rtc: New driver for RTC in Netronix embedded controller

With this driver, mainline Linux can keep its time and date in sync with
the vendor kernel.

Advanced functionality like alarm and automatic power-on is not yet
supported.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/rtc/Kconfig

@@ -1296,6 +1296,14 @@ config RTC_DRV_CROS_EC
 	  This driver can also be built as a module. If so, the module
 	  will be called rtc-cros-ec.
 
+config RTC_DRV_NTXEC
+	tristate "Netronix embedded controller RTC"
+	depends on MFD_NTXEC
+	help
+	  Say yes here if you want to support the RTC functionality of the
+	  embedded controller found in certain e-book readers designed by the
+	  original design manufacturer Netronix.
+
 comment "on-CPU RTC drivers"
 
 config RTC_DRV_ASM9260

drivers/rtc/Makefile

@@ -108,6 +108,7 @@ obj-$(CONFIG_RTC_DRV_MT7622)	+= rtc-mt7622.o
 obj-$(CONFIG_RTC_DRV_MV)	+= rtc-mv.o
 obj-$(CONFIG_RTC_DRV_MXC)	+= rtc-mxc.o
 obj-$(CONFIG_RTC_DRV_MXC_V2)	+= rtc-mxc_v2.o
+obj-$(CONFIG_RTC_DRV_NTXEC)	+= rtc-ntxec.o
 obj-$(CONFIG_RTC_DRV_OMAP)	+= rtc-omap.o
 obj-$(CONFIG_RTC_DRV_OPAL)	+= rtc-opal.o
 obj-$(CONFIG_RTC_DRV_PALMAS)	+= rtc-palmas.o

drivers/rtc/rtc-ntxec.c

@@ -0,0 +1,145 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * The Netronix embedded controller is a microcontroller found in some
+ * e-book readers designed by the original design manufacturer Netronix, Inc.
+ * It contains RTC, battery monitoring, system power management, and PWM
+ * functionality.
+ *
+ * This driver implements access to the RTC time and date.
+ *
+ * Copyright 2020 Jonathan Neuschäfer <j.neuschaefer@gmx.net>
+ */
+
+#include <linux/mfd/ntxec.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <linux/regmap.h>
+#include <linux/rtc.h>
+#include <linux/types.h>
+
+struct ntxec_rtc {
+	struct device *dev;
+	struct ntxec *ec;
+};
+
+#define NTXEC_REG_WRITE_YEAR	0x10
+#define NTXEC_REG_WRITE_MONTH	0x11
+#define NTXEC_REG_WRITE_DAY	0x12
+#define NTXEC_REG_WRITE_HOUR	0x13
+#define NTXEC_REG_WRITE_MINUTE	0x14
+#define NTXEC_REG_WRITE_SECOND	0x15
+
+#define NTXEC_REG_READ_YEAR_MONTH	0x20
+#define NTXEC_REG_READ_MDAY_HOUR	0x21
+#define NTXEC_REG_READ_MINUTE_SECOND	0x23
+
+static int ntxec_read_time(struct device *dev, struct rtc_time *tm)
+{
+	struct ntxec_rtc *rtc = dev_get_drvdata(dev);
+	unsigned int value;
+	int res;
+
+retry:
+	res = regmap_read(rtc->ec->regmap, NTXEC_REG_READ_MINUTE_SECOND, &value);
+	if (res < 0)
+		return res;
+
+	tm->tm_min = value >> 8;
+	tm->tm_sec = value & 0xff;
+
+	res = regmap_read(rtc->ec->regmap, NTXEC_REG_READ_MDAY_HOUR, &value);
+	if (res < 0)
+		return res;
+
+	tm->tm_mday = value >> 8;
+	tm->tm_hour = value & 0xff;
+
+	res = regmap_read(rtc->ec->regmap, NTXEC_REG_READ_YEAR_MONTH, &value);
+	if (res < 0)
+		return res;
+
+	tm->tm_year = (value >> 8) + 100;
+	tm->tm_mon = (value & 0xff) - 1;
+
+	/*
+	 * Read the minutes/seconds field again. If it changed since the first
+	 * read, we can't assume that the values read so far are consistent,
+	 * and should start from the beginning.
+	 */
+	res = regmap_read(rtc->ec->regmap, NTXEC_REG_READ_MINUTE_SECOND, &value);
+	if (res < 0)
+		return res;
+
+	if (tm->tm_min != value >> 8 || tm->tm_sec != (value & 0xff))
+		goto retry;
+
+	return 0;
+}
+
+static int ntxec_set_time(struct device *dev, struct rtc_time *tm)
+{
+	struct ntxec_rtc *rtc = dev_get_drvdata(dev);
+
+	/*
+	 * To avoid time overflows while we're writing the full date/time,
+	 * set the seconds field to zero before doing anything else. For the
+	 * next 59 seconds (plus however long it takes until the RTC's next
+	 * update of the second field), the seconds field will not overflow
+	 * into the other fields.
+	 */
+	struct reg_sequence regs[] = {
+		{ NTXEC_REG_WRITE_SECOND, ntxec_reg8(0) },
+		{ NTXEC_REG_WRITE_YEAR, ntxec_reg8(tm->tm_year - 100) },
+		{ NTXEC_REG_WRITE_MONTH, ntxec_reg8(tm->tm_mon + 1) },
+		{ NTXEC_REG_WRITE_DAY, ntxec_reg8(tm->tm_mday) },
+		{ NTXEC_REG_WRITE_HOUR, ntxec_reg8(tm->tm_hour) },
+		{ NTXEC_REG_WRITE_MINUTE, ntxec_reg8(tm->tm_min) },
+		{ NTXEC_REG_WRITE_SECOND, ntxec_reg8(tm->tm_sec) },
+	};
+
+	return regmap_multi_reg_write(rtc->ec->regmap, regs, ARRAY_SIZE(regs));
+}
+
+static const struct rtc_class_ops ntxec_rtc_ops = {
+	.read_time = ntxec_read_time,
+	.set_time = ntxec_set_time,
+};
+
+static int ntxec_rtc_probe(struct platform_device *pdev)
+{
+	struct rtc_device *dev;
+	struct ntxec_rtc *rtc;
+
+	pdev->dev.of_node = pdev->dev.parent->of_node;
+
+	rtc = devm_kzalloc(&pdev->dev, sizeof(*rtc), GFP_KERNEL);
+	if (!rtc)
+		return -ENOMEM;
+
+	rtc->dev = &pdev->dev;
+	rtc->ec = dev_get_drvdata(pdev->dev.parent);
+	platform_set_drvdata(pdev, rtc);
+
+	dev = devm_rtc_allocate_device(&pdev->dev);
+	if (IS_ERR(dev))
+		return PTR_ERR(dev);
+
+	dev->ops = &ntxec_rtc_ops;
+	dev->range_min = RTC_TIMESTAMP_BEGIN_2000;
+	dev->range_max = 9025257599LL; /* 2255-12-31 23:59:59 */
+
+	return devm_rtc_register_device(dev);
+}
+
+static struct platform_driver ntxec_rtc_driver = {
+	.driver = {
+		.name = "ntxec-rtc",
+	},
+	.probe = ntxec_rtc_probe,
+};
+module_platform_driver(ntxec_rtc_driver);
+
+MODULE_AUTHOR("Jonathan Neuschäfer <j.neuschaefer@gmx.net>");
+MODULE_DESCRIPTION("RTC driver for Netronix EC");
+MODULE_LICENSE("GPL");
+MODULE_ALIAS("platform:ntxec-rtc");