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>

5 files changed, 53 insertions, 90 deletions

diff --git a/src/avr/timer.c b/src/avr/timer.c
index a6da9b4f..06d8845e 100644
--- a/src/avr/timer.c
+++ b/src/avr/timer.c
@@ -39,13 +39,6 @@ timer_set(uint16_t next)
 }
 
 static inline void
-timer_set_clear(uint16_t next)
-{
-    OCR1A = next;
-    TIFR1 = 1<<OCF1A;
-}
-
-static inline void
 timer_repeat_set(uint16_t next)
 {
     // Timer1B is used to limit the number of timers run from a timer1A irq
@@ -78,6 +71,15 @@ timer_init(void)
 }
 DECL_INIT(timer_init);
 
+static void
+timer_shutdown(void)
+{
+    // Reenable timer irq
+    timer_set(timer_get() + 50);
+    TIFR1 = 1<<OCF1A;
+}
+DECL_SHUTDOWN(timer_shutdown);
+
 
 /****************************************************************
  * 32bit timer wrappers
@@ -120,35 +122,15 @@ timer_periodic(void)
     }
 }
 
-#define TIMER_MIN_TICKS 100
-
-// Set the next timer wake time (in absolute clock ticks).  Caller
-// must disable irqs.  The caller should not schedule a time more than
-// a few milliseconds in the future.
-uint8_t
-timer_set_next(uint32_t next)
-{
-    uint32_t cur = timer_read_time();
-    if ((int16_t)(OCR1A - (uint16_t)cur) < 0 && !(TIFR1 & (1<<OCF1A)))
-        // Already processing timer irqs
-        try_shutdown("timer_set_next called during timer dispatch");
-    uint32_t mintime = cur + TIMER_MIN_TICKS;
-    if (sched_is_before(mintime, next)) {
-        timer_set_clear(next);
-        return 0;
-    }
-    timer_set_clear(mintime);
-    return 1;
-}
-
 #define TIMER_IDLE_REPEAT_TICKS 8000
 #define TIMER_REPEAT_TICKS 3000
 
 #define TIMER_MIN_TRY_TICKS 60 // 40 ticks to exit irq; 20 ticks of progress
 #define TIMER_DEFER_REPEAT_TICKS 200
 
-// Similar to timer_set_next(), but wait for the given time if it is
-// in the near future.
+// Set the next timer wake time (in absolute clock ticks) or return 1
+// if the next timer is too close to schedule.  Caller must disable
+// irqs.
 uint8_t
 timer_try_set_next(uint32_t target)
 {
diff --git a/src/generic/misc.h b/src/generic/misc.h
index ce1c8f02..92d4d145 100644
--- a/src/generic/misc.h
+++ b/src/generic/misc.h
@@ -12,7 +12,6 @@ void console_push_output(uint8_t len);
 uint32_t timer_from_us(uint32_t us);
 void timer_periodic(void);
 uint32_t timer_read_time(void);
-uint8_t timer_set_next(uint32_t next);
 uint8_t timer_try_set_next(uint32_t next);
 
 size_t alloc_maxsize(size_t reqsize);
diff --git a/src/sam3x8e/timer.c b/src/sam3x8e/timer.c
index bab2512f..dbc7700e 100644
--- a/src/sam3x8e/timer.c
+++ b/src/sam3x8e/timer.c
@@ -44,13 +44,6 @@ timer_set(uint32_t value)
 }
 
 static void
-timer_set_clear(uint32_t value)
-{
-    TC0->TC_CHANNEL[0].TC_RA = value;
-    TC0->TC_CHANNEL[0].TC_SR; // read to clear irq pending
-}
-
-static void
 timer_init(void)
 {
     TcChannel *tc = &TC0->TC_CHANNEL[0];
@@ -69,6 +62,15 @@ timer_init(void)
 }
 DECL_INIT(timer_init);
 
+static void
+timer_shutdown(void)
+{
+    // Reenable timer irq
+    timer_set(timer_read_time() + 50);
+    TC0->TC_CHANNEL[0].TC_SR; // read to clear irq pending
+}
+DECL_SHUTDOWN(timer_shutdown);
+
 // Called by main code once every millisecond.  (IRQs disabled.)
 void
 timer_periodic(void)
@@ -82,28 +84,6 @@ timer_read_time(void)
     return TC0->TC_CHANNEL[0].TC_CV;
 }
 
-#define TIMER_MIN_TICKS 100
-
-// Set the next timer wake time (in absolute clock ticks).  Caller
-// must disable irqs.  The caller should not schedule a time more than
-// a few milliseconds in the future.
-uint8_t
-timer_set_next(uint32_t next)
-{
-    uint32_t cur = timer_read_time();
-    if (sched_is_before(TC0->TC_CHANNEL[0].TC_RA, cur)
-        && !(TC0->TC_CHANNEL[0].TC_SR & TC_SR_CPAS))
-        // Already processing timer irqs
-        try_shutdown("timer_set_next called during timer dispatch");
-    uint32_t mintime = cur + TIMER_MIN_TICKS;
-    if (sched_is_before(mintime, next)) {
-        timer_set_clear(next);
-        return 0;
-    }
-    timer_set_clear(mintime);
-    return 1;
-}
-
 static uint32_t timer_repeat_until;
 #define TIMER_IDLE_REPEAT_TICKS timer_from_us(500)
 #define TIMER_REPEAT_TICKS timer_from_us(100)
@@ -111,8 +91,9 @@ static uint32_t timer_repeat_until;
 #define TIMER_MIN_TRY_TICKS timer_from_us(1)
 #define TIMER_DEFER_REPEAT_TICKS timer_from_us(5)
 
-// Similar to timer_set_next(), but wait for the given time if it is
-// in the near future.
+// Set the next timer wake time (in absolute clock ticks) or return 1
+// if the next timer is too close to schedule.  Caller must disable
+// irqs.
 uint8_t
 timer_try_set_next(uint32_t next)
 {
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);
 
 
 /****************************************************************
diff --git a/src/simulator/main.c b/src/simulator/main.c
index 1bffc265..f45b5c45 100644
--- a/src/simulator/main.c
+++ b/src/simulator/main.c
@@ -70,12 +70,6 @@ timer_read_time(void)
 }
 
 uint8_t
-timer_set_next(uint32_t next)
-{
-    return 0;
-}
-
-uint8_t
 timer_try_set_next(uint32_t next)
 {
     return 1;