[PATCH v2] HID: wacom: Update last_slot_field during pre_report phase

[Jason Gerecke <killertofu@gmail.com>]

If a touchscreen contains both multitouch and single-touch reports in its
descriptor in that order, the driver may overwrite information it saved
about the format of the multitouch report. This can cause the report
processing code to get tripped up and send an incorrect event stream to
userspace.

In particular, this can cause last_slot_field to be overwritten with the
result that the driver prematurely assumes it has finished processing a
slot and sending the ABS_MT_SLOT event at the wrong point in time,
associating events for the current contact with the following contact
instead.

To prevent this from occurring, we update the value of last_slot_field
durring the pre_report phase to ensure that it is correct for the report
that is to be processed.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
---
Changes from v1:
 * The v1 patch cut off processing by wacom_wac_finger_usage_mapping once
   it saw a HID_DG_CONTACTCOUNT usage in any report. This method of
   handling the bug has two potential problems: 1) reports which place
   the HID_DG_CONTACTCOUNT usage near the beginning of the packet will
   not properly map the rest of the usages, and 2) devices which have
   multiple reports with the HID_DG_CONTACTCOUNT usage will only send
   reports formatted like the first discovered. Neither of these should
   cause issues for currently available hardware, but to maximize forward
   compatibility, the revised scheme contained here was devised.

 drivers/hid/wacom_wac.c | 62 +++++++++++++++++++++----------------------------
 drivers/hid/wacom_wac.h |  2 +-
 2 files changed, 27 insertions(+), 37 deletions(-)

diff --git a/drivers/hid/wacom_wac.c b/drivers/hid/wacom_wac.c
index fcf2264..d2611f3 100644
--- a/drivers/hid/wacom_wac.c
+++ b/drivers/hid/wacom_wac.c
@@ -1556,13 +1556,11 @@ static void wacom_wac_finger_usage_mapping(struct hid_device *hdev,
 {
 	struct wacom *wacom = hid_get_drvdata(hdev);
 	struct wacom_wac *wacom_wac = &wacom->wacom_wac;
-	struct wacom_features *features = &wacom_wac->features;
 	struct input_dev *input = wacom_wac->touch_input;
 	unsigned touch_max = wacom_wac->features.touch_max;
 
 	switch (usage->hid) {
 	case HID_GD_X:
-		features->last_slot_field = usage->hid;
 		if (touch_max == 1)
 			wacom_map_usage(input, usage, field, EV_ABS, ABS_X, 4);
 		else
@@ -1570,7 +1568,6 @@ static void wacom_wac_finger_usage_mapping(struct hid_device *hdev,
 					ABS_MT_POSITION_X, 4);
 		break;
 	case HID_GD_Y:
-		features->last_slot_field = usage->hid;
 		if (touch_max == 1)
 			wacom_map_usage(input, usage, field, EV_ABS, ABS_Y, 4);
 		else
@@ -1579,22 +1576,11 @@ static void wacom_wac_finger_usage_mapping(struct hid_device *hdev,
 		break;
 	case HID_DG_WIDTH:
 	case HID_DG_HEIGHT:
-		features->last_slot_field = usage->hid;
 		wacom_map_usage(input, usage, field, EV_ABS, ABS_MT_TOUCH_MAJOR, 0);
 		wacom_map_usage(input, usage, field, EV_ABS, ABS_MT_TOUCH_MINOR, 0);
 		input_set_abs_params(input, ABS_MT_ORIENTATION, 0, 1, 0, 0);
 		break;
-	case HID_DG_CONTACTID:
-		features->last_slot_field = usage->hid;
-		break;
-	case HID_DG_INRANGE:
-		features->last_slot_field = usage->hid;
-		break;
-	case HID_DG_INVERT:
-		features->last_slot_field = usage->hid;
-		break;
 	case HID_DG_TIPSWITCH:
-		features->last_slot_field = usage->hid;
 		wacom_map_usage(input, usage, field, EV_KEY, BTN_TOUCH, 0);
 		break;
 	case HID_DG_CONTACTCOUNT:
@@ -1672,7 +1658,7 @@ static int wacom_wac_finger_event(struct hid_device *hdev,
 
 
 	if (usage->usage_index + 1 == field->report_count) {
-		if (usage->hid == wacom_wac->features.last_slot_field)
+		if (usage->hid == wacom_wac->hid_data.last_slot_field)
 			wacom_wac_finger_slot(wacom_wac, wacom_wac->touch_input);
 	}
 
@@ -1685,31 +1671,35 @@ static void wacom_wac_finger_pre_report(struct hid_device *hdev,
 	struct wacom *wacom = hid_get_drvdata(hdev);
 	struct wacom_wac *wacom_wac = &wacom->wacom_wac;
 	struct hid_data* hid_data = &wacom_wac->hid_data;
+	int i;
 
-	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;
-				}
+	for (i = 0; i < report->maxfield; i++) {
+		struct hid_field *field = report->field[i];
+		int j;
+
+		for (j = 0; j < field->maxusage; j++) {
+			struct hid_usage *usage = &field->usage[j];
+
+			switch (usage->hid) {
+			case HID_GD_X:
+			case HID_GD_Y:
+			case HID_DG_WIDTH:
+			case HID_DG_HEIGHT:
+			case HID_DG_CONTACTID:
+			case HID_DG_INRANGE:
+			case HID_DG_INVERT:
+			case HID_DG_TIPSWITCH:
+				hid_data->last_slot_field = usage->hid;
+				break;
+			case HID_DG_CONTACTCOUNT:
+				hid_data->cc_report = report->id;
+				hid_data->cc_index = i;
+				hid_data->cc_value_index = j;
+				break;
 			}
 		}
 	}
+
 	if (hid_data->cc_report != 0 &&
 	    hid_data->cc_index >= 0) {
 		struct hid_field *field = report->field[hid_data->cc_index];
diff --git a/drivers/hid/wacom_wac.h b/drivers/hid/wacom_wac.h
index 8a8974c..267025c 100644
--- a/drivers/hid/wacom_wac.h
+++ b/drivers/hid/wacom_wac.h
@@ -185,7 +185,6 @@ struct wacom_features {
 	int pktlen;
 	bool check_for_hid_type;
 	int hid_type;
-	int last_slot_field;
 };
 
 struct wacom_shared {
@@ -214,6 +213,7 @@ struct hid_data {
 	int cc_report;
 	int cc_index;
 	int cc_value_index;
+	int last_slot_field;
 	int num_expected;
 	int num_received;
 };
-- 
2.9.0