From eb4eeb6f7313cbae93f7f996de283e9c28cb16ff Mon Sep 17 00:00:00 2001
From: Kevin O'Connor <kevin@koconnor.net>
Date: Tue, 28 Mar 2017 11:25:11 -0400
Subject: [PATCH] timer_irq: Integrate timer_try_set_next() into
 timer_dispatch_many()

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
---
 src/generic/timer_irq.c | 69 +++++++++++++++--------------------------
 src/generic/timer_irq.h |  2 +-
 src/sam3x8e/timer.c     | 26 +++++++++-------
 3 files changed, 40 insertions(+), 57 deletions(-)

diff --git a/src/generic/timer_irq.c b/src/generic/timer_irq.c
index a0587a72..69f4e64a 100644
--- a/src/generic/timer_irq.c
+++ b/src/generic/timer_irq.c
@@ -11,11 +11,6 @@
 #include "command.h" // shutdown
 #include "sched.h" // sched_timer_kick
 
-// In order to use this code the board must still define
-// timer_read_time().  In addition, it must also provide a timer_set()
-// function.
-void timer_set(uint32_t next);
-
 DECL_CONSTANT(CLOCK_FREQ, CONFIG_CLOCK_FREQ);
 
 // Return the number of clock ticks for a given number of microseconds
@@ -47,55 +42,41 @@ static uint32_t timer_repeat_until;
 #define TIMER_MIN_TRY_TICKS timer_from_us(1)
 #define TIMER_DEFER_REPEAT_TICKS timer_from_us(5)
 
-// 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.
-static int
-timer_try_set_next(unsigned int next)
+// Reschedule timers after a brief pause to prevent task starvation
+static uint32_t noinline
+force_defer(uint32_t next)
 {
     uint32_t now = timer_read_time();
-    int32_t diff = next - now;
-    if (diff > (int32_t)TIMER_MIN_TRY_TICKS)
-        // Schedule next timer normally.
-        goto done;
-
-    // Next timer is in the past or near future - can't reschedule to it
-    if (likely(timer_is_before(now, timer_repeat_until))) {
-        // Can run more timers from this irq; briefly allow irqs
-        irq_enable();
-        while (diff >= 0) {
-            // Next timer is in the near future - wait for time to occur
-            now = timer_read_time();
-            diff = next - now;
-        }
-        irq_disable();
-        return 0;
-    }
-    if (diff < (int32_t)(-timer_from_us(1000)))
-        goto fail;
-
-    // Too many repeat timers from a single interrupt - force a pause
+    if (timer_is_before(next + timer_from_us(1000), now))
+        shutdown("Rescheduled timer in the past");
     timer_repeat_until = now + TIMER_REPEAT_TICKS;
-    next = now + TIMER_DEFER_REPEAT_TICKS;
-
-done:
-    timer_set(next);
-    return 1;
-fail:
-    shutdown("Rescheduled timer in the past");
+    return now + TIMER_DEFER_REPEAT_TICKS;
 }
 
 // Invoke timers - called from board irq code.
-void
+uint32_t
 timer_dispatch_many(void)
 {
+    uint32_t tru = timer_repeat_until;
     for (;;) {
-        uint32_t next_waketime = sched_timer_dispatch();
+        // Run the next software timer
+        uint32_t next = sched_timer_dispatch();
 
-        // Schedule next timer event (or run next timer if it's ready)
-        int res = timer_try_set_next(next_waketime);
-        if (res)
-            break;
+        uint32_t now = timer_read_time();
+        int32_t diff = next - now;
+        if (diff > (int32_t)TIMER_MIN_TRY_TICKS)
+            // Schedule next timer normally.
+            return next;
+
+        if (unlikely(timer_is_before(tru, now)))
+            // Too many repeat timers from a single interrupt - force a pause
+            return force_defer(next);
+
+        // Next timer in the past or near future - wait for it to be ready
+        irq_enable();
+        while (unlikely(diff > 0))
+            diff = next - timer_read_time();
+        irq_disable();
     }
 }
 
diff --git a/src/generic/timer_irq.h b/src/generic/timer_irq.h
index 0421c98c..031f0d6b 100644
--- a/src/generic/timer_irq.h
+++ b/src/generic/timer_irq.h
@@ -1,6 +1,6 @@
 #ifndef __GENERIC_TIMER_IRQ_H
 #define __GENERIC_TIMER_IRQ_H
 
-void timer_dispatch_many(void);
+uint32_t timer_dispatch_many(void);
 
 #endif // timer_irq.h
diff --git a/src/sam3x8e/timer.c b/src/sam3x8e/timer.c
index d156588a..8548400a 100644
--- a/src/sam3x8e/timer.c
+++ b/src/sam3x8e/timer.c
@@ -11,19 +11,8 @@
 #include "sam3x8e.h" // TC0
 #include "sched.h" // sched_timer_kick
 
-// IRQ handler
-void __visible
-TC0_Handler(void)
-{
-    irq_disable();
-    uint32_t status = TC0->TC_CHANNEL[0].TC_SR; // read to clear irq pending
-    if (likely(status & TC_SR_CPAS))
-        timer_dispatch_many();
-    irq_enable();
-}
-
 // Set the next irq time
-void
+static void
 timer_set(uint32_t value)
 {
     TC0->TC_CHANNEL[0].TC_RA = value;
@@ -63,3 +52,16 @@ timer_shutdown(void)
     TC0->TC_CHANNEL[0].TC_SR; // read to clear irq pending
 }
 DECL_SHUTDOWN(timer_shutdown);
+
+// IRQ handler
+void __visible
+TC0_Handler(void)
+{
+    irq_disable();
+    uint32_t status = TC0->TC_CHANNEL[0].TC_SR; // read to clear irq pending
+    if (likely(status & TC_SR_CPAS)) {
+        uint32_t next = timer_dispatch_many();
+        timer_set(next);
+    }
+    irq_enable();
+}