cpuidle: psci: Enable the hierarchical topology for s2ram on PREEMPT_RT

The hierarchical PM domain topology are currently disabled on a PREEMPT_RT
based configuration. As a first step to enable it to be used, let's try to
attach the CPU devices to their PM domains on PREEMPT_RT. In this way the
syscore ops becomes available, allowing the PM domain topology to be
managed during s2ram.

For the moment let's leave the support for CPU hotplug outside PREEMPT_RT,
as it's depending on using runtime PM. For s2ram, this isn't a problem as
all CPUs are managed via the syscore ops.
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Raghavendra Kakarla <quic_rkakarla@quicinc.com>  # qcm6490 with PREEMPT_RT set
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/20240527142557.321610-7-ulf.hansson@linaro.org

drivers/cpuidle/cpuidle-psci.c

@@ -37,6 +37,7 @@ struct psci_cpuidle_data {
 
 static DEFINE_PER_CPU_READ_MOSTLY(struct psci_cpuidle_data, psci_cpuidle_data);
 static DEFINE_PER_CPU(u32, domain_state);
+static bool psci_cpuidle_use_syscore;
 static bool psci_cpuidle_use_cpuhp;
 
 void psci_set_domain_state(u32 state)
@@ -166,6 +167,12 @@ static struct syscore_ops psci_idle_syscore_ops = {
 	.resume = psci_idle_syscore_resume,
 };
 
+static void psci_idle_init_syscore(void)
+{
+	if (psci_cpuidle_use_syscore)
+		register_syscore_ops(&psci_idle_syscore_ops);
+}
+
 static void psci_idle_init_cpuhp(void)
 {
 	int err;
@@ -173,8 +180,6 @@ static void psci_idle_init_cpuhp(void)
 	if (!psci_cpuidle_use_cpuhp)
 		return;
 
-	register_syscore_ops(&psci_idle_syscore_ops);
-
 	err = cpuhp_setup_state_nocalls(CPUHP_AP_CPU_PM_STARTING,
 					"cpuidle/psci:online",
 					psci_idle_cpuhp_up,
@@ -222,13 +227,16 @@ static int psci_dt_cpu_init_topology(struct cpuidle_driver *drv,
 	if (!psci_has_osi_support())
 		return 0;
 
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		return 0;
-
 	data->dev = dt_idle_attach_cpu(cpu, "psci");
 	if (IS_ERR_OR_NULL(data->dev))
 		return PTR_ERR_OR_ZERO(data->dev);
 
+	psci_cpuidle_use_syscore = true;
+
+	/* The hierarchical topology is limited to s2ram on PREEMPT_RT. */
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		return 0;
+
 	/*
 	 * Using the deepest state for the CPU to trigger a potential selection
 	 * of a shared state for the domain, assumes the domain states are all
@@ -312,6 +320,7 @@ static void psci_cpu_deinit_idle(int cpu)
 	struct psci_cpuidle_data *data = per_cpu_ptr(&psci_cpuidle_data, cpu);
 
 	dt_idle_detach_cpu(data->dev);
+	psci_cpuidle_use_syscore = false;
 	psci_cpuidle_use_cpuhp = false;
 }
 
@@ -408,6 +417,7 @@ static int psci_cpuidle_probe(struct platform_device *pdev)
 			goto out_fail;
 	}
 
+	psci_idle_init_syscore();
 	psci_idle_init_cpuhp();
 	return 0;