nrf: rtc: persist rtc offset across reboots

Store the RTC value in the .uninitialized section, but make sure to
flank it with some known values.  That way we can determine if the RTC
value has been initialized, or if it's random uninitialized garbage.

As part of this, add a `common_hal_rtc_init()` routine to determine if
the value is correct, or reset it to 0 if it is not valid.

Signed-off-by: Sean Cross <[email protected]>

Author: xobs

ports/nrf/common-hal/rtc/RTC.c

@@ -34,19 +34,42 @@
 #include "supervisor/shared/translate.h"
 
 // This is the time in seconds since 2000 that the RTC was started.
-static uint32_t rtc_offset = 0;
+__attribute__((section(".uninitialized"))) static uint32_t rtc_offset[3];
+
+// These values are placed before and after the current RTC count.  They are
+// used to determine if the RTC count is valid.  These randomly-generated values
+// will be set when the RTC value is set in order to mark the RTC as valid. If
+// the system crashes or reboots, these values will remain undisturbed and the
+// RTC offset will remain valid.
+//
+// If Circuit Python is updated or these symbols shift around, the prefix and
+// suffix will no longer match, and the time will no longer be valid.
+#define RTC_OFFSET_CHECK_PREFIX 0x25ea7e2a
+#define RTC_OFFSET_CHECK_SUFFIX 0x2b80b69e
+
+void common_hal_rtc_init(void) {
+    // If the prefix and suffix are not valid, zero-initialize the RTC offset.
+    if ((rtc_offset[0] != RTC_OFFSET_CHECK_PREFIX) || (rtc_offset[2] != RTC_OFFSET_CHECK_SUFFIX))
+        rtc_offset[1] = 0;
+}
 
 void common_hal_rtc_get_time(timeutils_struct_time_t *tm) {
     uint64_t ticks_s = port_get_raw_ticks(NULL) / 1024;
-    timeutils_seconds_since_2000_to_struct_time(rtc_offset + ticks_s, tm);
+    timeutils_seconds_since_2000_to_struct_time(rtc_offset[1] + ticks_s, tm);
 }
 
 void common_hal_rtc_set_time(timeutils_struct_time_t *tm) {
     uint64_t ticks_s = port_get_raw_ticks(NULL) / 1024;
     uint32_t epoch_s = timeutils_seconds_since_2000(
         tm->tm_year, tm->tm_mon, tm->tm_mday, tm->tm_hour, tm->tm_min, tm->tm_sec
     );
-    rtc_offset = epoch_s - ticks_s;
+    rtc_offset[1] = epoch_s - ticks_s;
+
+    // Set the prefix and suffix in order to indicate the time is valid.  This
+    // must be done after the offset is updated, in case there is a crash or
+    // power failure.
+    rtc_offset[0] = RTC_OFFSET_CHECK_PREFIX;
+    rtc_offset[2] = RTC_OFFSET_CHECK_SUFFIX;
 }
 
 int common_hal_rtc_get_calibration(void) {

ports/nrf/common-hal/rtc/RTC.h

@@ -29,5 +29,6 @@
 
 extern void rtc_init(void);
 extern void rtc_reset(void);
+extern void common_hal_rtc_init(void);
 
 #endif  // MICROPY_INCLUDED_NRF_COMMON_HAL_RTC_RTC_H