pm: policy: default: Optimize power state selection

Update power state selection. Previously, it was iterating over states
starting from the last one so the most common short sleep periods were
taking the longest time to select. Order is now swapped so that short
sleeps will get power state as quick as possible.

When comparing current sleep time to power mode residency time only
min_residency_us is used as exit time can be neglected as typically
is much lower.

Signed-off-by: Krzysztof Chruściński <[email protected]>

Author: nordic-krch

subsys/pm/policy/policy_default.c

@@ -9,13 +9,12 @@
 #include <zephyr/sys_clock.h>
 #include <zephyr/pm/device.h>
 
-extern int32_t max_latency_cyc;
-
 const struct pm_state_info *pm_policy_next_state(uint8_t cpu, int32_t ticks)
 {
 	int64_t cyc = -1;
 	uint8_t num_cpu_states;
 	const struct pm_state_info *cpu_states;
+	const struct pm_state_info *out_state = NULL;
 
 #ifdef CONFIG_PM_NEED_ALL_DEVICES_IDLE
 	if (pm_device_is_any_busy()) {
@@ -29,29 +28,24 @@ const struct pm_state_info *pm_policy_next_state(uint8_t cpu, int32_t ticks)
 
 	num_cpu_states = pm_state_cpu_get_all(cpu, &cpu_states);
 
-	for (int16_t i = (int16_t)num_cpu_states - 1; i >= 0; i--) {
+	for (uint32_t i = 0; i < num_cpu_states; i++) {
 		const struct pm_state_info *state = &cpu_states[i];
-		uint32_t min_residency_cyc, exit_latency_cyc;
+		uint32_t min_residency_ticks;
 
-		/* check if there is a lock on state + substate */
-		if (pm_policy_state_lock_is_active(state->state, state->substate_id)) {
-			continue;
-		}
+		min_residency_ticks = k_us_to_ticks_ceil32(state->min_residency_us);
 
-		min_residency_cyc = k_us_to_cyc_ceil32(state->min_residency_us);
-		exit_latency_cyc = k_us_to_cyc_ceil32(state->exit_latency_us);
+		if (ticks < min_residency_ticks) {
+			/* If current state has higher residency then use the previous state; */
+			break;
+		}
 
-		/* skip state if it brings too much latency */
-		if ((max_latency_cyc >= 0) &&
-		    (exit_latency_cyc >= max_latency_cyc)) {
+		/* check if state is available. */
+		if (!pm_policy_state_is_available(state->state, state->substate_id)) {
 			continue;
 		}
 
-		if ((cyc < 0) ||
-		    (cyc >= (min_residency_cyc + exit_latency_cyc))) {
-			return state;
-		}
+		out_state = state;
 	}
 
-	return NULL;
+	return out_state;
 }