diff options
author | Aaron Odell <aaronodell@google.com> | 2023-04-04 20:38:32 -0700 |
---|---|---|
committer | Travis Geiselbrecht <travisg@gmail.com> | 2023-06-01 12:40:55 -0700 |
commit | 08111d4a37d72f7ae965a81095f7cbe30c7e9034 (patch) | |
tree | cbbd9903559b3a0a0e3a01259607aea8fa5ae78a | |
parent | 957b9033fbe6a8f384a30f0b3d261442344e27e4 (diff) | |
download | lk-08111d4a37d72f7ae965a81095f7cbe30c7e9034.tar.gz |
[arch][arm-m][systick] Update for monotonicity
Update arm-m systick current_time() and current_time_hires() to advance
monotonically even when interrupts are disabled.
Previous implementation relied on the systick exception triggering
immediately when the counter wrapped and incrementing the current_ticks
count. But if called when interrutps are disabled, then the systick has
not had a chance to trigger and increment the count, but the counter has
already wrapped around. This would result in the current_time() value
moving backwards.
The implementation in this commit is as follows:
- Access to the systick val csr is always done in conjunction with a
check of the COUNTFLAG bit, which indicates counter reaching zero.
- The global current_ticks count is incremented immediately when
wrap around is detected via COUNTFLAG, rather than waiting for the
systick exception to run.
- The check of the counter value, COUNTFLAG bit, and current_ticks
global count are always done in a critical section to avoid race
conditions between current_time and the systick handler.
- The critical section and workarounds are consolidated into a helper
function shared by current_time() and current_time_hires() to
atomically get the tick count and the cycles since the last tick.
The effect should be that current_time() always returns an accurate
monotonically increasing value, regardless of interrupt enablement
or not.
-rw-r--r-- | arch/arm/arm-m/systick/systick.c | 61 |
1 files changed, 38 insertions, 23 deletions
diff --git a/arch/arm/arm-m/systick/systick.c b/arch/arm/arm-m/systick/systick.c index 8917522a..6efd55d2 100644 --- a/arch/arm/arm-m/systick/systick.c +++ b/arch/arm/arm-m/systick/systick.c @@ -31,6 +31,8 @@ static uint32_t tick_rate = 0; static uint32_t tick_rate_mhz = 0; static lk_time_t tick_interval_ms; static lk_bigtime_t tick_interval_us; +static bool systick_configured; +static uint32_t reload; static platform_timer_callback cb; static void *cb_args; @@ -41,9 +43,10 @@ static void arm_cm_systick_set_periodic(lk_time_t period) { uint32_t ticks = tick_rate / (1000 / period); LTRACEF("ticks %d\n", ticks); - SysTick->LOAD = (ticks & SysTick_LOAD_RELOAD_Msk) - 1; + SysTick->LOAD = reload = (ticks - 1) & SysTick_LOAD_RELOAD_Msk; SysTick->VAL = 0; SysTick->CTRL = SysTick_CTRL_CLKSOURCE_Msk | SysTick_CTRL_TICKINT_Msk | SysTick_CTRL_ENABLE_Msk; + systick_configured = true; } static void arm_cm_systick_cancel_periodic(void) { @@ -52,10 +55,12 @@ static void arm_cm_systick_cancel_periodic(void) { /* main systick irq handler */ void _systick(void) { - current_ticks++; - arm_cm_irq_entry(); + if (SysTick->CTRL & SysTick_CTRL_COUNTFLAG_Msk) { + current_ticks++; + } + bool resched = false; if (cb) { lk_time_t now = current_time(); @@ -81,38 +86,48 @@ status_t platform_set_periodic_timer(platform_timer_callback callback, void *arg return NO_ERROR; } +/* Helper to atomically get the global ticks and the current counter value */ +static void systick_get_ticks_val(uint64_t *ticks, uint32_t *val) { + spin_lock_saved_state_t state; + arch_interrupt_save(&state, SPIN_LOCK_FLAG_INTERRUPTS); + + *val = SysTick->VAL; + if (unlikely(SysTick->CTRL & SysTick_CTRL_COUNTFLAG_Msk)) { + current_ticks++; + *val = SysTick->VAL; + } + + *ticks = current_ticks; + + arch_interrupt_restore(state, SPIN_LOCK_FLAG_INTERRUPTS); +} + lk_time_t current_time(void) { - uint32_t reload = SysTick->LOAD & SysTick_LOAD_RELOAD_Msk; + if (!systick_configured) + return 0; - uint64_t t; - uint32_t delta; - do { - t = current_ticks; - delta = (volatile uint32_t)SysTick->VAL; - DMB; - } while (current_ticks != t); + uint32_t val; + uint64_t ticks; + systick_get_ticks_val(&ticks, &val); /* convert ticks to msec */ - delta = (reload - delta) / (tick_rate_mhz * 1000); - lk_time_t res = (t * tick_interval_ms) + delta; + uint32_t delta = (reload - val) / (tick_rate_mhz * 1000); + lk_time_t res = (ticks * tick_interval_ms) + delta; return res; } lk_bigtime_t current_time_hires(void) { - uint32_t reload = SysTick->LOAD & SysTick_LOAD_RELOAD_Msk; + if (!systick_configured) + return 0; - uint64_t t; - uint32_t delta; - do { - t = current_ticks; - delta = (volatile uint32_t)SysTick->VAL; - DMB; - } while (current_ticks != t); + uint32_t val; + uint64_t ticks; + systick_get_ticks_val(&ticks, &val); /* convert ticks to usec */ - delta = (reload - delta) / tick_rate_mhz; - lk_bigtime_t res = (t * tick_interval_us) + delta; + uint32_t delta = (reload - val) / tick_rate_mhz; + lk_bigtime_t res = (ticks * tick_interval_us) + delta; return res; } |