doc: Improve timer documentation.

[bertos.git] / bertos / drv / timer.c

diff --git a/bertos/drv/timer.c b/bertos/drv/timer.c
index d380e5f1d458de07e0342281cbc84a934c80fdb2..a411dc4e8be29775aed3cc959f5af49b62f32c00 100644 (file)
--- a/bertos/drv/timer.c
+++ b/bertos/drv/timer.c
@@ -158,6 +158,8 @@ INLINE void timer_addToList(Timer *timer, List *queue)
  * When the delay indicated by the timer expires, the timer
  * device will execute the event associated with it.
  *
+ * You should not call this function on an already running timer.
+ *
  * \note Interrupt safe
  */
 void timer_add(Timer *timer)
@@ -252,14 +254,13 @@ void timer_delayTicks(ticks_t delay)
 
 #if CONFIG_KERN_SIGNALS
        Timer t;
-
+       DB(t.magic = TIMER_MAGIC_INACTIVE;)
        if (proc_preemptAllowed())
        {
-               ASSERT(!sig_check(SIG_SINGLE));
-               timer_setSignal(&t, proc_current(), SIG_SINGLE);
+               timer_setEvent(&t);
                timer_setDelay(&t, delay);
                timer_add(&t);
-               sig_wait(SIG_SINGLE);
+               timer_waitEvent(&t);
        }
        else
 #endif /* !CONFIG_KERN_SIGNALS */
@@ -286,15 +287,30 @@ void timer_busyWait(hptime_t delay)
        hptime_t now, prev = timer_hw_hpread();
        hptime_t delta;
 
-       for(;;)
+       for (;;)
        {
                now = timer_hw_hpread();
                /*
-                * We rely on hptime_t being unsigned here to
-                * reduce the modulo to an AND in the common
-                * case of TIMER_HW_CNT.
+                * The timer counter may wrap here and "prev" can become
+                * greater than "now". So, be sure to always evaluate a
+                * coherent timer difference:
+                *
+                * 0     prev            now   TIMER_HW_CNT
+                * |_____|_______________|_____|
+                *        ^^^^^^^^^^^^^^^
+                * delta = now - prev
+                *
+                * 0     now             prev  TIMER_HW_CNT
+                * |_____|_______________|_____|
+                *  ^^^^^                 ^^^^^
+                * delta = (TIMER_HW_CNT - prev) + now
+                *
+                * NOTE: TIMER_HW_CNT can be any value, not necessarily a power
+                * of 2. For this reason the "%" operator is not suitable for
+                * the generic case.
                 */
-               delta = (now - prev) % TIMER_HW_CNT;
+               delta = (now < prev) ? ((hptime_t)TIMER_HW_CNT - prev + now) :
+                                               (now - prev);
                if (delta >= delay)
                        break;
                delay -= delta;
@@ -357,7 +373,7 @@ DEFINE_TIMER_ISR
 
        /* Perform hw IRQ handling */
        timer_hw_irq();
-       
+
        TIMER_STROBE_OFF;
 }