text: // Copyright 2015-2017 Espressif Systems (Shanghai) PTE LTD
  //
  // Licensed under the Apache License, Version 2.0 (the "License");
  // you may not use this file except in compliance with the License.
  // You may obtain a copy of the License at
  //
  //     http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS IS" BASIS,
  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  // See the License for the specific language governing permissions and
  // limitations under the License.
  
  #include <errno.h>
  #include <stdlib.h>
  #include <time.h>
  #include <reent.h>
  #include <sys/types.h>
  #include <sys/reent.h>
  #include <sys/time.h>
  #include <sys/times.h>
  #include <sys/lock.h>
  #include <rom/rtc.h>
  #include "esp_attr.h"
  #include "esp_intr_alloc.h"
  #include "soc/soc.h"
  #include "soc/rtc.h"
  #include "esp_clk.h"
  #include "soc/rtc_cntl_reg.h"
  #include "soc/frc_timer_reg.h"
  #include "rom/ets_sys.h"
  #include "freertos/FreeRTOS.h"
  #include "freertos/xtensa_api.h"
  #include "freertos/task.h"
  #include "sdkconfig.h"
  
  #if defined( CONFIG_ESP32_TIME_SYSCALL_USE_RTC ) || defined( CONFIG_ESP32_TIME_SYSCALL_USE_RTC_FRC1 )
  #define WITH_RTC 1
  #endif
  
  #if defined( CONFIG_ESP32_TIME_SYSCALL_USE_FRC1 ) || defined( CONFIG_ESP32_TIME_SYSCALL_USE_RTC_FRC1 )
  #define WITH_FRC1 1
  #endif
  
  #ifdef WITH_RTC
  static uint64_t get_rtc_time_us()
  {
      const uint64_t ticks = rtc_time_get();
      const uint32_t cal = esp_clk_slowclk_cal_get();
      /* RTC counter result is up to 2^48, calibration factor is up to 2^24,
       * for a 32kHz clock. We need to calculate (assuming no overflow):
       *   (ticks * cal) >> RTC_CLK_CAL_FRACT
       *
       * An overflow in the (ticks * cal) multiplication would cause time to
       * wrap around after approximately 13 days, which is probably not enough
       * for some applications.
       * Therefore multiplication is split into two terms, for the lower 32-bit
       * and the upper 16-bit parts of "ticks", i.e.:
       *   ((ticks_low + 2^32 * ticks_high) * cal) >> RTC_CLK_CAL_FRACT
       */
      const uint64_t ticks_low = ticks & UINT32_MAX;
      const uint64_t ticks_high = ticks >> 32;
      return ((ticks_low * cal) >> RTC_CLK_CAL_FRACT) +
             ((ticks_high * cal) << (32 - RTC_CLK_CAL_FRACT));
  }
  #endif // WITH_RTC
  
  
  // s_boot_time: time from Epoch to the first boot time
  #ifdef WITH_RTC
  // when RTC is used to persist time, two RTC_STORE registers are used to store boot time
  #elif defined(WITH_FRC1)
  static uint64_t s_boot_time;
  #endif
  
  #if defined(WITH_RTC) || defined(WITH_FRC1)
  static _lock_t s_boot_time_lock;
  #endif
  
  #ifdef WITH_FRC1
  #define FRC1_PRESCALER 16
  #define FRC1_PRESCALER_CTL 2
  #define FRC1_TICK_FREQ (APB_CLK_FREQ / FRC1_PRESCALER)
  #define FRC1_TICKS_PER_US (FRC1_TICK_FREQ / 1000000)
  #define FRC1_ISR_PERIOD_US (FRC_TIMER_LOAD_VALUE(0) / FRC1_TICKS_PER_US)
  // Counter frequency will be APB_CLK_FREQ / 16 = 5 MHz
  // 1 tick = 0.2 us
  // Timer has 23 bit counter, so interrupt will fire each 1677721.6 microseconds.
  // This is not a whole number, so timer will drift by 0.3 ppm due to rounding error.
  
  static volatile uint64_t s_microseconds = 0;
  
  static void IRAM_ATTR frc_timer_isr()
  {
      // Write to FRC_TIMER_INT_REG may not take effect in some cases (root cause TBD)
      // This extra write works around this issue.
      // FRC_TIMER_LOAD_REG(0) is used here, but any other DPORT register address can also be used.
      WRITE_PERI_REG(FRC_TIMER_LOAD_REG(0), FRC_TIMER_LOAD_VALUE(0));
      WRITE_PERI_REG(FRC_TIMER_INT_REG(0), FRC_TIMER_INT_CLR);
      s_microseconds += FRC1_ISR_PERIOD_US;
  }
  
  #endif // WITH_FRC1
  
  #if defined(WITH_RTC) || defined(WITH_FRC1)
  static void set_boot_time(uint64_t time_us)
  {
      _lock_acquire(&s_boot_time_lock);
  #ifdef WITH_RTC
      REG_WRITE(RTC_BOOT_TIME_LOW_REG, (uint32_t) (time_us & 0xffffffff));
      REG_WRITE(RTC_BOOT_TIME_HIGH_REG, (uint32_t) (time_us >> 32));
  #else
      s_boot_time = time_us;
  #endif
      _lock_release(&s_boot_time_lock);
  }
  
  static uint64_t get_boot_time()
  {
      uint64_t result;
      _lock_acquire(&s_boot_time_lock);
  #ifdef WITH_RTC
      result = ((uint64_t) REG_READ(RTC_BOOT_TIME_LOW_REG)) + (((uint64_t) REG_READ(RTC_BOOT_TIME_HIGH_REG)) << 32);
  #else
      result = s_boot_time;
  #endif
      _lock_release(&s_boot_time_lock);
      return result;
  }
  #endif //defined(WITH_RTC) || defined(WITH_FRC1)
  
  
  void esp_clk_slowclk_cal_set(uint32_t new_cal)
  {
  #if defined(WITH_RTC)
      /* To force monotonic time values even when clock calibration value changes,
       * we adjust boot time, given current time and the new calibration value:
       *      T = boot_time_old + cur_cal * ticks / 2^19
       *      T = boot_time_adj + new_cal * ticks / 2^19
       * which results in:
       *      boot_time_adj = boot_time_old + ticks * (cur_cal - new_cal) / 2^19
       */
      const int64_t ticks = (int64_t) rtc_time_get();
      const uint32_t cur_cal = REG_READ(RTC_SLOW_CLK_CAL_REG);
      int32_t cal_diff = (int32_t) (cur_cal - new_cal);
      int64_t boot_time_diff = ticks * cal_diff / (1LL << RTC_CLK_CAL_FRACT);
      uint64_t boot_time_adj = get_boot_time() + boot_time_diff;
      set_boot_time(boot_time_adj);
  #endif // WITH_RTC
      REG_WRITE(RTC_SLOW_CLK_CAL_REG, new_cal);
  }
  
  uint32_t esp_clk_slowclk_cal_get()
  {
      return REG_READ(RTC_SLOW_CLK_CAL_REG);
  }
  
  void esp_setup_time_syscalls()
  {
  #if defined( WITH_FRC1 )
  #if defined( WITH_RTC )
      // initialize time from RTC clock
      s_microseconds = get_rtc_time_us();
  #endif //WITH_RTC
  
      // set up timer
      WRITE_PERI_REG(FRC_TIMER_CTRL_REG(0), \
              FRC_TIMER_AUTOLOAD | \
              (FRC1_PRESCALER_CTL << FRC_TIMER_PRESCALER_S) | \
              FRC_TIMER_EDGE_INT);
  
      WRITE_PERI_REG(FRC_TIMER_LOAD_REG(0), FRC_TIMER_LOAD_VALUE(0));
      SET_PERI_REG_MASK(FRC_TIMER_CTRL_REG(0),
              FRC_TIMER_ENABLE | \
              FRC_TIMER_INT_ENABLE);
      esp_intr_alloc(ETS_TIMER1_INTR_SOURCE, 0, &frc_timer_isr, NULL, NULL);
  #endif // WITH_FRC1
  }
  
  clock_t IRAM_ATTR _times_r(struct _reent *r, struct tms *ptms)
  {
      clock_t t = xTaskGetTickCount() * (portTICK_PERIOD_MS * CLK_TCK / 1000);
      ptms->tms_cstime = 0;
      ptms->tms_cutime = 0;
      ptms->tms_stime = t;
      ptms->tms_utime = 0;
      struct timeval tv = {0, 0};
      _gettimeofday_r(r, &tv, NULL);
      return (clock_t) tv.tv_sec;
  }
  
  #if defined( WITH_FRC1 ) || defined( WITH_RTC )
  static uint64_t get_time_since_boot()
  {
      uint64_t microseconds = 0;
  #ifdef WITH_FRC1
      uint32_t timer_ticks_before = READ_PERI_REG(FRC_TIMER_COUNT_REG(0));
      microseconds = s_microseconds;
      uint32_t timer_ticks_after = READ_PERI_REG(FRC_TIMER_COUNT_REG(0));
      if (timer_ticks_after > timer_ticks_before) {
          // overflow happened at some point between getting
          // timer_ticks_before and timer_ticks_after
          // microseconds value is ambiguous, get a new one
          microseconds = s_microseconds;
      }
      microseconds += (FRC_TIMER_LOAD_VALUE(0) - timer_ticks_after) / FRC1_TICKS_PER_US;
  #elif defined(WITH_RTC)
      microseconds = get_rtc_time_us();
  #endif
      return microseconds;
  }
  #endif // defined( WITH_FRC1 ) || defined( WITH_RTC )
  
  int IRAM_ATTR _gettimeofday_r(struct _reent *r, struct timeval *tv, void *tz)
  {
      (void) tz;
  #if defined( WITH_FRC1 ) || defined( WITH_RTC )
      if (tv) {
          uint64_t microseconds = get_boot_time() + get_time_since_boot();
          tv->tv_sec = microseconds / 1000000;
          tv->tv_usec = microseconds % 1000000;
      }
      return 0;
  #else
      __errno_r(r) = ENOSYS;
      return -1;
  #endif // defined( WITH_FRC1 ) || defined( WITH_RTC )
  }
  
  int settimeofday(const struct timeval *tv, const struct timezone *tz)
  {
      (void) tz;
  #if defined( WITH_FRC1 ) || defined( WITH_RTC )
      if (tv) {
          uint64_t now = ((uint64_t) tv->tv_sec) * 1000000LL + tv->tv_usec;
          uint64_t since_boot = get_time_since_boot();
          set_boot_time(now - since_boot);
      }
      return 0;
  #else
      errno = ENOSYS;
      return -1;
  #endif
  }
  
  uint32_t system_get_time(void)
  {
  #if defined( WITH_FRC1 ) || defined( WITH_RTC )
      return get_time_since_boot();
  #else
      return 0;
  #endif
  }
  
  uint32_t system_get_current_time(void) __attribute__((alias("system_get_time")));
  
  uint32_t system_relative_time(uint32_t current_time)
  {
      return system_get_time() - current_time;
  }
  
  uint64_t system_get_rtc_time(void)
  {
  #ifdef WITH_RTC
      return get_rtc_time_us();
  #else
      return 0;
  #endif
  }
  
  