Merge pull request #2924 from simmel-project/nrf-rtc-persist

NRF: Persist RTC offset across reboots

Author: tannewt

ports/nrf/boards/common.template.ld

@@ -103,7 +103,7 @@ SECTIONS
         _edata = .;        /* define a global symbol at data end; used by startup code in order to initialise the .data section in RAM */
     } >RAM
 
-    /* Uninitialized data section */
+    /* Zero-initialized data section */
     .bss :
     {
         . = ALIGN(4);
@@ -116,6 +116,19 @@ SECTIONS
         _ebss = .;         /* define a global symbol at bss end; used by startup code and GC */
     } >RAM
 
+    /* Uninitialized data section
+    Data placed into this section will remain unchanged across reboots. */
+    .uninitialized (NOLOAD) :
+    {
+        . = ALIGN(4);
+        _suninitialized = .; /* define a global symbol at uninitialized start; currently unused */
+        *(.uninitialized)
+        *(.uninitialized*)
+
+        . = ALIGN(4);
+        _euninitialized = .; /* define a global symbol at uninitialized end; currently unused */
+    } >RAM
+
     /* this is to define the start of the heap, and make sure we have a minimum size */
     .heap :
     {

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

ports/nrf/supervisor/port.c

@@ -120,6 +120,10 @@ safe_mode_t port_init(void) {
     // Configure millisecond timer initialization.
     tick_init();
 
+#if CIRCUITPY_RTC
+    common_hal_rtc_init();
+#endif
+
 #if CIRCUITPY_ANALOGIO
     analogin_init();
 #endif
@@ -177,30 +181,35 @@ void reset_cpu(void) {
     NVIC_SystemReset();
 }
 
+// The uninitialized data section is placed directly after BSS, under the theory
+// that Circuit Python has a lot more .data and .bss than the bootloader.  As a
+// result, this section is less likely to be tampered with by the bootloader.
+extern uint32_t _euninitialized;
+
 uint32_t *port_heap_get_bottom(void) {
-    return port_stack_get_limit();
+    return &_euninitialized;
 }
 
 uint32_t *port_heap_get_top(void) {
     return port_stack_get_top();
 }
 
 uint32_t *port_stack_get_limit(void) {
-    return &_ebss;
+    return &_euninitialized;
 }
 
 uint32_t *port_stack_get_top(void) {
     return &_estack;
 }
 
-extern uint32_t _ebss;
-// Place the word to save just after our BSS section that gets blanked.
+// Place the word in the uninitialized section so it won't get overwritten.
+__attribute__((section(".uninitialized"))) uint32_t _saved_word;
 void port_set_saved_word(uint32_t value) {
-    _ebss = value;
+    _saved_word = value;
 }
 
 uint32_t port_get_saved_word(void) {
-    return _ebss;
+    return _saved_word;
 }
 
 uint64_t port_get_raw_ticks(uint8_t* subticks) {