rtc: pcf2127: add BSM support

The pcf2127 encodes BSM, BLD and power fail detection in the same set of
bits so it is necessary to do some calculation when changing BSM to keep
the rest of the configuration as-is. However, when BSM is disabled, there
is no configuration with BLD enabled so this will be lost when coming back
to a mode with BSM enabled.

Link: https://lore.kernel.org/r/20250127162728.86234-1-alexandre.belloni@bootlin.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Author: alexandrebelloni

drivers/rtc/rtc-pcf2127.c

@@ -20,6 +20,7 @@
 #include <linux/i2c.h>
 #include <linux/spi/spi.h>
 #include <linux/bcd.h>
+#include <linux/bitfield.h>
 #include <linux/rtc.h>
 #include <linux/slab.h>
 #include <linux/module.h>
@@ -48,6 +49,7 @@
 #define PCF2127_BIT_CTRL3_BLF			BIT(2)
 #define PCF2127_BIT_CTRL3_BF			BIT(3)
 #define PCF2127_BIT_CTRL3_BTSE			BIT(4)
+#define PCF2127_CTRL3_PM			GENMASK(7, 5)
 /* Time and date registers */
 #define PCF2127_REG_TIME_BASE		0x03
 #define PCF2127_BIT_SC_OSF			BIT(7)
@@ -331,6 +333,84 @@ static int pcf2127_rtc_set_time(struct device *dev, struct rtc_time *tm)
 	return 0;
 }
 
+static int pcf2127_param_get(struct device *dev, struct rtc_param *param)
+{
+	struct pcf2127 *pcf2127 = dev_get_drvdata(dev);
+	u32 value;
+	int ret;
+
+	switch (param->param) {
+	case RTC_PARAM_BACKUP_SWITCH_MODE:
+		ret = regmap_read(pcf2127->regmap, PCF2127_REG_CTRL3, &value);
+		if (ret < 0)
+			return ret;
+
+		value = FIELD_GET(PCF2127_CTRL3_PM, value);
+
+		if (value < 0x3)
+			param->uvalue = RTC_BSM_LEVEL;
+		else if (value < 0x6)
+			param->uvalue = RTC_BSM_DIRECT;
+		else
+			param->uvalue = RTC_BSM_DISABLED;
+
+		break;
+
+	default:
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
+static int pcf2127_param_set(struct device *dev, struct rtc_param *param)
+{
+	struct pcf2127 *pcf2127 = dev_get_drvdata(dev);
+	u8 mode = 0;
+	u32 value;
+	int ret;
+
+	switch (param->param) {
+	case RTC_PARAM_BACKUP_SWITCH_MODE:
+		ret = regmap_read(pcf2127->regmap, PCF2127_REG_CTRL3, &value);
+		if (ret < 0)
+			return ret;
+
+		value = FIELD_GET(PCF2127_CTRL3_PM, value);
+
+		if (value > 5)
+			value -= 5;
+		else if (value > 2)
+			value -= 3;
+
+		switch (param->uvalue) {
+		case RTC_BSM_LEVEL:
+			break;
+		case RTC_BSM_DIRECT:
+			mode = 3;
+			break;
+		case RTC_BSM_DISABLED:
+			if (value == 0)
+				value = 1;
+			mode = 5;
+			break;
+		default:
+			return -EINVAL;
+		}
+
+		return regmap_update_bits(pcf2127->regmap, PCF2127_REG_CTRL3,
+					  PCF2127_CTRL3_PM,
+					  FIELD_PREP(PCF2127_CTRL3_PM, mode + value));
+
+		break;
+
+	default:
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
 static int pcf2127_rtc_ioctl(struct device *dev,
 				unsigned int cmd, unsigned long arg)
 {
@@ -741,6 +821,8 @@ static const struct rtc_class_ops pcf2127_rtc_ops = {
 	.read_alarm       = pcf2127_rtc_read_alarm,
 	.set_alarm        = pcf2127_rtc_set_alarm,
 	.alarm_irq_enable = pcf2127_rtc_alarm_irq_enable,
+	.param_get        = pcf2127_param_get,
+	.param_set        = pcf2127_param_set,
 };
 
 /* sysfs interface */