HID: wacom: Tie cached HID_DG_CONTACTCOUNT indices to report ID

The cached indicies 'cc_index' and 'cc_value_index' introduced in 1b5d514
are only valid for a single report ID. If a touchscreen has multiple
reports with a HID_DG_CONTACTCOUNT usage, its possible that the values
will not be correct for the report we're handling, resulting in an
incorrect value for 'num_expected'. This has been observed with the Cintiq
Companion 2.

To address this, we store the ID of the report those indicies are valid
for in a new  'cc_report' variable. Before using them to get the expected
contact count, we first check if the ID of the report we're processing
matches 'cc_report'. If it doesn't, we update the indicies to point to
the HID_DG_CONTACTCOUNT usage of the current report (if it has one).

Cc: stable@vger.kernel.org
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/wacom_wac.c

@@ -1628,6 +1628,7 @@ static void wacom_wac_finger_usage_mapping(struct hid_device *hdev,
 		wacom_map_usage(input, usage, field, EV_KEY, BTN_TOUCH, 0);
 		break;
 	case HID_DG_CONTACTCOUNT:
+		wacom_wac->hid_data.cc_report = field->report->id;
 		wacom_wac->hid_data.cc_index = field->index;
 		wacom_wac->hid_data.cc_value_index = usage->usage_index;
 		break;
@@ -1715,6 +1716,31 @@ static void wacom_wac_finger_pre_report(struct hid_device *hdev,
 	struct wacom_wac *wacom_wac = &wacom->wacom_wac;
 	struct hid_data* hid_data = &wacom_wac->hid_data;
 
+	if (hid_data->cc_report != 0 &&
+	    hid_data->cc_report != report->id) {
+		int i;
+
+		hid_data->cc_report = report->id;
+		hid_data->cc_index = -1;
+		hid_data->cc_value_index = -1;
+
+		for (i = 0; i < report->maxfield; i++) {
+			struct hid_field *field = report->field[i];
+			int j;
+
+			for (j = 0; j < field->maxusage; j++) {
+				if (field->usage[j].hid == HID_DG_CONTACTCOUNT) {
+					hid_data->cc_index = i;
+					hid_data->cc_value_index = j;
+
+					/* break */
+					i = report->maxfield;
+					j = field->maxusage;
+				}
+			}
+		}
+	}
+
 	if (hid_data->cc_index >= 0) {
 		struct hid_field *field = report->field[hid_data->cc_index];
 		int value = field->value[hid_data->cc_value_index];

drivers/hid/wacom_wac.h

@@ -198,6 +198,7 @@ struct hid_data {
 	int width;
 	int height;
 	int id;
+	int cc_report;
 	int cc_index;
 	int cc_value_index;
 	int num_expected;