rtc: add driver for Marvell 88PM886 PMIC RTC

RTC lives on the chip's base register page. Add the relevant register
definitions and implement a basic set/read time functionality. Tested
with the samsung,coreprimevelte smartphone which contains this PMIC and
whose vendor kernel tree has also served as the sole reference for this.

Signed-off-by: Karel Balej <[e-mail]>
Acked-by: Lee Jones <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexandre Belloni <[e-mail]>

MAINTAINERS

···
 F:	drivers/input/misc/88pm886-onkey.c
 F:	drivers/mfd/88pm886.c
 F:	drivers/regulator/88pm886-regulator.c
+F:	drivers/rtc/rtc-88pm886.c
 F:	include/linux/mfd/88pm886.h
 
 MARVELL ARMADA 3700 PHY DRIVERS

drivers/rtc/Kconfig

···
 	  This driver can also be built as a module. If so, the module
 	  will be called rtc-88pm80x.
 
+config RTC_DRV_88PM886
+	tristate "Marvell 88PM886 RTC driver"
+	depends on MFD_88PM886_PMIC
+	help
+	  If you say yes here you will get support for the RTC function in the
+	  Marvell 88PM886 chip.
+
+	  This driver can also be built as a module. If so, the module
+	  will be called rtc-88pm886.
+
 config RTC_DRV_ABB5ZES3
 	select REGMAP_I2C
 	tristate "Abracon AB-RTCMC-32.768kHz-B5ZE-S3"

drivers/rtc/Makefile

···
 
 obj-$(CONFIG_RTC_DRV_88PM80X)	+= rtc-88pm80x.o
 obj-$(CONFIG_RTC_DRV_88PM860X)	+= rtc-88pm860x.o
+obj-$(CONFIG_RTC_DRV_88PM886)	+= rtc-88pm886.o
 obj-$(CONFIG_RTC_DRV_AB8500)	+= rtc-ab8500.o
 obj-$(CONFIG_RTC_DRV_ABB5ZES3)	+= rtc-ab-b5ze-s3.o
 obj-$(CONFIG_RTC_DRV_ABEOZ9)	+= rtc-ab-eoz9.o

drivers/rtc/rtc-88pm886.c

···
+// SPDX-License-Identifier: GPL-2.0-only
+#include <linux/limits.h>
+#include <linux/mod_devicetable.h>
+#include <linux/platform_device.h>
+#include <linux/rtc.h>
+
+#include <linux/mfd/88pm886.h>
+
+/*
+ * Time is calculated as the sum of a 32-bit read-only advancing counter and a
+ * writeable constant offset stored in the chip's spare registers.
+ */
+
+static int pm886_rtc_read_time(struct device *dev, struct rtc_time *tm)
+{
+	struct regmap *regmap = dev_get_drvdata(dev);
+	u32 time;
+	u32 buf;
+	int ret;
+
+	ret = regmap_bulk_read(regmap, PM886_REG_RTC_SPARE1, &buf, 4);
+	if (ret)
+		return ret;
+	time = buf;
+
+	ret = regmap_bulk_read(regmap, PM886_REG_RTC_CNT1, &buf, 4);
+	if (ret)
+		return ret;
+	time += buf;
+
+	rtc_time64_to_tm(time, tm);
+
+	return 0;
+}
+
+static int pm886_rtc_set_time(struct device *dev, struct rtc_time *tm)
+{
+	struct regmap *regmap = dev_get_drvdata(dev);
+	u32 buf;
+	int ret;
+
+	ret = regmap_bulk_read(regmap, PM886_REG_RTC_CNT1, &buf, 4);
+	if (ret)
+		return ret;
+
+	buf = rtc_tm_to_time64(tm) - buf;
+
+	return regmap_bulk_write(regmap, PM886_REG_RTC_SPARE1, &buf, 4);
+}
+
+static const struct rtc_class_ops pm886_rtc_ops = {
+	.read_time = pm886_rtc_read_time,
+	.set_time = pm886_rtc_set_time,
+};
+
+static int pm886_rtc_probe(struct platform_device *pdev)
+{
+	struct pm886_chip *chip = dev_get_drvdata(pdev->dev.parent);
+	struct device *dev = &pdev->dev;
+	struct rtc_device *rtc;
+	int ret;
+
+	platform_set_drvdata(pdev, chip->regmap);
+
+	rtc = devm_rtc_allocate_device(dev);
+	if (IS_ERR(rtc))
+		return dev_err_probe(dev, PTR_ERR(rtc),
+				"Failed to allocate RTC device\n");
+
+	rtc->ops = &pm886_rtc_ops;
+	rtc->range_max = U32_MAX;
+
+	ret = devm_rtc_register_device(rtc);
+	if (ret)
+		return dev_err_probe(dev, ret, "Failed to register RTC device\n");
+
+	return 0;
+}
+
+static const struct platform_device_id pm886_rtc_id_table[] = {
+	{ "88pm886-rtc", },
+	{ }
+};
+MODULE_DEVICE_TABLE(platform, pm886_rtc_id_table);
+
+static struct platform_driver pm886_rtc_driver = {
+	.driver = {
+		.name = "88pm886-rtc",
+	},
+	.probe = pm886_rtc_probe,
+	.id_table = pm886_rtc_id_table,
+};
+module_platform_driver(pm886_rtc_driver);
+
+MODULE_DESCRIPTION("Marvell 88PM886 RTC driver");
+MODULE_AUTHOR("Karel Balej <[e-mail]>");
+MODULE_LICENSE("GPL");

include/linux/mfd/88pm886.h

···
 #define PM886_INT_WC			BIT(1)
 #define PM886_INT_MASK_MODE		BIT(2)
 
+#define PM886_REG_RTC_CNT1		0xd1
+#define PM886_REG_RTC_CNT2		0xd2
+#define PM886_REG_RTC_CNT3		0xd3
+#define PM886_REG_RTC_CNT4		0xd4
+#define PM886_REG_RTC_SPARE1		0xea
+#define PM886_REG_RTC_SPARE2		0xeb
+#define PM886_REG_RTC_SPARE3		0xec
+#define PM886_REG_RTC_SPARE4		0xed
+#define PM886_REG_RTC_SPARE5		0xee
 #define PM886_REG_RTC_SPARE6		0xef
 
 #define PM886_REG_BUCK_EN		0x08