rtc: ds1307: add offset sysfs for mt41txx chips.

m41txx chips can hold a calibration value to get correct clock bias.

Add offset handling (ranging between -63ppm and 126ppm) via sysfs.
Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-ds1307.c

@@ -114,6 +114,20 @@ enum ds_type {
 #	define RX8025_BIT_VDET		0x40
 #	define RX8025_BIT_XST		0x20
 
+#define M41TXX_REG_CONTROL	0x07
+#	define M41TXX_BIT_OUT		BIT(7)
+#	define M41TXX_BIT_FT		BIT(6)
+#	define M41TXX_BIT_CALIB_SIGN	BIT(5)
+#	define M41TXX_M_CALIBRATION	GENMASK(4, 0)
+
+/* negative offset step is -2.034ppm */
+#define M41TXX_NEG_OFFSET_STEP_PPB	2034
+/* positive offset step is +4.068ppm */
+#define M41TXX_POS_OFFSET_STEP_PPB	4068
+/* Min and max values supported with 'offset' interface by M41TXX */
+#define M41TXX_MIN_OFFSET	((-31) * M41TXX_NEG_OFFSET_STEP_PPB)
+#define M41TXX_MAX_OFFSET	((31) * M41TXX_POS_OFFSET_STEP_PPB)
+
 struct ds1307 {
 	enum ds_type		type;
 	unsigned long		flags;
@@ -146,6 +160,9 @@ struct chip_desc {
 
 static int ds1307_get_time(struct device *dev, struct rtc_time *t);
 static int ds1307_set_time(struct device *dev, struct rtc_time *t);
+static int ds1337_read_alarm(struct device *dev, struct rtc_wkalrm *t);
+static int ds1337_set_alarm(struct device *dev, struct rtc_wkalrm *t);
+static int ds1307_alarm_irq_enable(struct device *dev, unsigned int enabled);
 static u8 do_trickle_setup_ds1339(struct ds1307 *, u32 ohms, bool diode);
 static irqreturn_t rx8130_irq(int irq, void *dev_id);
 static int rx8130_read_alarm(struct device *dev, struct rtc_wkalrm *t);
@@ -155,6 +172,8 @@ static irqreturn_t mcp794xx_irq(int irq, void *dev_id);
 static int mcp794xx_read_alarm(struct device *dev, struct rtc_wkalrm *t);
 static int mcp794xx_set_alarm(struct device *dev, struct rtc_wkalrm *t);
 static int mcp794xx_alarm_irq_enable(struct device *dev, unsigned int enabled);
+static int m41txx_rtc_read_offset(struct device *dev, long *offset);
+static int m41txx_rtc_set_offset(struct device *dev, long offset);
 
 static const struct rtc_class_ops rx8130_rtc_ops = {
 	.read_time      = ds1307_get_time,
@@ -172,6 +191,16 @@ static const struct rtc_class_ops mcp794xx_rtc_ops = {
 	.alarm_irq_enable = mcp794xx_alarm_irq_enable,
 };
 
+static const struct rtc_class_ops m41txx_rtc_ops = {
+	.read_time      = ds1307_get_time,
+	.set_time       = ds1307_set_time,
+	.read_alarm	= ds1337_read_alarm,
+	.set_alarm	= ds1337_set_alarm,
+	.alarm_irq_enable = ds1307_alarm_irq_enable,
+	.read_offset	= m41txx_rtc_read_offset,
+	.set_offset	= m41txx_rtc_set_offset,
+};
+
 static const struct chip_desc chips[last_ds_type] = {
 	[ds_1307] = {
 		.nvram_offset	= 8,
@@ -228,10 +257,17 @@ static const struct chip_desc chips[last_ds_type] = {
 		.irq_handler = rx8130_irq,
 		.rtc_ops = &rx8130_rtc_ops,
 	},
+	[m41t0] = {
+		.rtc_ops	= &m41txx_rtc_ops,
+	},
+	[m41t00] = {
+		.rtc_ops	= &m41txx_rtc_ops,
+	},
 	[m41t11] = {
 		/* this is battery backed SRAM */
 		.nvram_offset	= 8,
 		.nvram_size	= 56,
+		.rtc_ops	= &m41txx_rtc_ops,
 	},
 	[mcp794xx] = {
 		.alarm		= 1,
@@ -973,6 +1009,47 @@ static int mcp794xx_alarm_irq_enable(struct device *dev, unsigned int enabled)
 				  enabled ? MCP794XX_BIT_ALM0_EN : 0);
 }
 
+static int m41txx_rtc_read_offset(struct device *dev, long *offset)
+{
+	struct ds1307 *ds1307 = dev_get_drvdata(dev);
+	unsigned int ctrl_reg;
+	u8 val;
+
+	regmap_read(ds1307->regmap, M41TXX_REG_CONTROL, &ctrl_reg);
+
+	val = ctrl_reg & M41TXX_M_CALIBRATION;
+
+	/* check if positive */
+	if (ctrl_reg & M41TXX_BIT_CALIB_SIGN)
+		*offset = (val * M41TXX_POS_OFFSET_STEP_PPB);
+	else
+		*offset = -(val * M41TXX_NEG_OFFSET_STEP_PPB);
+
+	return 0;
+}
+
+static int m41txx_rtc_set_offset(struct device *dev, long offset)
+{
+	struct ds1307 *ds1307 = dev_get_drvdata(dev);
+	unsigned int ctrl_reg;
+
+	if ((offset < M41TXX_MIN_OFFSET) || (offset > M41TXX_MAX_OFFSET))
+		return -ERANGE;
+
+	if (offset >= 0) {
+		ctrl_reg = DIV_ROUND_CLOSEST(offset,
+					     M41TXX_POS_OFFSET_STEP_PPB);
+		ctrl_reg |= M41TXX_BIT_CALIB_SIGN;
+	} else {
+		ctrl_reg = DIV_ROUND_CLOSEST(abs(offset),
+					     M41TXX_NEG_OFFSET_STEP_PPB);
+	}
+
+	return regmap_update_bits(ds1307->regmap, M41TXX_REG_CONTROL,
+				  M41TXX_M_CALIBRATION | M41TXX_BIT_CALIB_SIGN,
+				  ctrl_reg);
+}
+
 /*----------------------------------------------------------------------*/
 
 static int ds1307_nvram_read(void *priv, unsigned int offset, void *val,