sched: Avoid rescheduling the currently active timer

It's tricky to reschedule the timer irq correctly (due to race
conditions with the irq) and in practice it's very rarely needed.
Handle the special cases in the generic sched.c code so that the board
code doesn't have to handle it.

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

1 files changed, 29 insertions, 22 deletions

diff --git a/src/sched.c b/src/sched.c
index 19e428be..7e4a86a1 100644
--- a/src/sched.c
+++ b/src/sched.c
@@ -101,33 +101,39 @@ sched_timer(struct timer *add)
 {
     uint32_t waketime = add->waketime;
     irqstatus_t flag = irq_save();
-    if (sched_is_before(waketime, timer_list->waketime)) {
-        // This timer is the next - insert at front of list and reschedule
-        add->next = timer_list;
-        timer_list = add;
-        uint8_t ret = timer_set_next(waketime);
-        if (ret)
-            shutdown("Timer too close");
-    } else {
-        // Find position in list and insert
-        struct timer *pos = timer_list;
-        while (!sched_is_before(waketime, pos->next->waketime))
-            pos = pos->next;
-        add->next = pos->next;
-        pos->next = add;
-    }
+    if (sched_is_before(waketime, timer_list->waketime))
+        // Timer in past (or very near future)
+        shutdown("Timer too close");
+    // Find position in list and insert
+    struct timer *pos = timer_list;
+    while (!sched_is_before(waketime, pos->next->waketime))
+        pos = pos->next;
+    add->next = pos->next;
+    pos->next = add;
     irq_restore(flag);
 }
 
+// The deleted timer is used when deleting an active timer.
+static uint_fast8_t
+deleted_event(struct timer *t)
+{
+    return SF_DONE;
+}
+
+static struct timer deleted_timer = {
+    .func = deleted_event,
+};
+
 // Remove a timer that may be live.
 void
 sched_del_timer(struct timer *del)
 {
     irqstatus_t flag = irq_save();
     if (timer_list == del) {
-        // Deleting the next active timer - delete and reschedule
-        timer_list = del->next;
-        timer_set_next(timer_list->waketime);
+        // Deleting the next active timer - replace with deleted_timer
+        deleted_timer.waketime = del->waketime;
+        deleted_timer.next = del->next;
+        timer_list = &deleted_timer;
     } else {
         // Find and remove from timer list (if present)
         struct timer *pos;
@@ -197,13 +203,14 @@ sched_timer_kick(void)
 
 // Shutdown all user timers on an emergency stop.
 static void
-timer_shutdown(void)
+sched_timer_shutdown(void)
 {
-    timer_list = &ms_timer;
+    timer_list = &deleted_timer;
+    deleted_timer.waketime = ms_timer.waketime;
+    deleted_timer.next = &ms_timer;
     ms_timer.next = &sentinel_timer;
-    timer_set_next(timer_list->waketime);
 }
-DECL_SHUTDOWN(timer_shutdown);
+DECL_SHUTDOWN(sched_timer_shutdown);
 
 
 /****************************************************************