timers/oneshot: Fix converted tv_nsec > NSEC_PER_SEC.

In rare case, the round-nearest behavior in the clkcnt_best_multshift
may result in converted tv_nsec > NSEC_PER_SEC. This commit fixed the
issue.

Signed-off-by: ouyangxiangzhen <ouyangxiangzhen@xiaomi.com>

Author: Fix-Point

include/nuttx/timers/clkcnt.h

@@ -84,7 +84,7 @@ void clkcnt_best_multshift(uint32_t freq, uint32_t scale,
 {
   uint32_t logfreq = log2floor(freq);
 
-  /* Be careful of the round-nearest behavior here.
+  /* Do not use the round-nearest behavior here.
    * It may lead to the converted result is larger than the
    * integer division.
    * In rare cases, this can cause the converted tick to be greater
@@ -103,7 +103,7 @@ void clkcnt_best_multshift(uint32_t freq, uint32_t scale,
    * affecting the accuracy of time compensation.
    */
 
-  *mult  = (((uint64_t)scale << logfreq) + (freq >> 1)) / freq;
+  *mult  = ((uint64_t)scale << logfreq) / freq;
   *shift = logfreq;
 }
 

include/nuttx/timers/oneshot.h

@@ -44,6 +44,8 @@
  * Pre-processor Definitions
  ****************************************************************************/
 
+#define ONESHOT_NSEC_TOLERANT 5
+
 /* IOCTL commands ***********************************************************/
 
 /* These commands are used by applications to access the oneshot lower-half
@@ -293,12 +295,15 @@ void oneshot_count_init(FAR struct oneshot_lowerhalf_s *lower,
                         &lower->cnt2nsec_mult,
                         &lower->cnt2nsec_shift);
 
-  /* Ensure the maximum error of the mult-shift is less than 5ns. */
+  /* Ensure the maximum error of the mult-shift is less than
+   * ONESHOT_NSEC_TOLERANT.
+   */
 
   result = clkcnt_delta_cnt2nsec_fast(frequency, lower->cnt2nsec_mult,
                                       lower->cnt2nsec_shift);
 
-  ASSERT(NSEC_PER_SEC - 5 <= result && NSEC_PER_SEC + 5 >= result);
+  ASSERT(NSEC_PER_SEC - ONESHOT_NSEC_TOLERANT <= result &&
+         NSEC_PER_SEC >= result);
 
 #  ifdef CONFIG_ONESHOT_FAST_DIVISION
   /* invdiv requires the invariant-divsor > 1. */
@@ -402,6 +407,8 @@ int oneshot_current(FAR struct oneshot_lowerhalf_s *lower,
   cnt          -= sec * freq;
   ts->tv_nsec   = oneshot_delta_cnt2nsec(lower, cnt);
   ts->tv_sec    = sec;
+
+  DEBUGASSERT(cnt < freq && ts->tv_nsec < NSEC_PER_SEC);
 #else
   ret = lower->ops->current(lower, ts);
 #endif