HID: magicmouse: disable and add module param for scroll acceleration

Scroll acceleration is unique to the magicmouse driver, and is
unintuitive to a user who is unaware of the functionality. Thus, disable
it by default, but add a module parameter to enable it for power users
who want it.
Signed-off-by: Chase Douglas <chase.douglas@canonical.com>
Acked-by: Michael Poole <mdpoole@troilus.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/hid-magicmouse.c

@@ -30,6 +30,10 @@ static bool emulate_scroll_wheel = true;
 module_param(emulate_scroll_wheel, bool, 0644);
 MODULE_PARM_DESC(emulate_scroll_wheel, "Emulate a scroll wheel");
 
+static bool scroll_acceleration = false;
+module_param(scroll_acceleration, bool, 0644);
+MODULE_PARM_DESC(scroll_acceleration, "Accelerate sequential scroll events");
+
 static bool report_touches = true;
 module_param(report_touches, bool, 0644);
 MODULE_PARM_DESC(report_touches, "Emit touch records (otherwise, only use them for emulation)");
@@ -177,7 +181,9 @@ static void magicmouse_emit_touch(struct magicmouse_sc *msc, int raw_id, u8 *tda
 		switch (tdata[7] & TOUCH_STATE_MASK) {
 		case TOUCH_STATE_START:
 			msc->touches[id].scroll_y = y;
-			msc->scroll_accel = min_t(int, msc->scroll_accel + 1,
+			if (scroll_acceleration)
+				msc->scroll_accel = min_t(int,
+						msc->scroll_accel + 1,
 						ARRAY_SIZE(accel_profile) - 1);
 			break;
 		case TOUCH_STATE_DRAG: