Input: add infrastructure for selecting clockid for event time stamps

As noted by Arve and others, since wall time can jump backwards, it is
difficult to use for input because one cannot determine if one event
occurred before another or for how long a key was pressed.

However, the timestamp field is part of the kernel ABI, and cannot be
changed without possibly breaking existing users.

This patch adds a new IOCTL that allows a clockid to be set in the
evdev_client struct that will specify which time base to use for event
timestamps (ie: CLOCK_MONOTONIC instead of CLOCK_REALTIME).

For now we only support CLOCK_MONOTONIC and CLOCK_REALTIME, but
in the future we could support other clockids if appropriate.

The default remains CLOCK_REALTIME, so we don't change the ABI.
Signed-off-by: John Stultz <john.stultz@linaro.org>
Reviewed-by: Daniel Kurtz <djkurtz@google.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

drivers/input/evdev.c

@@ -46,6 +46,7 @@ struct evdev_client {
 	struct fasync_struct *fasync;
 	struct evdev *evdev;
 	struct list_head node;
+	int clkid;
 	unsigned int bufsize;
 	struct input_event buffer[];
 };
@@ -54,8 +55,12 @@ static struct evdev *evdev_table[EVDEV_MINORS];
 static DEFINE_MUTEX(evdev_table_mutex);
 
 static void evdev_pass_event(struct evdev_client *client,
-			     struct input_event *event)
+			     struct input_event *event,
+			     ktime_t mono, ktime_t real)
 {
+	event->time = ktime_to_timeval(client->clkid == CLOCK_MONOTONIC ?
+					mono : real);
+
 	/* Interrupts are disabled, just acquire the lock. */
 	spin_lock(&client->buffer_lock);
 
@@ -94,8 +99,11 @@ static void evdev_event(struct input_handle *handle,
 	struct evdev *evdev = handle->private;
 	struct evdev_client *client;
 	struct input_event event;
+	ktime_t time_mono, time_real;
+
+	time_mono = ktime_get();
+	time_real = ktime_sub(time_mono, ktime_get_monotonic_offset());
 
-	do_gettimeofday(&event.time);
 	event.type = type;
 	event.code = code;
 	event.value = value;
@@ -103,11 +111,12 @@ static void evdev_event(struct input_handle *handle,
 	rcu_read_lock();
 
 	client = rcu_dereference(evdev->grab);
+
 	if (client)
-		evdev_pass_event(client, &event);
+		evdev_pass_event(client, &event, time_mono, time_real);
 	else
 		list_for_each_entry_rcu(client, &evdev->client_list, node)
-			evdev_pass_event(client, &event);
+			evdev_pass_event(client, &event, time_mono, time_real);
 
 	rcu_read_unlock();
 
@@ -685,6 +694,14 @@ static long evdev_do_ioctl(struct file *file, unsigned int cmd,
 		else
 			return evdev_ungrab(evdev, client);
 
+	case EVIOCSCLOCKID:
+		if (copy_from_user(&i, p, sizeof(unsigned int)))
+			return -EFAULT;
+		if (i != CLOCK_MONOTONIC && i != CLOCK_REALTIME)
+			return -EINVAL;
+		client->clkid = i;
+		return 0;
+
 	case EVIOCGKEYCODE:
 		return evdev_handle_get_keycode(dev, p);
 

include/linux/input.h

@@ -129,6 +129,8 @@ struct input_keymap_entry {
 
 #define EVIOCGRAB		_IOW('E', 0x90, int)			/* Grab/Release device */
 
+#define EVIOCSCLOCKID		_IOW('E', 0xa0, int)			/* Set clockid to be used for timestamps */
+
 /*
  * Device properties and quirks
  */

kernel/time/timekeeping.c

@@ -1140,6 +1140,8 @@ ktime_t ktime_get_monotonic_offset(void)
 	} while (read_seqretry(&xtime_lock, seq));
 	return timespec_to_ktime(wtom);
 }
+EXPORT_SYMBOL_GPL(ktime_get_monotonic_offset);
+
 
 /**
  * xtime_update() - advances the timekeeping infrastructure