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 60494413276918af234d3a802dbb8177c6fd413d..d75f15abcf85324e525444bd3ad060404f196c56 100644 (file)
--- a/bertos/drv/timer.c
+++ b/bertos/drv/timer.c
@@ -52,6 +52,8 @@
 #include <cpu/irq.h>
 #include <cpu/power.h> // cpu_relax()
 
+#include <kern/proc_p.h> // proc_decQuantun()
+
 /*
  * Include platform-specific binding code if we're hosted.
  * Try the CPU specific one for bare-metal environments.
@@ -240,30 +242,34 @@ void synctimer_poll(List *queue)
 
 /**
  * Wait for the specified amount of timer ticks.
+ *
+ * \note Sleeping while preemption is disabled fallbacks to a busy wait sleep.
  */
 void timer_delayTicks(ticks_t delay)
 {
        /* We shouldn't sleep with interrupts disabled */
        IRQ_ASSERT_ENABLED();
 
-#if defined(CONFIG_KERN_SIGNALS) && CONFIG_KERN_SIGNALS
+#if CONFIG_KERN_SIGNALS
        Timer t;
 
-       ASSERT(!sig_check(SIG_SINGLE));
-       timer_setSignal(&t, proc_current(), SIG_SINGLE);
-       timer_setDelay(&t, delay);
-       timer_add(&t);
-       sig_wait(SIG_SINGLE);
-
-#else /* !CONFIG_KERN_SIGNALS */
-
-       ticks_t start = timer_clock();
-
-       /* Busy wait */
-       while (timer_clock() - start < delay)
-               cpu_relax();
-
+       if (proc_preemptAllowed())
+       {
+               ASSERT(!sig_check(SIG_SINGLE));
+               timer_setSignal(&t, proc_current(), SIG_SINGLE);
+               timer_setDelay(&t, delay);
+               timer_add(&t);
+               sig_wait(SIG_SINGLE);
+       }
+       else
 #endif /* !CONFIG_KERN_SIGNALS */
+       {
+               ticks_t start = timer_clock();
+
+               /* Busy wait */
+               while (timer_clock() - start < delay)
+                       cpu_relax();
+       }
 }
 
 
@@ -280,15 +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;
@@ -315,7 +336,6 @@ void timer_delayHp(hptime_t delay)
 }
 #endif /* CONFIG_TIMER_UDELAY */
 
-
 /**
  * Timer interrupt handler. Find soft timers expired and
  * trigger corresponding events.
@@ -340,16 +360,19 @@ DEFINE_TIMER_ISR
 
        TIMER_STROBE_ON;
 
-       /* Perform hw IRQ handling */
-       timer_hw_irq();
-
        /* Update the master ms counter */
        ++_clock;
 
+       /* Update the current task's quantum (if enabled). */
+       proc_decQuantum();
+
        #if CONFIG_TIMER_EVENTS
                timer_poll(&timers_queue);
        #endif
 
+       /* Perform hw IRQ handling */
+       timer_hw_irq();
+
        TIMER_STROBE_OFF;
 }