sched/wdog: improve accuracy of wd_gettime() in tickless mode

sched/wdog/wd_gettime.c

@@ -88,6 +88,7 @@ int wd_gettime(WDOG_ID wdog)
           delay += curr->lag;
           if (curr == wdog)
             {
+              delay -= wd_elapse();
               leave_critical_section(flags);
               return delay;
             }

sched/wdog/wd_initialize.c

@@ -67,6 +67,14 @@ sq_queue_t g_wdactivelist;
 
 uint16_t g_wdnfree;
 
+/* This is wdog tickbase, for wd_gettime() may called many times
+ * between 2 times of wd_timer(), we use it to update wd_gettime().
+ */
+
+#ifdef CONFIG_SCHED_TICKLESS
+clock_t g_wdtickbase;
+#endif
+
 /****************************************************************************
  * Private Data
  ****************************************************************************/

sched/wdog/wd_start.c

@@ -292,6 +292,12 @@ int wd_start(WDOG_ID wdog, int32_t delay, wdentry_t wdentry,  int argc, ...)
 
   if (g_wdactivelist.head == NULL)
     {
+#ifdef CONFIG_SCHED_TICKLESS
+      /* Update clock tickbase */
+
+      g_wdtickbase = clock_systimer();
+#endif
+
       /* Add the watchdog to the head == tail of the queue. */
 
       sq_addlast((FAR sq_entry_t *)wdog, &g_wdactivelist);
@@ -461,14 +467,19 @@ unsigned int wd_timer(int ticks)
 
       /* There are.  Decrement the lag counter */
 
-      wdog->lag -= decr;
-      ticks     -= decr;
+      wdog->lag    -= decr;
+      ticks        -= decr;
+      g_wdtickbase += decr;
 
       /* Check if the watchdog at the head of the list is ready to run */
 
       wd_expiration();
     }
 
+  /* Update clock tickbase */
+
+  g_wdtickbase += ticks;
+
   /* Return the delay for the next watchdog to expire */
 
   ret = g_wdactivelist.head ?

sched/wdog/wdog.h

@@ -46,8 +46,30 @@
 #include <stdbool.h>
 
 #include <nuttx/compiler.h>
+#include <nuttx/clock.h>
 #include <nuttx/wdog.h>
 
+/****************************************************************************
+ * Pre-processor Definitions
+ ****************************************************************************/
+
+/****************************************************************************
+ * Name: wd_elapse
+ *
+ * Description:
+ *   This function is used to get time-elapse from last time wd_timer() be
+ *   called. In case of CONFIG_SCHED_TICKLESS configured, wd_timer() may
+ *   take lots of ticks, during this time, wd_start()/wd_cancel() may
+ *   called, so we need wd_elapse() to correct the delay/lag.
+ *
+ ****************************************************************************/
+
+#ifdef CONFIG_SCHED_TICKLESS
+#  define wd_elapse() (clock_systimer() - g_wdtickbase)
+#else
+#  define wd_elapse() (0)
+#endif
+
 /****************************************************************************
  * Public Data
  ****************************************************************************/
@@ -80,6 +102,14 @@ extern sq_queue_t g_wdactivelist;
 
 extern uint16_t g_wdnfree;
 
+/* This is wdog tickbase, for wd_gettime() may called many times
+ * between 2 times of wd_timer(), we use it to update wd_gettime().
+ */
+
+#ifdef CONFIG_SCHED_TICKLESS
+extern clock_t g_wdtickbase;
+#endif
+
 /****************************************************************************
  * Public Function Prototypes
  ****************************************************************************/