HID: wacom: insert timestamp to packed Bluetooth (BT) events

To fully utilize the BT polling/refresh rate, a few input events
are sent together to reduce event delay. This causes issue to the
timestamp generated by input_sync since all the events in the same
packet would pretty much have the same timestamp. This patch inserts
time interval to the events by averaging the total time used for
sending the packet.

This decision was mainly based on observing the actual time interval
between each BT polling. The interval doesn't seem to be constant,
due to the network and system environment. So, using solutions other
than averaging doesn't end up with valid timestamps.
Signed-off-by: Ping Cheng <ping.cheng@wacom.com>
Reviewed-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/wacom_wac.c

@@ -1308,6 +1308,9 @@ static void wacom_intuos_pro2_bt_pen(struct wacom_wac *wacom)
 
 	struct input_dev *pen_input = wacom->pen_input;
 	unsigned char *data = wacom->data;
+	int number_of_valid_frames = 0;
+	int time_interval = 15000000;
+	ktime_t time_packet_received = ktime_get();
 	int i;
 
 	if (wacom->features.type == INTUOSP2_BT ||
@@ -1328,12 +1331,30 @@ static void wacom_intuos_pro2_bt_pen(struct wacom_wac *wacom)
 		wacom->id[0] |= (wacom->serial[0] >> 32) & 0xFFFFF;
 	}
 
+	/* number of valid frames */
 	for (i = 0; i < pen_frames; i++) {
 		unsigned char *frame = &data[i*pen_frame_len + 1];
 		bool valid = frame[0] & 0x80;
+
+		if (valid)
+			number_of_valid_frames++;
+	}
+
+	if (number_of_valid_frames) {
+		if (wacom->hid_data.time_delayed)
+			time_interval = ktime_get() - wacom->hid_data.time_delayed;
+		time_interval /= number_of_valid_frames;
+		wacom->hid_data.time_delayed = time_packet_received;
+	}
+
+	for (i = 0; i < number_of_valid_frames; i++) {
+		unsigned char *frame = &data[i*pen_frame_len + 1];
+		bool valid = frame[0] & 0x80;
 		bool prox = frame[0] & 0x40;
 		bool range = frame[0] & 0x20;
 		bool invert = frame[0] & 0x10;
+		int frames_number_reversed = number_of_valid_frames - i - 1;
+		int event_timestamp = time_packet_received - frames_number_reversed * time_interval;
 
 		if (!valid)
 			continue;
@@ -1346,6 +1367,7 @@ static void wacom_intuos_pro2_bt_pen(struct wacom_wac *wacom)
 			wacom->tool[0] = 0;
 			wacom->id[0] = 0;
 			wacom->serial[0] = 0;
+			wacom->hid_data.time_delayed = 0;
 			return;
 		}
 
@@ -1382,6 +1404,7 @@ static void wacom_intuos_pro2_bt_pen(struct wacom_wac *wacom)
 						 get_unaligned_le16(&frame[11]));
 			}
 		}
+
 		if (wacom->tool[0]) {
 			input_report_abs(pen_input, ABS_PRESSURE, get_unaligned_le16(&frame[5]));
 			if (wacom->features.type == INTUOSP2_BT ||
@@ -1405,6 +1428,9 @@ static void wacom_intuos_pro2_bt_pen(struct wacom_wac *wacom)
 
 		wacom->shared->stylus_in_proximity = prox;
 
+		/* add timestamp to unpack the frames */
+		input_set_timestamp(pen_input, event_timestamp);
+
 		input_sync(pen_input);
 	}
 }

drivers/hid/wacom_wac.h

@@ -324,6 +324,7 @@ struct hid_data {
 	int ps_connected;
 	bool pad_input_event_flag;
 	unsigned short sequence_number;
+	int time_delayed;
 };
 
 struct wacom_remote_data {