Input: elants_i2c - provide an attribute to show calibration count

There is an non-touch case by non-calibration after update firmware.
Elan could know calibrate or not by calibration count.
The value of '0xffff' means we didn't calibrate after update firmware.
If calibrate success, it will plus one and change to '0x0000'.
Signed-off-by: Johnny Chuang <johnny.chuang@emc.com.tw>
Link: https://lore.kernel.org/r/1588754932-5902-1-git-send-email-johnny.chuang.emc@gmail.comSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/touchscreen/elants_i2c.c

@@ -87,6 +87,7 @@
 /* FW read command, 0x53 0x?? 0x0, 0x01 */
 #define E_ELAN_INFO_FW_VER	0x00
 #define E_ELAN_INFO_BC_VER	0x10
+#define E_ELAN_INFO_REK		0xE0
 #define E_ELAN_INFO_TEST_VER	0xE0
 #define E_ELAN_INFO_FW_ID	0xF0
 #define E_INFO_OSR		0xD6
@@ -1056,8 +1057,32 @@ static ssize_t show_iap_mode(struct device *dev,
 				"Normal" : "Recovery");
 }
 
+static ssize_t show_calibration_count(struct device *dev,
+				      struct device_attribute *attr, char *buf)
+{
+	struct i2c_client *client = to_i2c_client(dev);
+	const u8 cmd[] = { CMD_HEADER_READ, E_ELAN_INFO_REK, 0x00, 0x01 };
+	u8 resp[HEADER_SIZE];
+	u16 rek_count;
+	int error;
+
+	error = elants_i2c_execute_command(client, cmd, sizeof(cmd),
+						resp, sizeof(resp));
+	if (error) {
+		dev_err(&client->dev,
+			"read ReK status error=%d, buf=%*phC\n",
+			error, (int)sizeof(resp), resp);
+		return sprintf(buf, "%d\n", error);
+	}
+
+	rek_count = get_unaligned_be16(&resp[2]);
+
+	return sprintf(buf, "0x%04x\n", rek_count);
+}
+
 static DEVICE_ATTR_WO(calibrate);
 static DEVICE_ATTR(iap_mode, S_IRUGO, show_iap_mode, NULL);
+static DEVICE_ATTR(calibration_count, S_IRUGO, show_calibration_count, NULL);
 static DEVICE_ATTR(update_fw, S_IWUSR, NULL, write_update_fw);
 
 struct elants_version_attribute {
@@ -1113,6 +1138,7 @@ static struct attribute *elants_attributes[] = {
 	&dev_attr_calibrate.attr,
 	&dev_attr_update_fw.attr,
 	&dev_attr_iap_mode.attr,
+	&dev_attr_calibration_count.attr,
 
 	&elants_ver_attr_fw_version.dattr.attr,
 	&elants_ver_attr_hw_version.dattr.attr,