timer: fix a build warning and add a comment

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

diff --git a/bertos/drv/timer.c b/bertos/drv/timer.c
index d380e5f1d458de07e0342281cbc84a934c80fdb2..d75f15abcf85324e525444bd3ad060404f196c56 100644 (file)
--- a/bertos/drv/timer.c
+++ b/bertos/drv/timer.c
@@ -286,15 +286,30 @@ 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 +372,7 @@ DEFINE_TIMER_ISR
 
        /* Perform hw IRQ handling */
        timer_hw_irq();
-       
+
        TIMER_STROBE_OFF;
 }