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[arch][arm-m][systick] fix elapsed cycle calculation in current_time #545

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[arch][arm-m][systick] fix elapsed cycle calculation in current_time #545

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6 changes: 4 additions & 2 deletions arch/arm/arm-m/systick/systick.c
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Original file line number Diff line number Diff line change
Expand Up @@ -110,7 +110,8 @@ lk_time_t current_time(void) {
systick_get_ticks_val(&ticks, &val);

/* convert ticks to msec */
uint32_t delta = (reload - val) / (tick_rate_mhz * 1000);
uint32_t delta_ticks = val ? (reload + 1 - val) : 0;

@travisg travisg Oct 23, 2026

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nit: delta_ticks reads oddly next to current_ticks / *ticks, which mean timer periods in this file. elapsed_cycles or similar would avoid the overload.

Also worth a one-line comment here that VAL == 0 is the cycle between the 1->0 transition and the reload, so the next reader doesn't have to go to the ARM ARM to see why 0 maps to 0 and reload maps to 1.

uint32_t delta = delta_ticks / (tick_rate_mhz * 1000);
lk_time_t res = (ticks * tick_interval_ms) + delta;

return res;
Expand All @@ -125,7 +126,8 @@ lk_bigtime_t current_time_hires(void) {
systick_get_ticks_val(&ticks, &val);

/* convert ticks to usec */
uint32_t delta = (reload - val) / tick_rate_mhz;
uint32_t delta_ticks = val ? (reload + 1 - val) : 0;

@travisg travisg Oct 23, 2026

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nit: same expression as in current_time(). It could live once, e.g. have systick_get_ticks_val() return elapsed cycles instead of the raw VAL, so the two callers only differ in the divisor.

@travisg travisg Oct 23, 2026

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Though these are AI generated this one actually makes the most sense to me. the others are pretty optional.

uint32_t delta = delta_ticks / tick_rate_mhz;
lk_bigtime_t res = (ticks * tick_interval_us) + delta;

return res;
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