sched: Implement generic sleep mechanism based on tasks pending

Track when tasks are pending and spin in irq_wait() when no tasks are
pending.  This improves the mechanism for sleeping the processor -
it's simpler for the board specific code and it reduces the
possibility of the processor sleeping when tasks are busy.

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

src/generic/timer_irq.c

@@ -8,7 +8,6 @@
 #include "board/irq.h" // irq_disable
 #include "board/misc.h" // timer_from_us
 #include "board/timer_irq.h" // timer_dispatch_many
-#include "basecmd.h" // stats_note_sleep
 #include "command.h" // shutdown
 #include "sched.h" // sched_timer_dispatch
 
@@ -80,23 +79,9 @@ timer_dispatch_many(void)
 void
 timer_task(void)
 {
-    static uint32_t last_timer;
-    uint32_t lst = last_timer;
     irq_disable();
-    uint32_t next = timer_get_next(), cur = timer_read_time();
-    if (lst != next) {
-        timer_repeat_until = cur + TIMER_IDLE_REPEAT_TICKS;
-        irq_enable();
-        last_timer = next;
-        return;
-    }
-
-    // Sleep the processor
-    irq_wait();
-    uint32_t post_sleep = timer_read_time();
-    timer_repeat_until = post_sleep + TIMER_IDLE_REPEAT_TICKS;
+    timer_repeat_until = timer_read_time() + TIMER_IDLE_REPEAT_TICKS;
     irq_enable();
-    stats_note_sleep(post_sleep - cur);
 }
 DECL_TASK(timer_task);
 

src/generic/timer_irq.h

@@ -2,6 +2,5 @@
 #define __GENERIC_TIMER_IRQ_H
 
 uint32_t timer_dispatch_many(void);
-uint32_t timer_get_next(void);
 
 #endif // timer_irq.h