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// Timer based on ARM Cortex-M3/M4 SysTick and DWT logic
//
// Copyright (C) 2017-2019 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/internal.h" // SysTick
#include "board/irq.h" // irq_disable
#include "board/misc.h" // timer_from_us
#include "command.h" // shutdown
#include "sched.h" // sched_timer_dispatch
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);
}
// Return true if time1 is before time2. Always use this function to
// compare times as regular C comparisons can fail if the counter
// rolls over.
uint8_t
timer_is_before(uint32_t time1, uint32_t time2)
{
return (int32_t)(time1 - time2) < 0;
}
// Set the next irq time
static void
timer_set_diff(uint32_t value)
{
SysTick->LOAD = value;
SysTick->VAL = 0;
SysTick->LOAD = 0;
}
// Return the current time (in absolute clock ticks).
uint32_t
timer_read_time(void)
{
return DWT->CYCCNT;
}
// Activate timer dispatch as soon as possible
void
timer_kick(void)
{
SysTick->LOAD = 0;
SysTick->VAL = 0;
SCB->ICSR = SCB_ICSR_PENDSTSET_Msk;
}
// Implement simple early-boot delay mechanism
void
udelay(uint32_t usecs)
{
if (!(CoreDebug->DEMCR & CoreDebug_DEMCR_TRCENA_Msk)) {
CoreDebug->DEMCR |= CoreDebug_DEMCR_TRCENA_Msk;
DWT->CTRL |= DWT_CTRL_CYCCNTENA_Msk;
}
uint32_t end = timer_read_time() + timer_from_us(usecs);
while (timer_is_before(timer_read_time(), end))
;
}
void
timer_init(void)
{
// Enable Debug Watchpoint and Trace (DWT) for its 32bit timer
CoreDebug->DEMCR |= CoreDebug_DEMCR_TRCENA_Msk;
DWT->CTRL |= DWT_CTRL_CYCCNTENA_Msk;
DWT->CYCCNT = 0;
// Enable SysTick
irqstatus_t flag = irq_save();
NVIC_SetPriority(SysTick_IRQn, 2);
SysTick->CTRL = (SysTick_CTRL_CLKSOURCE_Msk | SysTick_CTRL_TICKINT_Msk
| SysTick_CTRL_ENABLE_Msk);
timer_kick();
irq_restore(flag);
}
DECL_INIT(timer_init);
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(2)
#define TIMER_DEFER_REPEAT_TICKS timer_from_us(5)
// Invoke timers
static uint32_t
timer_dispatch_many(void)
{
uint32_t tru = timer_repeat_until;
for (;;) {
// Run the next software timer
uint32_t next = sched_timer_dispatch();
uint32_t now = timer_read_time();
int32_t diff = next - now;
if (diff > (int32_t)TIMER_MIN_TRY_TICKS)
// Schedule next timer normally.
return diff;
if (unlikely(timer_is_before(tru, now))) {
// Check if there are too many repeat timers
if (diff < (int32_t)(-timer_from_us(1000)))
try_shutdown("Rescheduled timer in the past");
if (sched_tasks_busy()) {
timer_repeat_until = now + TIMER_REPEAT_TICKS;
return TIMER_DEFER_REPEAT_TICKS;
}
timer_repeat_until = tru = now + TIMER_IDLE_REPEAT_TICKS;
}
// 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();
}
}
// IRQ handler
void __visible __aligned(16) // aligning helps stabilize perf benchmarks
SysTick_Handler(void)
{
irq_disable();
uint32_t diff = timer_dispatch_many();
timer_set_diff(diff);
irq_enable();
}
// Make sure timer_repeat_until doesn't wrap 32bit comparisons
void
timer_task(void)
{
uint32_t now = timer_read_time();
irq_disable();
if (timer_is_before(timer_repeat_until, now))
timer_repeat_until = now;
irq_enable();
}
DECL_TASK(timer_task);
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