motion_lid: Make tablet mode great (again).

Chrome seems to pay attention to the tablet mode switch reported by the
EC.  However, for some devices that don't actually have a switch and use
the lid angle as a "virtual" switch, it's possible for invalid tablet
mode change events to be reported.  This is because a single reading
could flip the switch.

This commit adds some debouncing to the tablet mode event changes.
Instead of having a single event be able to change the tablet mode
switch, we will now perform TABLET_MODE_DEBOUNCE_COUNT number of
calculations of the new tablet mode value.  If those calculations all
agree, then we'll flip the switch.  This should help mitigate false
tablet mode change events caused by spurious forces.

BUG=chrome-os-partner:59203
BUG=chrome-os-partner:59480
BRANCH=gru
TEST=flash kevin; open lid to pi/2 rad; rotate device counter-clockwise
pi/2 rad; shake device and verify that tablet mode doesn't change
easily.
TEST=verify that tablet mode still works.

Change-Id: Id020f7db28e93e53b276b3f0d28a40251f035f0b
Signed-off-by: Aseda Aboagye <aaboagye@google.com>
Reviewed-on: https://chromium-review.googlesource.com/410942
Commit-Ready: Gwendal Grignou <gwendal@chromium.org>
Tested-by: Aseda Aboagye <aaboagye@chromium.org>
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>

1 files changed, 35 insertions, 7 deletions

diff --git a/common/motion_lid.c b/common/motion_lid.c
index 3209492a65..f2c6863b80 100644
--- a/common/motion_lid.c
+++ b/common/motion_lid.c
@@ -58,6 +58,16 @@
 #define TABLET_ZONE_LID_ANGLE FLOAT_TO_FP(300)
 #define LAPTOP_ZONE_LID_ANGLE FLOAT_TO_FP(240)
 
+/*
+ * We will change our tablet mode status when we are "convinced" that it has
+ * changed.  This means we will have to consecutively calculate our new tablet
+ * mode while the angle is stable and come to the same conclusion.  The number
+ * of consecutive calculations is the debounce count with an interval between
+ * readings set by the motion_sense task.  This should avoid spurious forces
+ * that may trigger false transitions of the tablet mode switch.
+ */
+#define TABLET_MODE_DEBOUNCE_COUNT 3
+static int tablet_mode_debounce_cnt = TABLET_MODE_DEBOUNCE_COUNT;
 #endif
 
 #ifdef CONFIG_LID_ANGLE_INVALID_CHECK
@@ -230,13 +240,31 @@ static int calculate_lid_angle(const vector_3_t base, const vector_3_t lid,
 	*lid_angle = FP_TO_INT(last_lid_angle_fp + FLOAT_TO_FP(0.5));
 
 #ifdef CONFIG_LID_ANGLE_TABLET_MODE
-	if (last_lid_angle_fp > TABLET_ZONE_LID_ANGLE)
-		new_tablet_mode = 1;
-	else if (last_lid_angle_fp < LAPTOP_ZONE_LID_ANGLE)
-		new_tablet_mode = 0;
-	if (tablet_get_mode() != new_tablet_mode) {
-		tablet_set_mode(new_tablet_mode);
-		hook_notify(HOOK_TABLET_MODE_CHANGE);
+	if (reliable) {
+		if (last_lid_angle_fp > TABLET_ZONE_LID_ANGLE)
+			new_tablet_mode = 1;
+		else if (last_lid_angle_fp < LAPTOP_ZONE_LID_ANGLE)
+			new_tablet_mode = 0;
+
+		/* Only change tablet mode if we're sure. */
+		if (tablet_get_mode() != new_tablet_mode) {
+			if (tablet_mode_debounce_cnt == 0) {
+				/* Alright, we're convinced. */
+				tablet_mode_debounce_cnt =
+					TABLET_MODE_DEBOUNCE_COUNT;
+				tablet_set_mode(new_tablet_mode);
+				hook_notify(HOOK_TABLET_MODE_CHANGE);
+				return reliable;
+			}
+			tablet_mode_debounce_cnt--;
+			return reliable;
+		}
+
+		/*
+		 * Since it hasn't changed, clear any pending tablet mode
+		 * change.
+		 */
+		tablet_mode_debounce_cnt = TABLET_MODE_DEBOUNCE_COUNT;
 	}
 #endif   /* CONFIG_LID_ANGLE_TABLET_MODE */
 #else    /* CONFIG_LID_ANGLE_INVALID_CHECK */