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, 12 insertions, 31 deletions

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)
 {