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drivers: nrf_rtc_timer: Rework set_absolute_alarm()
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Eliminate waiting for a potential COMPARE event when setting a CC
value close to the previously set one and rely instead on checking
target time when processing channel events in the ISR.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>
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@anangl @carlescufi
anangl authored and carlescufi committed Jan 17, 2023
1 parent bf1d3db commit 205e684
Showing 1 changed file with 55 additions and 52 deletions.
107 changes: 55 additions & 52 deletions drivers/timer/nrf_rtc_timer.c
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Original file line number Diff line number Diff line change
Expand Up @@ -71,6 +71,11 @@ static uint32_t get_comparator(int32_t chan)
return nrf_rtc_cc_get(RTC, chan);
}

static bool event_check(int32_t chan)
{
return nrf_rtc_event_check(RTC, RTC_CHANNEL_EVENT_ADDR(chan));
}

static void event_clear(int32_t chan)
{
nrf_rtc_event_clear(RTC, RTC_CHANNEL_EVENT_ADDR(chan));
Expand Down Expand Up @@ -225,63 +230,56 @@ uint64_t z_nrf_rtc_timer_get_ticks(k_timeout_t t)
* @param[in] chan A channel for which a new CC value is to be set.
*
* @param[in] abs_val An absolute value of CC register to be set.
*
* @returns CC value that was actually set. It is equal to @p abs_val or
* shifted ahead if @p abs_val was too near in the future (+1 case).
*/
static uint32_t set_absolute_alarm(int32_t chan, uint32_t abs_val)
static void set_absolute_alarm(int32_t chan, uint32_t abs_val)
{
uint32_t now;
uint32_t now2;
uint32_t cc_val = abs_val & COUNTER_MAX;
uint32_t prev_cc = get_comparator(chan);
uint32_t tick_inc = 2;
uint32_t cc_inc = 2;

/* Disable event routing for the channel to avoid getting a COMPARE
* event for the previous CC value before the new one takes effect
* (however, even if such spurious event was generated, it would be
* properly filtered out in process_channel(), where the target time
* is checked).
* Clear also the event as it may already be generated at this point.
*/
event_disable(chan);
event_clear(chan);

do {
now = counter();
for (;;) {
uint32_t now;

/* Handle case when previous event may generate an event.
* It is handled by setting CC to now (far in the future),
* in case previous event was set for next tick wait for half
* LF tick and clear event that may have been generated.
set_comparator(chan, cc_val);
/* Enable event routing after the required CC value was set.
* Even though the above operation may get repeated (see below),
* there is no need to disable event routing in every iteration
* of the loop, as the COMPARE event resulting from any attempt
* of setting the CC register is acceptable (as mentioned above,
* process_channel() does the proper filtering).
*/
set_comparator(chan, now);
if (counter_sub(prev_cc, now) == 1) {
/* It should wait for half of RTC tick 15.26us. As
* busy wait runs from different clock source thus
* wait longer to cover for discrepancy.
*/
k_busy_wait(19);
}
event_enable(chan);

/* RTC may not generate event if CC is set for 1 tick from now.
* Because of that if requested cc_val is in the past or next tick,
* set CC to further in future. Start with 2 ticks from now but
* if that fails go even futher. It may fail if operation got
* interrupted and RTC counter progressed or if optimization is
* turned off.
*/
if (counter_sub(cc_val, now + 2) > COUNTER_HALF_SPAN) {
cc_val = now + tick_inc;
tick_inc++;
}
now = counter();

event_clear(chan);
event_enable(chan);
set_comparator(chan, cc_val);
now2 = counter();
prev_cc = cc_val;
/* Rerun the algorithm if counter progressed during execution
* and cc_val is in the past or one tick from now. In such
* scenario, it is possible that event will not be generated.
* Rerunning the algorithm will delay the alarm but ensure that
* event will be generated at the moment indicated by value in
* CC register.
/* RTC may not generate a COMPARE event if its COUNTER value
* is N and a given CC register is set to N or N+1. If it turns
* out that the above configuration of the comparator resulted
* in such CC value or even in a value that is considered to be
* from the past, repeat the operation using a CC value that is
* guaranteed to generate the event. Start with 2 RTC ticks from
* now and if that fails (because the operation gets delayed),
* go even futher in the next attempt.
* But if the COMPARE event turns out to be already generated,
* there is obviously no need to continue the loop.
*/
} while ((now2 != now) &&
(counter_sub(cc_val, now2 + 2) > COUNTER_HALF_SPAN));

return cc_val;
if ((counter_sub(cc_val, now + 2) > COUNTER_HALF_SPAN) &&
!event_check(chan)) {
cc_val = now + cc_inc;
cc_inc++;
} else {
break;
}
}
}

static int compare_set_nolocks(int32_t chan, uint64_t target_time,
Expand All @@ -302,9 +300,7 @@ static int compare_set_nolocks(int32_t chan, uint64_t target_time,
/* Target time is valid and is different than currently set.
* Set CC value.
*/
uint32_t cc_set = set_absolute_alarm(chan, cc_value);

target_time += counter_sub(cc_set, cc_value);
set_absolute_alarm(chan, cc_value);
}
} else {
/* Force ISR handling when exiting from critical section. */
Expand Down Expand Up @@ -454,7 +450,7 @@ static bool channel_processing_check_and_clear(int32_t chan)
* or be forced.
*/
result = atomic_and(&force_isr_mask, ~BIT(chan)) ||
nrf_rtc_event_check(RTC, RTC_CHANNEL_EVENT_ADDR(chan));
event_check(chan);

if (result) {
event_clear(chan);
Expand Down Expand Up @@ -493,6 +489,13 @@ static void process_channel(int32_t chan)
cc_data[chan].callback = NULL;
cc_data[chan].target_time = TARGET_TIME_INVALID;
event_disable(chan);
/* Because of the way set_absolute_alarm() sets the CC
* register, it may turn out that another COMPARE event
* has been generated for the same alarm. Make sure the
* event is cleared, so that the ISR is not executed
* again unnecessarily.
*/
event_clear(chan);
}

full_int_unlock(mcu_critical_state);
Expand Down

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