rtc: rv3029: add RTC_VL_READ and RTC_VL_CLEAR support

The RV3029 can report three different conditions: power on, voltage dropped
and data is lost and voltage is low and temperature compensation has
stopped. The first two conditions amount to the same status, the RTC data
is invalid.

VLOW1 has to be cleared manually to resume temperature compensation, this
is achieved using RTC_VL_CLEAR.

Link: https://lore.kernel.org/r/20191214221022.622482-11-alexandre.belloni@bootlin.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-rv3029c2.c

@@ -459,6 +459,35 @@ static int rv3029_set_time(struct device *dev, struct rtc_time *tm)
 				  RV3029_STATUS_PON, 0);
 }
 
+static int rv3029_ioctl(struct device *dev, unsigned int cmd, unsigned long arg)
+{
+	struct rv3029_data *rv3029 = dev_get_drvdata(dev);
+	unsigned long vl = 0;
+	int sr, ret = 0;
+
+	switch (cmd) {
+	case RTC_VL_READ:
+		ret = regmap_read(rv3029->regmap, RV3029_STATUS, &sr);
+		if (ret < 0)
+			return ret;
+
+		if (sr & RV3029_STATUS_VLOW1)
+			vl = RTC_VL_ACCURACY_LOW;
+
+		if (sr & (RV3029_STATUS_VLOW2 | RV3029_STATUS_PON))
+			vl |= RTC_VL_DATA_INVALID;
+
+		return put_user(vl, (unsigned int __user *)arg);
+
+	case RTC_VL_CLR:
+		return regmap_update_bits(rv3029->regmap, RV3029_STATUS,
+					  RV3029_STATUS_VLOW1, 0);
+
+	default:
+		return -ENOIOCTLCMD;
+	}
+}
+
 static const struct rv3029_trickle_tab_elem {
 	u32 r;		/* resistance in ohms */
 	u8 conf;	/* trickle config bits */
@@ -672,6 +701,7 @@ static void rv3029_hwmon_register(struct device *dev, const char *name)
 static struct rtc_class_ops rv3029_rtc_ops = {
 	.read_time	= rv3029_read_time,
 	.set_time	= rv3029_set_time,
+	.ioctl		= rv3029_ioctl,
 };
 
 static int rv3029_probe(struct device *dev, struct regmap *regmap, int irq,