avr: Implement internal avr specific timer to handle 16bit overflows

Don't rely on the generic scheduler code to always have a timer no
more than 1ms in the future.  Instead, create an avr specific timer
that will be called every 0x8000 ticks.  This simplifies the generic
code and it reduces the amount of code that needs to be run every
millisecond.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/avr/timer.c

@@ -85,6 +85,15 @@ timer_kick(void)
     TIFR1 = 1<<OCF1A;
 }
 
+static struct timer wrap_timer;
+
+void
+timer_reset(void)
+{
+    sched_add_timer(&wrap_timer);
+}
+DECL_SHUTDOWN(timer_reset);
+
 void
 timer_init(void)
 {
@@ -96,6 +105,7 @@ timer_init(void)
     irqstatus_t flag = irq_save();
     timer_kick();
     timer_repeat_set(timer_get() + 50);
+    timer_reset();
     TIFR1 = 1<<TOV1;
     // enable interrupt
     TIMSK1 = 1<<OCIE1A;
@@ -115,25 +125,33 @@ uint32_t
 timer_read_time(void)
 {
     irqstatus_t flag = irq_save();
-    union u32_u calc;
-    calc.val = timer_get();
+    union u32_u calc = { .val = timer_get() };
     calc.hi = timer_high;
-    if (TIFR1 & (1<<TOV1) && calc.lo < 0x8000)
-        calc.hi++;
+    if (unlikely(TIFR1 & (1<<TOV1))) {
+        irq_restore(flag);
+        if (calc.b1 < 0xff)
+            calc.hi++;
+        return calc.val;
+    }
     irq_restore(flag);
     return calc.val;
 }
 
-// Called by main code once every millisecond.  (IRQs disabled.)
-void
-timer_periodic(void)
+// Timer that runs every ~2ms - allows 16bit comparison optimizations
+static uint_fast8_t
+timer_event(struct timer *t)
 {
     if (TIFR1 & (1<<TOV1)) {
         // Hardware timer has overflowed - update overflow counter
         TIFR1 = 1<<TOV1;
         timer_high++;
     }
+    wrap_timer.waketime += 0x8000;
+    return SF_RESCHEDULE;
 }
+static struct timer wrap_timer = {
+    .func = timer_event,
+};
 
 #define TIMER_IDLE_REPEAT_TICKS 8000
 #define TIMER_REPEAT_TICKS 3000

src/generic/misc.h

@@ -12,7 +12,6 @@ uint32_t timer_from_us(uint32_t us);
 uint8_t timer_is_before(uint32_t time1, uint32_t time2);
 uint32_t timer_read_time(void);
 void timer_kick(void);
-void timer_periodic(void);
 
 void *dynmem_start(void);
 void *dynmem_end(void);

src/generic/timer_irq.c

@@ -30,12 +30,6 @@ timer_is_before(uint32_t time1, uint32_t time2)
     return (int32_t)(time1 - time2) < 0;
 }
 
-// Called by main code once every millisecond.  (IRQs disabled.)
-void
-timer_periodic(void)
-{
-}
-
 static uint32_t timer_repeat_until;
 #define TIMER_IDLE_REPEAT_TICKS timer_from_us(500)
 #define TIMER_REPEAT_TICKS timer_from_us(100)

src/sched.c

@@ -24,12 +24,11 @@ static struct timer sentinel_timer, deleted_timer;
 
 // The periodic_timer simplifies the timer code by ensuring there is
 // always a timer on the timer list and that there is always a timer
-// not more than 1ms in the future.
+// not far in the future.
 static uint_fast8_t
 periodic_event(struct timer *t)
 {
-    timer_periodic();
-    periodic_timer.waketime += timer_from_us(1000);
+    periodic_timer.waketime += timer_from_us(100000);
     sentinel_timer.waketime = periodic_timer.waketime + 0x80000000;
     return SF_RESCHEDULE;
 }
@@ -162,9 +161,9 @@ sched_timer_dispatch(void)
     return next_waketime;
 }
 
-// Shutdown all user timers on an emergency stop.
-void
-sched_timer_shutdown(void)
+// Remove all user timers on a shutdown
+static void
+sched_timer_reset(void)
 {
     timer_list = &deleted_timer;
     deleted_timer.waketime = periodic_timer.waketime;
@@ -172,7 +171,6 @@ sched_timer_shutdown(void)
     periodic_timer.next = &sentinel_timer;
     timer_kick();
 }
-DECL_SHUTDOWN(sched_timer_shutdown);
 
 
 /****************************************************************
@@ -230,6 +228,7 @@ run_shutdown(int reason)
     if (!shutdown_status)
         shutdown_reason = reason;
     shutdown_status = 2;
+    sched_timer_reset();
     extern void ctr_run_shutdownfuncs(void);
     ctr_run_shutdownfuncs();
     shutdown_status = 1;

src/simulator/main.c

@@ -85,11 +85,6 @@ timer_kick(void)
 {
 }
 
-void
-timer_periodic(void)
-{
-}
-
 
 /****************************************************************
  * Turn stdin/stdout into serial console