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

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Date:	Mon,  4 Jan 2016 21:58:48 +0530
From:	Aniroop Mathur <a.mathur@...sung.com>
To:	dmitry.torokhov@...il.com, benjamin.tissoires@...il.com,
	peter.hutterer@...-t.net
Cc:	dh.herrmann@...il.com, rydberg@...math.org,
	aniroop.mathur@...il.com, linux-input@...r.kernel.org,
	linux-kernel@...r.kernel.org, Aniroop Mathur <a.mathur@...sung.com>
Subject: [PATCH] [v2]Input: evdev - drop partial events after emptying the buffer

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@...sung.com>
---
 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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