rtc: initialize emulated RTC from environment variable

Up to now the emulated RTC is initialized using the U-Boot build time.

With this patch the environment variable 'rtc_emul_epoch' can be used to
provide a better initial time. The variable is a decimal string with
the number of seconds since 1970-01-01. Here is an example where the RTC
had not been probed yet:

    => setenv rtc_emul_epoch 1610109000
    => date
    Date: 2021-01-08 (Friday)    Time: 12:30:00

If the variable does not exist, the U-Boot build time is used as fallback.

The environment variable may be set when shutting down the operating system
if the U-Boot environment is exposed to the OS (cf. ENV_IS_IN_FAT and
ENV_IS_IN_EXT4).

Suggested-by: Pablo Sebastián Greco <pgreco@centosproject.org>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

2 files changed, 21 insertions, 8 deletions

diff --git a/drivers/rtc/Kconfig b/drivers/rtc/Kconfig
index d06d272e14..cad667a404 100644
--- a/drivers/rtc/Kconfig
+++ b/drivers/rtc/Kconfig
@@ -68,11 +68,14 @@ config RTC_EMULATION
 	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.
+	  used. The initial time may be provided via the environment variable
+	  'rtc_emul_epoch' as a decimal string indicating seconds since
+	  1970-01-01. If the environment variable is missing, the build time is
+	  used to initialize the RTC. The time can be adjusted manually via the
+	  'date' command or the 'sntp' command can be used 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"
diff --git a/drivers/rtc/emul_rtc.c b/drivers/rtc/emul_rtc.c
index 209b4965c1..7e522103fd 100644
--- a/drivers/rtc/emul_rtc.c
+++ b/drivers/rtc/emul_rtc.c
@@ -8,6 +8,7 @@
 #include <common.h>
 #include <div64.h>
 #include <dm.h>
+#include <env.h>
 #include <generated/timestamp_autogenerated.h>
 #include <rtc.h>
 
@@ -60,9 +61,18 @@ static int emul_rtc_set(struct udevice *dev, const struct rtc_time *time)
 int emul_rtc_probe(struct udevice *dev)
 {
 	struct emul_rtc *priv = dev_get_priv(dev);
-
-	/* Use the build date as initial time */
-	priv->offset_us = U_BOOT_EPOCH * 1000000ULL - timer_get_us();
+	const char *epoch_str;
+	u64 epoch;
+
+	epoch_str = env_get("rtc_emul_epoch");
+
+	if (epoch_str) {
+		epoch = simple_strtoull(epoch_str, NULL, 10);
+	} else {
+		/* Use the build date as initial time */
+		epoch = U_BOOT_EPOCH;
+	}
+	priv->offset_us = epoch * 1000000ULL - timer_get_us();
 	priv->isdst = -1;
 
 	return 0;