HID: multitouch: do not use pointers towards hid-core

The previous implementation registered a pointer towards hid-core
to the value of contact count. This is not safe and may be difficult
to debug if hid-core ever changes its implementation.
The use of regular indexes is a better choice.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/hid-multitouch.c

@@ -84,7 +84,8 @@ struct mt_device {
 	struct mt_class mtclass;	/* our mt device class */
 	struct mt_fields *fields;	/* temporary placeholder for storing the
 					   multitouch fields */
-	__s32 *contactcount;		/* contact count value in the report */
+	int cc_index;	/* contact count field index in the report */
+	int cc_value_index;	/* contact count value index in the field */
 	unsigned last_field_index;	/* last field index of the report */
 	unsigned last_slot_field;	/* the last field of a slot */
 	unsigned mt_report_id;	/* the report ID of the multitouch device */
@@ -269,7 +270,7 @@ static ssize_t mt_set_quirks(struct device *dev,
 
 	td->mtclass.quirks = val;
 
-	if (!td->contactcount)
+	if (td->cc_index < 0)
 		td->mtclass.quirks &= ~MT_QUIRK_CONTACT_CNT_ACCURATE;
 
 	return count;
@@ -483,7 +484,8 @@ static int mt_input_mapping(struct hid_device *hdev, struct hid_input *hi,
 			td->last_field_index = field->index;
 			return 1;
 		case HID_DG_CONTACTCOUNT:
-			td->contactcount = field->value + usage->usage_index;
+			td->cc_index = field->index;
+			td->cc_value_index = usage->usage_index;
 			td->last_field_index = field->index;
 			return 1;
 		case HID_DG_CONTACTMAX:
@@ -701,8 +703,12 @@ static void mt_report(struct hid_device *hid, struct hid_report *report)
 	 * Includes multi-packet support where subsequent
 	 * packets are sent with zero contactcount.
 	 */
-	if (td->contactcount && *td->contactcount)
-		td->num_expected = *td->contactcount;
+	if (td->cc_index >= 0) {
+		struct hid_field *field = report->field[td->cc_index];
+		int value = field->value[td->cc_value_index];
+		if (value)
+			td->num_expected = value;
+	}
 
 	for (r = 0; r < report->maxfield; r++) {
 		field = report->field[r];
@@ -786,7 +792,7 @@ static void mt_post_parse(struct mt_device *td)
 		td->last_slot_field = f->usages[field_count_per_touch - 1];
 	}
 
-	if (!td->contactcount)
+	if (td->cc_index < 0)
 		cls->quirks &= ~MT_QUIRK_CONTACT_CNT_ACCURATE;
 }
 
@@ -845,6 +851,7 @@ static int mt_probe(struct hid_device *hdev, const struct hid_device_id *id)
 	td->mtclass = *mtclass;
 	td->inputmode = -1;
 	td->maxcontact_report_id = -1;
+	td->cc_index = -1;
 	hid_set_drvdata(hdev, td);
 
 	td->fields = kzalloc(sizeof(struct mt_fields), GFP_KERNEL);