da1469x/os-tick: Fix slightly incorrect time-keeping

If the LP clock's frequency is not an integer multiple of the specified OS tick
frequency, the period of OS ticks will be incorrect due to an integer division
truncation error (to the tune of almost 1% for RCX), resulting in OS Time
gradually creeping away from the actual time. This commit fixes the issue, by
maintaining the fractional OS tick value and advance OS Time accordingly.

hw/mcu/dialog/da1469x/include/mcu/da1469x_clock.h

@@ -173,7 +173,7 @@ void da1469x_clock_lp_rcx_disable(void);
 /**
  * Set the RTC dividers
  */
-void da1469x_clock_lp_set_rtc_divs(void);
+void da1469x_clock_lp_set_rtc_divs(uint32_t rtc_clock_freq);
 
 /**
  * Enable AMBA clock(s)

hw/mcu/dialog/da1469x/include/mcu/da1469x_hal.h

@@ -87,6 +87,13 @@ extern const int qspi_flash_config_array_size;
 
 const struct qspi_flash_config *da1469x_qspi_get_config(void);
 
+/**
+ * Calculate the OS tick parameters.
+ *
+ * @param cycles_per_sec Input frequency of timer generating OS ticks
+ */
+void hal_os_tick_calc_params(uint32_t cycles_per_sec);
+
 #ifdef __cplusplus
 }
 #endif

hw/mcu/dialog/da1469x/src/da1469x_clock.c

@@ -476,14 +476,14 @@ da1469x_clock_lp_rcx_disable(void)
 }
 
 void
-da1469x_clock_lp_set_rtc_divs(void)
+da1469x_clock_lp_set_rtc_divs(uint32_t rtc_clock_freq)
 {
-    /* Please see the DA1469x desig doc section 2.20.4 for details */
+    /* Please see the DA1469x Datasheet section 34.3 for details */
     uint32_t reg;
 
-    reg = ((da1469x_clock_lp_freq_get() % RTC_IN_FREQ_HZ) * RTC_DIV_FRAC_ADJ) <<
+    reg = ((rtc_clock_freq % RTC_IN_FREQ_HZ) * RTC_DIV_FRAC_ADJ) <<
           CRG_TOP_CLK_RTCDIV_REG_RTC_DIV_FRAC_Pos;
-    reg |= ((da1469x_clock_lp_freq_get() / RTC_IN_FREQ_HZ)) <<
+    reg |= ((rtc_clock_freq / RTC_IN_FREQ_HZ)) <<
            CRG_TOP_CLK_RTCDIV_REG_RTC_DIV_INT_Pos;
     reg |= DA1469X_RTC_DIV_DENOM_SEL << CRG_TOP_CLK_RTCDIV_REG_RTC_DIV_DENOM_Pos;
     reg |= CRG_TOP_CLK_RTCDIV_REG_RTC_DIV_ENABLE_Msk;

hw/mcu/dialog/da1469x/src/da1469x_lpclk.c

@@ -57,7 +57,8 @@ da1469x_lpclk_notify(void)
             g_da1469x_lpclk_cmac_cb(lp_curr_freq);
         }
 
-        da1469x_clock_lp_set_rtc_divs();
+        da1469x_clock_lp_set_rtc_divs(lp_curr_freq);
+        hal_os_tick_calc_params(lp_curr_freq);
     }
 }
 

hw/mcu/dialog/da1469x/src/hal_os_tick.c

@@ -28,7 +28,10 @@
 #include "mcu/mcu.h"
 
 struct hal_os_tick {
-    int ticks_per_ostick;
+    uint32_t os_ticks_per_sec;          /* Configured upon init */
+    uint32_t cycles_per_ostick;         /* For generating the OS ticks */
+    uint32_t cycles_per_256_osticks;    /* For more precise OS Time calculation */
+    uint32_t os_tick_residual;
     os_time_t max_idle_ticks;
     uint32_t last_trigger_val;
 };
@@ -88,7 +91,7 @@ hal_os_tick_set_timer_trigger_val(uint32_t trigger_val)
             break;
         }
 
-        trigger_val += g_hal_os_tick.ticks_per_ostick;
+        trigger_val += g_hal_os_tick.cycles_per_ostick;
     }
 }
 
@@ -97,28 +100,28 @@ hal_os_tick_handler(void)
 {
     uint32_t primask;
     uint32_t timer_val;
-    int delta;
-    int ticks;
+    uint32_t delta_x256;
+    uint32_t ticks;
 
     __HAL_DISABLE_INTERRUPTS(primask);
 
-    /* Calculate elapsed ticks and advance OS time. */
+    /* Calculate elapsed cycles of timer & record its current value */
     timer_val = hal_os_tick_get_timer_val();
-    delta = sub24(timer_val, g_hal_os_tick.last_trigger_val);
-    ticks = delta / g_hal_os_tick.ticks_per_ostick;
-    os_time_advance(ticks);
+    delta_x256 = ((timer_val - g_hal_os_tick.last_trigger_val) & 0xffffff) << 8;
+    g_hal_os_tick.last_trigger_val = timer_val;
 
     /* Clear timer interrupt */
     TIMER2->TIMER2_CLEAR_IRQ_REG = 1;
 
-    /* Update the time associated with the most recent tick */
-    g_hal_os_tick.last_trigger_val = (g_hal_os_tick.last_trigger_val +
-                                      (ticks * g_hal_os_tick.ticks_per_ostick)) &
-                                     0xffffff;
+    /* Re-arm timer for the next OS tick */
+    hal_os_tick_set_timer_trigger_val(timer_val + g_hal_os_tick.cycles_per_ostick);
 
-    /* Update timer trigger value for interrupt at the next tick */
-    hal_os_tick_set_timer_trigger_val(g_hal_os_tick.last_trigger_val +
-                                      g_hal_os_tick.ticks_per_ostick);
+    /* Update OS Time */
+    ticks = delta_x256 / g_hal_os_tick.cycles_per_256_osticks;
+    g_hal_os_tick.os_tick_residual += delta_x256 % g_hal_os_tick.cycles_per_256_osticks;
+    ticks += g_hal_os_tick.os_tick_residual / g_hal_os_tick.cycles_per_256_osticks;
+    g_hal_os_tick.os_tick_residual %= g_hal_os_tick.cycles_per_256_osticks;
+    os_time_advance(ticks);
 
     __HAL_ENABLE_INTERRUPTS(primask);
 }
@@ -133,6 +136,19 @@ hal_os_tick_timer2_isr(void)
     os_trace_isr_exit();
 }
 
+void
+hal_os_tick_calc_params(uint32_t cycles_per_sec)
+{
+    /* Upon imit, `os_ticks_per_sec` becomes available only after clock setup - skip for now */
+    if (g_hal_os_tick.os_ticks_per_sec == 0) {
+        return;
+    }
+
+    g_hal_os_tick.cycles_per_256_osticks = (cycles_per_sec << 8) / g_hal_os_tick.os_ticks_per_sec;
+    g_hal_os_tick.cycles_per_ostick = g_hal_os_tick.cycles_per_256_osticks >> 8;
+    g_hal_os_tick.max_idle_ticks = (1UL << 22) / g_hal_os_tick.cycles_per_ostick;
+}
+
 void
 os_tick_idle(os_time_t ticks)
 {
@@ -146,7 +162,7 @@ os_tick_idle(os_time_t ticks)
         }
 
         new_trigger_val = g_hal_os_tick.last_trigger_val +
-                          (ticks * g_hal_os_tick.ticks_per_ostick);
+                          (ticks * g_hal_os_tick.cycles_per_ostick);
 
         hal_os_tick_set_timer_trigger_val(new_trigger_val);
     }
@@ -163,13 +179,10 @@ os_tick_init(uint32_t os_ticks_per_sec, int prio)
 {
     uint32_t primask;
 
+    g_hal_os_tick.os_ticks_per_sec = os_ticks_per_sec;
     g_hal_os_tick.last_trigger_val = 0;
-#if MYNEWT_VAL_CHOICE(MCU_LPCLK_SOURCE, RCX)
-    g_hal_os_tick.ticks_per_ostick = da1469x_clock_lp_rcx_freq_get() / os_ticks_per_sec;
-#else
-    g_hal_os_tick.ticks_per_ostick = 32768 / os_ticks_per_sec;
-#endif
-    g_hal_os_tick.max_idle_ticks = (1UL << 22) / g_hal_os_tick.ticks_per_ostick;
+    g_hal_os_tick.os_tick_residual = 0;
+    hal_os_tick_calc_params(da1469x_clock_lp_freq_get());
 
     TIMER2->TIMER2_CTRL_REG = 0;
     TIMER2->TIMER2_PRESCALER_REG = 0;