m41txx chips can hold a calibration value to get really near to real
tick value.

Add calibration property(ranging between (-31) and 31), so on every probe
calibration value will be written to rtc.
This is because ic could loose supply due to low battery.

Signed-off-by: Giulio Benetti <redacted>
---
 .../devicetree/bindings/rtc/rtc-ds1307.txt    |  2 ++
 drivers/rtc/rtc-ds1307.c                      | 33 +++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/Documentation/devicetree/bindings/rtc/rtc-ds1307.txt b/Documentation/devicetree/bindings/rtc/rtc-ds1307.txt
index ce6469c1a516..d3d70a5495c5 100644
--- a/Documentation/devicetree/bindings/rtc/rtc-ds1307.txt
+++ b/Documentation/devicetree/bindings/rtc/rtc-ds1307.txt

@@ -34,6 +34,8 @@ Optional properties:
 - trickle-diode-disable : ds1339, ds1340 and ds 1388 only
 	Do not use internal trickle charger diode
 	Should be given if internal trickle charger diode should be disabled
+- calibration: m41t0, m41t00, m41t11 only
+	Set calibration value to correct external bias, ranging between (-31) and 31.
 
 Example:
 	rtc1: ds1339@68 {

diff --git a/drivers/rtc/rtc-ds1307.c b/drivers/rtc/rtc-ds1307.c
index 0ab0c166da83..9cda52589c0f 100644
--- a/drivers/rtc/rtc-ds1307.c
+++ b/drivers/rtc/rtc-ds1307.c

@@ -114,6 +114,12 @@ enum ds_type {
 #	define RX8025_BIT_VDET		0x40
 #	define RX8025_BIT_XST		0x20
 
+#define M41TXX_REG_CONTROL	0x07
+#	define M41TXX_BIT_OUT		0x80
+#	define M41TXX_BIT_FT		0x40
+#	define M41TXX_BIT_CALIB_SIGN	0x20
+#	define M41TXX_M_CALIBRATION	0x1f
+
 struct ds1307 {
 	enum ds_type		type;
 	unsigned long		flags;

@@ -1397,6 +1403,7 @@ static int ds1307_probe(struct i2c_client *client,
 	unsigned char		regs[8];
 	struct ds1307_platform_data *pdata = dev_get_platdata(&client->dev);
 	u8			trickle_charger_setup = 0;
+	s32			calib;
 
 	ds1307 = devm_kzalloc(&client->dev, sizeof(struct ds1307), GFP_KERNEL);
 	if (!ds1307)

@@ -1460,6 +1467,32 @@ static int ds1307_probe(struct i2c_client *client,
 	if (chip->alarm && of_property_read_bool(client->dev.of_node,
 						 "wakeup-source"))
 		ds1307_can_wakeup_device = true;
+
+	/* retrieve calibration value if provided */
+	if (!of_property_read_s32(client->dev.of_node, "calibration",
+				 &calib)) {
+		switch (ds1307->type) {
+		case m41t0:
+		case m41t00:
+		case m41t11:
+		{
+			/*
+			 * Set calibration value every power-on since rtc
+			 * could have shut off(low battery)
+			 */
+			u8 out_byte = abs(calib) & M41TXX_M_CALIBRATION;
+
+			if (calib >= 0)
+				out_byte |= M41TXX_BIT_CALIB_SIGN;
+
+			regmap_write(ds1307->regmap, M41TXX_REG_CONTROL,
+					out_byte);
+		}
+			break;
+		default:
+			break;
+		}
+	}
 #endif
 
 	switch (ds1307->type) {

-- 
2.17.0