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

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Date:	Thu, 31 Dec 2015 03:36:47 +0530
From:	Aniroop Mathur <a.mathur@...sung.com>
To:	dmitry.torokhov@...il.com
Cc:	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	aniroop.mathur@...il.com, Aniroop Mathur <a.mathur@...sung.com>
Subject: [PATCH] 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 | 49 +++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 45 insertions(+), 4 deletions(-)

diff --git a/drivers/input/evdev.c b/drivers/input/evdev.c
index e9ae3d5..e7b612e 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) {
 
@@ -218,6 +220,17 @@ 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) {
+
+			/*
+			 * Set drop_pevent to true if last event packet is
+			 * not stored completely in buffer.
+			 */
+			client->head--;
+			client->head &= client->bufsize - 1;
+			ev = &client->buffer[client->head];
+			if (!(ev->type == EV_SYN && ev->code == SYN_REPORT))
+				client->drop_pevent = true;
+
 			client->packet_head = client->head = client->tail;
 			__evdev_queue_syn_dropped(client);
 		}
@@ -228,31 +241,51 @@ static int evdev_set_clk_type(struct evdev_client *client, unsigned int clkid)
 	return 0;
 }
 
-static void __pass_event(struct evdev_client *client,
+static bool __pass_event(struct evdev_client *client,
 			 const struct input_event *event)
 {
+	struct input_event *prev_ev;
+
 	client->buffer[client->head++] = *event;
 	client->head &= client->bufsize - 1;
 
 	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 not part of partial packet.
+		 * Set drop_pevent to true if last event packet is not stored
+		 * completely in buffer and set to false if SYN_REPORT occurs.
 		 */
+
 		client->tail = (client->head - 2) & (client->bufsize - 1);
 
+		prev_ev = &client->buffer[client->tail];
+		if (!(prev_ev->type == EV_SYN && prev_ev->code == SYN_REPORT)) {
+			client->drop_pevent = true;
+			client->head--;
+			client->head &= 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->packet_head = client->tail;
+
+		if (event->type == EV_SYN && event->code == SYN_REPORT) {
+			client->drop_pevent = false;
+			return true;
+		}
 	}
 
 	if (event->type == EV_SYN && event->code == SYN_REPORT) {
 		client->packet_head = client->head;
 		kill_fasync(&client->fasync, SIGIO, POLL_IN);
 	}
+
+	return false;
 }
 
 static void evdev_pass_values(struct evdev_client *client,
@@ -284,10 +317,18 @@ static void evdev_pass_values(struct evdev_client *client,
 			wakeup = true;
 		}
 
+		/* drop partial events until SYN_REPORT occurs */
+		if (client->drop_pevent) {
+			if (v->type == EV_SYN && v->code == SYN_REPORT)
+				client->drop_pevent = false;
+			continue;
+		}
+
 		event.type = v->type;
 		event.code = v->code;
 		event.value = v->value;
-		__pass_event(client, &event);
+		if (__pass_event(client, &event))
+			wakeup = false;
 	}
 
 	spin_unlock(&client->buffer_lock);
-- 
2.6.2

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