rtc: provide an emulated RTC

On a board without hardware clock this software real time clock can be
used. The build time is used to initialize the RTC. So you will have
to adjust the time either manually using the 'date' command  or use
the 'sntp' to update the RTC with the time from a network time server.
See CONFIG_CMD_SNTP and CONFIG_BOOTP_NTPSERVER. The RTC time is
advanced according to CPU ticks.
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>

MAINTAINERS

@@ -678,6 +678,7 @@ S:	Maintained
 T:	git https://gitlab.denx.de/u-boot/custodians/u-boot-efi.git
 F:	doc/api/efi.rst
 F:	doc/uefi/*
+F:	drivers/rtc/emul_rtc.c
 F:	include/capitalization.h
 F:	include/charset.h
 F:	include/cp1250.h

drivers/rtc/Kconfig

@@ -63,6 +63,17 @@ config RTC_DS3232
 	  Support for Dallas Semiconductor (now Maxim) DS3232 compatible
 	  Real Time Clock devices.
 
+config RTC_EMULATION
+	bool "Enable emulated RTC"
+	depends on DM_RTC
+	help
+	  On a board without hardware clock this software real time clock can be
+	  used. The build time is used to initialize the RTC. So you will have
+	  to adjust the time either manually using the 'date' command  or use
+	  the 'sntp' to update the RTC with the time from a network time server.
+	  See CONFIG_CMD_SNTP and CONFIG_BOOTP_NTPSERVER. The RTC time is
+	  advanced according to CPU ticks.
+
 config RTC_ISL1208
 	bool "Enable ISL1208 driver"
 	depends on DM_RTC

drivers/rtc/Makefile

@@ -22,6 +22,7 @@ obj-$(CONFIG_RTC_DS164x) += ds164x.o
 obj-$(CONFIG_RTC_DS174x) += ds174x.o
 obj-$(CONFIG_RTC_DS3231) += ds3231.o
 obj-$(CONFIG_RTC_DS3232) += ds3232.o
+obj-$(CONFIG_RTC_EMULATION) += emul_rtc.o
 obj-$(CONFIG_RTC_FTRTC010) += ftrtc010.o
 obj-$(CONFIG_SANDBOX) += i2c_rtc_emul.o
 obj-$(CONFIG_RTC_IMXDI) += imxdi.o

drivers/rtc/emul_rtc.c

+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright 2020, Heinrich Schuchardt <xypron.glpk@gmx.de>
+ *
+ * This driver emulates a real time clock based on timer ticks.
+ */
+
+#include <common.h>
+#include <div64.h>
+#include <dm.h>
+#include <generated/timestamp_autogenerated.h>
+#include <rtc.h>
+
+/**
+ * struct emul_rtc - private data for emulated RTC driver
+ */
+struct emul_rtc {
+	/**
+	 * @offset_us: microseconds from 1970-01-01 to timer_get_us() base
+	 */
+	u64 offset_us;
+	/**
+	 * @isdst: daylight saving time
+	 */
+	int isdst;
+};
+
+static int emul_rtc_get(struct udevice *dev, struct rtc_time *time)
+{
+	struct emul_rtc *priv = dev_get_priv(dev);
+	u64 now;
+
+	if (!priv->offset_us) {
+		/* Use the build date as initial time */
+		priv->offset_us = U_BOOT_EPOCH * 1000000ULL - timer_get_us();
+		priv->isdst = -1;
+	}
+
+	now = timer_get_us() + priv->offset_us;
+	do_div(now, 1000000);
+	rtc_to_tm(now, time);
+	time->tm_isdst = priv->isdst;
+
+	return 0;
+}
+
+static int emul_rtc_set(struct udevice *dev, const struct rtc_time *time)
+{
+	struct emul_rtc *priv = dev_get_priv(dev);
+
+	if (time->tm_year < 1970)
+		return -EINVAL;
+
+	priv->offset_us = rtc_mktime(time) * 1000000ULL - timer_get_us();
+
+	if (time->tm_isdst > 0)
+		priv->isdst = 1;
+	else if (time->tm_isdst < 0)
+		priv->isdst = -1;
+	else
+		priv->isdst = 0;
+
+	return 0;
+}
+
+static const struct rtc_ops emul_rtc_ops = {
+	.get = emul_rtc_get,
+	.set = emul_rtc_set,
+};
+
+U_BOOT_DRIVER(rtc_emul) = {
+	.name	= "rtc_emul",
+	.id	= UCLASS_RTC,
+	.ops	= &emul_rtc_ops,
+	.priv_auto_alloc_size = sizeof(struct emul_rtc),
+};
+
+U_BOOT_DEVICE(rtc_emul) = {
+	.name	= "rtc_emul",
+};