100644
--- a/include/linux/hid.h
+++ b/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