Input: silead - add support for capactive home button found on some x86 tablets

On some x86 tablets with a silead touchscreen the windows logo on the
front is a capacitive home button. Touching this button results in a touch
with bits 12-15 of the Y coordinates set, while normally only the lower 12
are used.

Detect this and report a KEY_LEFTMETA press when this happens. Note for
now we only respond to the Y coordinate bits 12-15 containing 0x01, on some
tablets *without* a capacative button I've noticed these bits containing
0x04 when crossing the edges of the screen.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/devicetree/bindings/input/touchscreen/silead_gsl1680.txt

@@ -23,6 +23,8 @@ Optional properties:
 - touchscreen-inverted-y  : See touchscreen.txt
 - touchscreen-swapped-x-y : See touchscreen.txt
 - silead,max-fingers	  : maximum number of fingers the touchscreen can detect
+- silead,home-button	  : Boolean, set to true on devices which have a
+			    capacitive home-button build into the touchscreen
 - vddio-supply		  : regulator phandle for controller VDDIO
 - avdd-supply		  : regulator phandle for controller AVDD
 

drivers/input/touchscreen/silead.c

@@ -56,7 +56,7 @@
 #define SILEAD_POINT_Y_MSB_OFF	0x01
 #define SILEAD_POINT_X_OFF	0x02
 #define SILEAD_POINT_X_MSB_OFF	0x03
-#define SILEAD_TOUCH_ID_MASK	0xF0
+#define SILEAD_EXTRA_DATA_MASK	0xF0
 
 #define SILEAD_CMD_SLEEP_MIN	10000
 #define SILEAD_CMD_SLEEP_MAX	20000
@@ -109,6 +109,9 @@ static int silead_ts_request_input_dev(struct silead_ts_data *data)
 			    INPUT_MT_DIRECT | INPUT_MT_DROP_UNUSED |
 			    INPUT_MT_TRACK);
 
+	if (device_property_read_bool(dev, "silead,home-button"))
+		input_set_capability(data->input, EV_KEY, KEY_LEFTMETA);
+
 	data->input->name = SILEAD_TS_NAME;
 	data->input->phys = "input/ts";
 	data->input->id.bustype = BUS_I2C;
@@ -139,7 +142,8 @@ static void silead_ts_read_data(struct i2c_client *client)
 	struct input_dev *input = data->input;
 	struct device *dev = &client->dev;
 	u8 *bufp, buf[SILEAD_TS_DATA_LEN];
-	int touch_nr, error, i;
+	int touch_nr, softbutton, error, i;
+	bool softbutton_pressed = false;
 
 	error = i2c_smbus_read_i2c_block_data(client, SILEAD_REG_DATA,
 					      SILEAD_TS_DATA_LEN, buf);
@@ -148,21 +152,40 @@ static void silead_ts_read_data(struct i2c_client *client)
 		return;
 	}
 
-	touch_nr = buf[0];
-	if (touch_nr > data->max_fingers) {
+	if (buf[0] > data->max_fingers) {
 		dev_warn(dev, "More touches reported then supported %d > %d\n",
-			 touch_nr, data->max_fingers);
-		touch_nr = data->max_fingers;
+			 buf[0], data->max_fingers);
+		buf[0] = data->max_fingers;
 	}
 
+	touch_nr = 0;
 	bufp = buf + SILEAD_POINT_DATA_LEN;
-	for (i = 0; i < touch_nr; i++, bufp += SILEAD_POINT_DATA_LEN) {
-		/* Bits 4-7 are the touch id */
-		data->id[i] = (bufp[SILEAD_POINT_X_MSB_OFF] &
-			       SILEAD_TOUCH_ID_MASK) >> 4;
-		touchscreen_set_mt_pos(&data->pos[i], &data->prop,
+	for (i = 0; i < buf[0]; i++, bufp += SILEAD_POINT_DATA_LEN) {
+		softbutton = (bufp[SILEAD_POINT_Y_MSB_OFF] &
+			      SILEAD_EXTRA_DATA_MASK) >> 4;
+
+		if (softbutton) {
+			/*
+			 * For now only respond to softbutton == 0x01, some
+			 * tablets *without* a capacative button send 0x04
+			 * when crossing the edges of the screen.
+			 */
+			if (softbutton == 0x01)
+				softbutton_pressed = true;
+
+			continue;
+		}
+
+		/*
+		 * Bits 4-7 are the touch id, note not all models have
+		 * hardware touch ids so atm we don't use these.
+		 */
+		data->id[touch_nr] = (bufp[SILEAD_POINT_X_MSB_OFF] &
+				      SILEAD_EXTRA_DATA_MASK) >> 4;
+		touchscreen_set_mt_pos(&data->pos[touch_nr], &data->prop,
 			get_unaligned_le16(&bufp[SILEAD_POINT_X_OFF]) & 0xfff,
 			get_unaligned_le16(&bufp[SILEAD_POINT_Y_OFF]) & 0xfff);
+		touch_nr++;
 	}
 
 	input_mt_assign_slots(input, data->slots, data->pos, touch_nr, 0);
@@ -178,6 +201,7 @@ static void silead_ts_read_data(struct i2c_client *client)
 	}
 
 	input_mt_sync_frame(input);
+	input_report_key(input, KEY_LEFTMETA, softbutton_pressed);
 	input_sync(input);
 }