generic: Move generic parts of sam3x8e timer.c to generic directory

Most of sam3x8e/timer.c is going to be platform agnostic for any board
with standard irq handling.  Move the generic code into a new file
generic/timer.c.

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

1 files changed, 87 insertions, 0 deletions

diff --git a/src/generic/timer.c b/src/generic/timer.c
new file mode 100644
index 00000000..13a8bfe5
--- /dev/null
+++ b/src/generic/timer.c
@@ -0,0 +1,87 @@
+// Generic interrupt based timer helper functions
+//
+// Copyright (C) 2017  Kevin O'Connor <kevin@koconnor.net>
+//
+// This file may be distributed under the terms of the GNU GPLv3 license.
+
+#include "autoconf.h" // CONFIG_CLOCK_FREQ
+#include "board/irq.h" // irq_disable
+#include "board/misc.h" // timer_from_us
+#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
+uint32_t
+timer_from_us(uint32_t us)
+{
+    return us * (CONFIG_CLOCK_FREQ / 1000000);
+}
+
+// Called by main code once every millisecond.  (IRQs disabled.)
+void
+timer_periodic(void)
+{
+}
+
+static uint32_t timer_repeat_until;
+#define TIMER_IDLE_REPEAT_TICKS timer_from_us(500)
+#define TIMER_REPEAT_TICKS timer_from_us(100)
+
+#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.
+uint8_t
+timer_try_set_next(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(sched_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
+    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");
+}
+
+// Periodic background task that temporarily boosts priority of
+// timers.  This helps prioritize timers when tasks are idling.
+static void
+timer_task(void)
+{
+    irq_disable();
+    timer_repeat_until = timer_read_time() + TIMER_IDLE_REPEAT_TICKS;
+    irq_enable();
+}
+DECL_TASK(timer_task);