rtc: spacemit: support the SpacemiT P1 RTC

Add support for the RTC found in the SpacemiT P1 PMIC.  Initially
only setting and reading the time are supported.

The PMIC is implemented as a multi-function device.  This RTC is
probed based on this driver being named in a MFD cell in the simple
MFD I2C driver.

Signed-off-by: Alex Elder <elder@riscstar.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20250825172057.163883-5-elder@riscstar.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/Kconfig

@@ -406,6 +406,16 @@ config RTC_DRV_MAX77686
 	  This driver can also be built as a module. If so, the module
 	  will be called rtc-max77686.
 
+config RTC_DRV_SPACEMIT_P1
+	tristate "SpacemiT P1 RTC"
+	depends on ARCH_SPACEMIT || COMPILE_TEST
+	select MFD_SPACEMIT_P1
+	default ARCH_SPACEMIT
+	help
+	  Enable support for the RTC function in the SpacemiT P1 PMIC.
+	  This driver can also be built as a module, which will be called
+	  "spacemit-p1-rtc".
+
 config RTC_DRV_NCT3018Y
 	tristate "Nuvoton NCT3018Y"
 	depends on OF

drivers/rtc/Makefile

@@ -171,6 +171,7 @@ obj-$(CONFIG_RTC_DRV_SD2405AL)	+= rtc-sd2405al.o
 obj-$(CONFIG_RTC_DRV_SD3078)	+= rtc-sd3078.o
 obj-$(CONFIG_RTC_DRV_SH)	+= rtc-sh.o
 obj-$(CONFIG_RTC_DRV_SNVS)	+= rtc-snvs.o
+obj-$(CONFIG_RTC_DRV_SPACEMIT_P1)	+= rtc-spacemit-p1.o
 obj-$(CONFIG_RTC_DRV_SPEAR)	+= rtc-spear.o
 obj-$(CONFIG_RTC_DRV_STARFIRE)	+= rtc-starfire.o
 obj-$(CONFIG_RTC_DRV_STK17TA8)	+= rtc-stk17ta8.o

drivers/rtc/rtc-spacemit-p1.c

@@ -0,0 +1,167 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Driver for the RTC found in the SpacemiT P1 PMIC
+ *
+ * Copyright (C) 2025 by RISCstar Solutions Corporation.  All rights reserved.
+ */
+
+#include <linux/bits.h>
+#include <linux/device.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <linux/regmap.h>
+#include <linux/rtc.h>
+
+#define MOD_NAME	"spacemit-p1-rtc"
+
+/*
+ * Six consecutive 1-byte registers hold the seconds, minutes, hours,
+ * day-of-month, month, and year (respectively).
+ *
+ * The range of values in these registers is:
+ *    seconds	0-59
+ *    minutes	0-59
+ *    hours	0-59
+ *    day	0-30 (struct tm is 1-31)
+ *    month	0-11
+ *    year	years since 2000 (struct tm is since 1900)
+ *
+ * Note that the day and month must be converted after reading and
+ * before writing.
+ */
+#define RTC_TIME		0x0d	/* Offset of the seconds register */
+
+#define RTC_CTRL		0x1d
+#define RTC_EN		BIT(2)
+
+/* Number of attempts to read a consistent time stamp before giving up */
+#define RTC_READ_TRIES		20	/* At least 1 */
+
+struct p1_rtc {
+	struct regmap *regmap;
+	struct rtc_device *rtc;
+};
+
+/*
+ * The P1 hardware documentation states that the register values are
+ * latched to ensure a consistent time snapshot within the registers,
+ * but these are in fact unstable due to a bug in the hardware design.
+ * So we loop until we get two identical readings.
+ */
+static int p1_rtc_read_time(struct device *dev, struct rtc_time *t)
+{
+	struct p1_rtc *p1 = dev_get_drvdata(dev);
+	struct regmap *regmap = p1->regmap;
+	u32 count = RTC_READ_TRIES;
+	u8 seconds;
+	u8 time[6];
+	int ret;
+
+	if (!regmap_test_bits(regmap, RTC_CTRL, RTC_EN))
+		return -EINVAL;		/* RTC is disabled */
+
+	ret = regmap_bulk_read(regmap, RTC_TIME, time, sizeof(time));
+	if (ret)
+		return ret;
+
+	do {
+		seconds = time[0];
+		ret = regmap_bulk_read(regmap, RTC_TIME, time, sizeof(time));
+		if (ret)
+			return ret;
+	} while (time[0] != seconds && --count);
+
+	if (!count)
+		return -EIO;		/* Unable to get a consistent result */
+
+	t->tm_sec = time[0] & GENMASK(5, 0);
+	t->tm_min = time[1] & GENMASK(5, 0);
+	t->tm_hour = time[2] & GENMASK(4, 0);
+	t->tm_mday = (time[3] & GENMASK(4, 0)) + 1;
+	t->tm_mon = time[4] & GENMASK(3, 0);
+	t->tm_year = (time[5] & GENMASK(5, 0)) + 100;
+
+	return 0;
+}
+
+/*
+ * The P1 hardware documentation states that values in the registers are
+ * latched so when written they represent a consistent time snapshot.
+ * Nevertheless, this is not guaranteed by the implementation, so we must
+ * disable the RTC while updating it.
+ */
+static int p1_rtc_set_time(struct device *dev, struct rtc_time *t)
+{
+	struct p1_rtc *p1 = dev_get_drvdata(dev);
+	struct regmap *regmap = p1->regmap;
+	u8 time[6];
+	int ret;
+
+	time[0] = t->tm_sec;
+	time[1] = t->tm_min;
+	time[2] = t->tm_hour;
+	time[3] = t->tm_mday - 1;
+	time[4] = t->tm_mon;
+	time[5] = t->tm_year - 100;
+
+	/* Disable the RTC to update; re-enable again when done */
+	ret = regmap_clear_bits(regmap, RTC_CTRL, RTC_EN);
+	if (ret)
+		return ret;
+
+	/* If something goes wrong, leave the RTC disabled */
+	ret = regmap_bulk_write(regmap, RTC_TIME, time, sizeof(time));
+	if (ret)
+		return ret;
+
+	return regmap_set_bits(regmap, RTC_CTRL, RTC_EN);
+}
+
+static const struct rtc_class_ops p1_rtc_class_ops = {
+	.read_time = p1_rtc_read_time,
+	.set_time = p1_rtc_set_time,
+};
+
+static int p1_rtc_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct rtc_device *rtc;
+	struct p1_rtc *p1;
+
+	p1 = devm_kzalloc(dev, sizeof(*p1), GFP_KERNEL);
+	if (!p1)
+		return -ENOMEM;
+	dev_set_drvdata(dev, p1);
+
+	p1->regmap = dev_get_regmap(dev->parent, NULL);
+	if (!p1->regmap)
+		return dev_err_probe(dev, -ENODEV, "failed to get regmap\n");
+
+	rtc = devm_rtc_allocate_device(dev);
+	if (IS_ERR(rtc))
+		return dev_err_probe(dev, PTR_ERR(rtc),
+				     "error allocating device\n");
+	p1->rtc = rtc;
+
+	rtc->ops = &p1_rtc_class_ops;
+	rtc->range_min = RTC_TIMESTAMP_BEGIN_2000;
+	rtc->range_max = RTC_TIMESTAMP_END_2063;
+
+	clear_bit(RTC_FEATURE_ALARM, rtc->features);
+	clear_bit(RTC_FEATURE_UPDATE_INTERRUPT, rtc->features);
+
+	return devm_rtc_register_device(rtc);
+}
+
+static struct platform_driver p1_rtc_driver = {
+	.probe = p1_rtc_probe,
+	.driver = {
+		.name = MOD_NAME,
+	},
+};
+
+module_platform_driver(p1_rtc_driver);
+
+MODULE_DESCRIPTION("SpacemiT P1 RTC driver");
+MODULE_LICENSE("GPL");
+MODULE_ALIAS("platform:" MOD_NAME);