Input: synaptics - fix middle button on Lenovo 2015 products

On the X1 Carbon 3rd gen (with a 2015 broadwell cpu), the physical middle
button of the trackstick (attached to the touchpad serio device, of course)
seems to get lost.

Actually, the touchpads reports 3 extra buttons, which falls in the switch
below to the '2' case. Let's handle the case of odd numbers also, so that
the middle button finds its way back.

Cc: stable@vger.kernel.org
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/mouse/synaptics.c

@@ -658,6 +658,18 @@ static void synaptics_parse_agm(const unsigned char buf[],
 	priv->agm_pending = true;
 }
 
+static void synaptics_parse_ext_buttons(const unsigned char buf[],
+					struct synaptics_data *priv,
+					struct synaptics_hw_state *hw)
+{
+	unsigned int ext_bits =
+		(SYN_CAP_MULTI_BUTTON_NO(priv->ext_cap) + 1) >> 1;
+	unsigned int ext_mask = GENMASK(ext_bits - 1, 0);
+
+	hw->ext_buttons = buf[4] & ext_mask;
+	hw->ext_buttons |= (buf[5] & ext_mask) << ext_bits;
+}
+
 static bool is_forcepad;
 
 static int synaptics_parse_hw_state(const unsigned char buf[],
@@ -744,28 +756,9 @@ static int synaptics_parse_hw_state(const unsigned char buf[],
 			hw->down = ((buf[0] ^ buf[3]) & 0x02) ? 1 : 0;
 		}
 
-		if (SYN_CAP_MULTI_BUTTON_NO(priv->ext_cap) &&
+		if (SYN_CAP_MULTI_BUTTON_NO(priv->ext_cap) > 0 &&
 		    ((buf[0] ^ buf[3]) & 0x02)) {
-			switch (SYN_CAP_MULTI_BUTTON_NO(priv->ext_cap) & ~0x01) {
-			default:
-				/*
-				 * if nExtBtn is greater than 8 it should be
-				 * considered invalid and treated as 0
-				 */
-				break;
-			case 8:
-				hw->ext_buttons |= ((buf[5] & 0x08)) ? 0x80 : 0;
-				hw->ext_buttons |= ((buf[4] & 0x08)) ? 0x40 : 0;
-			case 6:
-				hw->ext_buttons |= ((buf[5] & 0x04)) ? 0x20 : 0;
-				hw->ext_buttons |= ((buf[4] & 0x04)) ? 0x10 : 0;
-			case 4:
-				hw->ext_buttons |= ((buf[5] & 0x02)) ? 0x08 : 0;
-				hw->ext_buttons |= ((buf[4] & 0x02)) ? 0x04 : 0;
-			case 2:
-				hw->ext_buttons |= ((buf[5] & 0x01)) ? 0x02 : 0;
-				hw->ext_buttons |= ((buf[4] & 0x01)) ? 0x01 : 0;
-			}
+			synaptics_parse_ext_buttons(buf, priv, hw);
 		}
 	} else {
 		hw->x = (((buf[1] & 0x1f) << 8) | buf[2]);
@@ -832,6 +825,7 @@ static void synaptics_report_buttons(struct psmouse *psmouse,
 {
 	struct input_dev *dev = psmouse->dev;
 	struct synaptics_data *priv = psmouse->private;
+	int ext_bits = (SYN_CAP_MULTI_BUTTON_NO(priv->ext_cap) + 1) >> 1;
 	int i;
 
 	input_report_key(dev, BTN_LEFT, hw->left);
@@ -845,8 +839,12 @@ static void synaptics_report_buttons(struct psmouse *psmouse,
 		input_report_key(dev, BTN_BACK, hw->down);
 	}
 
-	for (i = 0; i < SYN_CAP_MULTI_BUTTON_NO(priv->ext_cap); i++)
-		input_report_key(dev, BTN_0 + i, hw->ext_buttons & (1 << i));
+	for (i = 0; i < ext_bits; i++) {
+		input_report_key(dev, BTN_0 + 2 * i,
+				 hw->ext_buttons & (1 << i));
+		input_report_key(dev, BTN_1 + 2 * i,
+				 hw->ext_buttons & (1 << (i + ext_bits)));
+	}
 }
 
 static void synaptics_report_slot(struct input_dev *dev, int slot,