[PATCH] [v2]Input: evdev - drop partial events after emptying the buffer

[Aniroop Mathur <a.mathur@xxxxxxxxxxx>]

This patch introduces concept to drop partial events in evdev handler
itself after emptying the buffer which are dropped by all evdev
clients in userspace after SYN_DROPPED occurs.
This in turn saves space of partial events in evdev handler buffer
and reduces evdev client reading requests.

Signed-off-by: Aniroop Mathur <a.mathur@xxxxxxxxxxx>
---
 drivers/input/evdev.c | 56 ++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 47 insertions(+), 9 deletions(-)

diff --git a/drivers/input/evdev.c b/drivers/input/evdev.c
index e9ae3d5..0a5ead8 100644
--- a/drivers/input/evdev.c
+++ b/drivers/input/evdev.c
@@ -58,6 +58,7 @@ struct evdev_client {
 	struct list_head node;
 	unsigned int clk_type;
 	bool revoked;
+	bool drop_pevent; /* specifies whether partial events need to be dropped */
 	unsigned long *evmasks[EV_CNT];
 	unsigned int bufsize;
 	struct input_event buffer[];
@@ -192,6 +193,7 @@ static int evdev_set_clk_type(struct evdev_client *client, unsigned int clkid)
 {
 	unsigned long flags;
 	unsigned int clk_type;
+	struct input_event ev;
 
 	switch (clkid) {
 
@@ -218,8 +220,26 @@ static int evdev_set_clk_type(struct evdev_client *client, unsigned int clkid)
 		spin_lock_irqsave(&client->buffer_lock, flags);
 
 		if (client->head != client->tail) {
-			client->packet_head = client->head = client->tail;
+
+			/* Store previous event */
+			client->head--;
+			client->head &= client->bufsize - 1;
+			ev = client->buffer[client->head];
+
+			client->packet_head = client->head = client->tail = 0;
 			__evdev_queue_syn_dropped(client);
+
+			/*
+			 * If last packet is fully stored, queue SYN_REPORT
+			 * so that clients do not ignore next full packet.
+			 * And if last packet is NOT fully stored,
+			 * set drop_pevent to true to ignore partial events.
+			 */
+			if (ev.type == EV_SYN && ev.code == SYN_REPORT) {
+				ev.time = client->buffer[client->head - 1].time;
+				client->buffer[client->head++] = ev;
+			} else if (ev.type != EV_SYN && ev.code != SYN_REPORT)
+				client->drop_pevent = true;
 		}
 
 		spin_unlock_irqrestore(&client->buffer_lock, flags);
@@ -236,17 +256,27 @@ static void __pass_event(struct evdev_client *client,
 
 	if (unlikely(client->head == client->tail)) {
 		/*
-		 * This effectively "drops" all unconsumed events, leaving
-		 * EV_SYN/SYN_DROPPED plus the newest event in the queue.
+		 * This effectively "drops" all unconsumed events, storing
+		 * EV_SYN/SYN_DROPPED and the newest event in the queue but
+		 * only if it is EV_SYN/SYN_REPORT so that clients can read
+		 * the next full event packet. And set drop_pevent to true
+		 * if last event packet is not stored completely in buffer
+		 * to ignore upcoming partial events.
 		 */
-		client->tail = (client->head - 2) & (client->bufsize - 1);
 
-		client->buffer[client->tail].time = event->time;
-		client->buffer[client->tail].type = EV_SYN;
-		client->buffer[client->tail].code = SYN_DROPPED;
-		client->buffer[client->tail].value = 0;
+		client->tail = client->head = client->packet_head;
 
-		client->packet_head = client->tail;
+		client->buffer[client->head].time = event->time;
+		client->buffer[client->head].type = EV_SYN;
+		client->buffer[client->head].code = SYN_DROPPED;
+		client->buffer[client->head].value = 0;
+		client->head = (client->head + 1) & (client->bufsize -1);
+
+		if (event->type == EV_SYN && event->code == SYN_REPORT) {
+			client->buffer[client->head++] = *event;
+			client->head &= client->bufsize - 1;
+		} else if (event->type != EV_SYN && event->code != SYN_REPORT)
+			client->drop_pevent = true;
 	}
 
 	if (event->type == EV_SYN && event->code == SYN_REPORT) {
@@ -284,6 +314,14 @@ static void evdev_pass_values(struct evdev_client *client,
 			wakeup = true;
 		}
 
+		/* drop partial events of last packet but queue SYN_REPORT */
+		if (client->drop_pevent) {
+			if (v->type == EV_SYN && v->code == SYN_REPORT)
+				client->drop_pevent = false;
+			else
+				continue;
+		}
+
 		event.type = v->type;
 		event.code = v->code;
 		event.value = v->value;
-- 
2.6.2

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