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>

3 files changed, 28 insertions, 12 deletions

diff --git a/Documentation/input/event-codes.txt b/Documentation/input/event-codes.txt
index f13aee550409..23fcb05175be 100644
--- a/Documentation/input/event-codes.txt
+++ b/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
diff --git a/drivers/input/evdev.c b/drivers/input/evdev.c
index 7f42d3a454d2..88d8e4cb419a 100644
--- a/drivers/input/evdev.c
+++ b/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)
diff --git a/include/linux/input.h b/include/linux/input.h
index 0cc25e4ce2ab..73a8c6ee595b 100644
--- a/include/linux/input.h
+++ b/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