[PATCH] HID: wiimote: Fix wiimote mp scale linearization

[Cyan Ogilvie <cyan.ogilvie@gmail.com>]

The wiimote motion plus gyros use two scales to report fast and slow
rotation - below 440 deg/s uses 8192/440 units / deg/s, and above uses
8192/2000 units / deg/s.

Previously this driver attempted to linearize the two by scaling the fast
rate by 18 and the slow by 9, but this results in a scale of
8192*9/440 = ~167.564 for slow and 8192*18/2000 = 73.728 for fast.

Correct the fast motion scale factor so that both report ~167.564
units / deg/s

Signed-off-by: Cyan Ogilvie <cyan.ogilvie@gmail.com>
---
 drivers/hid/hid-wiimote-modules.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/drivers/hid/hid-wiimote-modules.c b/drivers/hid/hid-wiimote-modules.c
index 4390eee..c830ed3 100644
--- a/drivers/hid/hid-wiimote-modules.c
+++ b/drivers/hid/hid-wiimote-modules.c
@@ -2049,9 +2049,11 @@ static void wiimod_mp_in_mp(struct wiimote_data *wdata, const __u8 *ext)
 	 *   -----+------------------------------+-----+-----+
 	 * The single bits Yaw, Roll, Pitch in the lower right corner specify
 	 * whether the wiimote is rotating fast (0) or slow (1). Speed for slow
-	 * roation is 440 deg/s and for fast rotation 2000 deg/s. To get a
-	 * linear scale we multiply by 2000/440 = ~4.5454 which is 18 for fast
-	 * and 9 for slow.
+	 * roation is 8192/440 units / deg/s and for fast rotation 8192/2000
+	 * units / deg/s. To get a linear scale for fast rotation we multiply
+	 * by 2000/440 = ~4.5454 and scale both fast and slow by 9 to match the
+	 * previous scale reported by this driver.
+	 * This leaves a linear scale with 8192*9/440 (~167.564) units / deg/s.
 	 * If the wiimote is not rotating the sensor reports 2^13 = 8192.
 	 * Ext specifies whether an extension is connected to the motionp.
 	 * which is parsed by wiimote-core.
@@ -2070,15 +2072,15 @@ static void wiimod_mp_in_mp(struct wiimote_data *wdata, const __u8 *ext)
 	z -= 8192;
 
 	if (!(ext[3] & 0x02))
-		x *= 18;
+		x = (x * 2000 * 9) / 440;
 	else
 		x *= 9;
 	if (!(ext[4] & 0x02))
-		y *= 18;
+		y = (y * 2000 * 9) / 440;
 	else
 		y *= 9;
 	if (!(ext[3] & 0x01))
-		z *= 18;
+		z = (z * 2000 * 9) / 440;
 	else
 		z *= 9;
 
-- 
2.1.4