Input: evdev - indicate buffer overrun with SYN_DROPPED

Add a new EV_SYN code, SYN_DROPPED, to inform the client when input
events have been dropped from the evdev input buffer due to a
buffer overrun.  The client should use this event as a hint to
reset its state or ignore all following events until the next
packet begins.

Signed-off-by: Jeff Brown <jeffbrown@android.com>
[dtor@mail.ru: Implement Henrik's suggestion and drop old events in
 case of overflow.]
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

Documentation/input/event-codes.txt

@@ -85,6 +85,12 @@ sent in the evdev event stream.
   - Used to synchronize and separate touch events. See the
     multi-touch-protocol.txt document for more information.
 
+* SYN_DROPPED:
+  - Used to indicate buffer overrun in the evdev client's event queue.
+    Client should ignore all events up to and including next SYN_REPORT
+    event and query the device (using EVIOCG* ioctls) to obtain its
+    current state.
+
 EV_KEY:
 ----------
 EV_KEY events take the form KEY_<name> or BTN_<name>. For example, KEY_A is used

drivers/input/evdev.c

@@ -39,13 +39,13 @@ struct evdev {
 };
 
 struct evdev_client {
-	int head;
-	int tail;
+	unsigned int head;
+	unsigned int tail;
 	spinlock_t buffer_lock; /* protects access to buffer, head and tail */
 	struct fasync_struct *fasync;
 	struct evdev *evdev;
 	struct list_head node;
-	int bufsize;
+	unsigned int bufsize;
 	struct input_event buffer[];
 };
 
@@ -55,16 +55,25 @@ static DEFINE_MUTEX(evdev_table_mutex);
 static void evdev_pass_event(struct evdev_client *client,
 			     struct input_event *event)
 {
-	/*
-	 * Interrupts are disabled, just acquire the lock.
-	 * Make sure we don't leave with the client buffer
-	 * "empty" by having client->head == client->tail.
-	 */
+	/* Interrupts are disabled, just acquire the lock. */
 	spin_lock(&client->buffer_lock);
-	do {
-		client->buffer[client->head++] = *event;
-		client->head &= client->bufsize - 1;
-	} while (client->head == client->tail);
+
+	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.
+		 */
+		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;
+	}
+
 	spin_unlock(&client->buffer_lock);
 
 	if (event->type == EV_SYN)

include/linux/input.h

@@ -167,6 +167,7 @@ struct input_keymap_entry {
 #define SYN_REPORT		0
 #define SYN_CONFIG		1
 #define SYN_MT_REPORT		2
+#define SYN_DROPPED		3
 
 /*
  * Keys and buttons