HID: wacom: Reset expected and received contact counts at the same time

These two values go hand-in-hand and must be valid for the driver to
behave correctly. We are currently lazy about updating the values and
rely on the "expected" code flow to take care of making sure they're
valid at the point they're needed. The "expected" flow changed somewhat
with commit f8b6a74 ("HID: wacom: generic: Support multiple tools
per report"), however. This led to problems with the DTH-2452 due (in
part) to *all* contacts being fully processed -- even those past the
expected contact count. Specifically, the received count gets reset to
0 once all expected fingers are processed, but not the expected count.
The rest of the contacts in the report are then *also* processed since
now the driver thinks we've only processed 0 of N expected contacts.

Later commits such as 7fb0413 (HID: wacom: Use "Confidence" flag to
prevent reporting invalid contacts) worked around the DTH-2452 issue by
skipping the invalid contacts at the end of the report, but this is not
a complete fix. The confidence flag cannot be relied on when a contact
is removed (see the following patch), and dealing with that condition
re-introduces the DTH-2452 issue unless we also address this contact
count laziness. By resetting expected and received counts at the same
time we ensure the driver understands that there are 0 more contacts
expected in the report. Similarly, we also make sure to reset the
received count if for some reason we're out of sync in the pre-report
phase.

Link: https://github.com/linuxwacom/input-wacom/issues/288
Fixes: f8b6a74 ("HID: wacom: generic: Support multiple tools per report")
CC: stable@vger.kernel.org
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Ping Cheng <ping.cheng@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/wacom_wac.c

@@ -2692,11 +2692,14 @@ static void wacom_wac_finger_pre_report(struct hid_device *hdev,
 	    hid_data->cc_index >= 0) {
 		struct hid_field *field = report->field[hid_data->cc_index];
 		int value = field->value[hid_data->cc_value_index];
-		if (value)
+		if (value) {
 			hid_data->num_expected = value;
+			hid_data->num_received = 0;
+		}
 	}
 	else {
 		hid_data->num_expected = wacom_wac->features.touch_max;
+		hid_data->num_received = 0;
 	}
 }
 
@@ -2724,6 +2727,7 @@ static void wacom_wac_finger_report(struct hid_device *hdev,
 
 	input_sync(input);
 	wacom_wac->hid_data.num_received = 0;
+	wacom_wac->hid_data.num_expected = 0;
 
 	/* keep touch state for pen event */
 	wacom_wac->shared->touch_down = wacom_wac_finger_count_touches(wacom_wac);