rtc: isl12022: implement RTC_VL_READ ioctl

Hook up support for reading the values of the SR_LBAT85 and SR_LBAT75
bits. Translate the former to "battery low", and the latter to
"battery empty or not-present".
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/r/20230615105826.411953-6-linux@rasmusvillemoes.dkSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-isl12022.c

@@ -204,7 +204,34 @@ static int isl12022_rtc_set_time(struct device *dev, struct rtc_time *tm)
 	return regmap_bulk_write(regmap, ISL12022_REG_SC, buf, sizeof(buf));
 }
 
+static int isl12022_rtc_ioctl(struct device *dev, unsigned int cmd, unsigned long arg)
+{
+	struct regmap *regmap = dev_get_drvdata(dev);
+	u32 user, val;
+	int ret;
+
+	switch (cmd) {
+	case RTC_VL_READ:
+		ret = regmap_read(regmap, ISL12022_REG_SR, &val);
+		if (ret)
+			return ret;
+
+		user = 0;
+		if (val & ISL12022_SR_LBAT85)
+			user |= RTC_VL_BACKUP_LOW;
+
+		if (val & ISL12022_SR_LBAT75)
+			user |= RTC_VL_BACKUP_EMPTY;
+
+		return put_user(user, (u32 __user *)arg);
+
+	default:
+		return -ENOIOCTLCMD;
+	}
+}
+
 static const struct rtc_class_ops isl12022_rtc_ops = {
+	.ioctl		= isl12022_rtc_ioctl,
 	.read_time	= isl12022_rtc_read_time,
 	.set_time	= isl12022_rtc_set_time,
 };