cpuidle: psci: Do not suspend topology CPUs on PREEMPT_RT

The runtime Power Management of CPU topology is not compatible with
PREEMPT_RT:

 1. Core cpuidle path disables IRQs.
 2. Core cpuidle calls cpuidle-psci.
 3. cpuidle-psci in __psci_enter_domain_idle_state() calls
    pm_runtime_put_sync_suspend() and pm_runtime_get_sync() which use
    spinlocks (which are sleeping on PREEMPT_RT).

Deep sleep modes are not a priority of Realtime kernels because the
latencies might become unpredictable.  On the other hand the PSCI CPU
idle power domain is a parent of other devices and power domain
controllers, thus it cannot be simply skipped (e.g. on Qualcomm SM8250).

Disable the idle callbacks in cpuidle-psci and mark the domain as
always on.  This is a trade-off between making PREEMPT_RT working and
still having a proper power domain hierarchy in the system.
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Adrien Thierry <athierry@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/cpuidle/Kconfig.arm

@@ -24,6 +24,14 @@ config ARM_PSCI_CPUIDLE
 	  It provides an idle driver that is capable of detecting and
 	  managing idle states through the PSCI firmware interface.
 
+	  The driver has limitations when used with PREEMPT_RT:
+	  - If the idle states are described with the non-hierarchical layout,
+	    all idle states are still available.
+
+	  - If the idle states are described with the hierarchical layout,
+	    only the idle states defined per CPU are available, but not the ones
+	    being shared among a group of CPUs (aka cluster idle states).
+
 config ARM_PSCI_CPUIDLE_DOMAIN
 	bool "PSCI CPU idle Domain"
 	depends on ARM_PSCI_CPUIDLE

drivers/cpuidle/cpuidle-psci-domain.c

@@ -64,8 +64,11 @@ static int psci_pd_init(struct device_node *np, bool use_osi)
 
 	pd->flags |= GENPD_FLAG_IRQ_SAFE | GENPD_FLAG_CPU_DOMAIN;
 
-	/* Allow power off when OSI has been successfully enabled. */
-	if (use_osi)
+	/*
+	 * Allow power off when OSI has been successfully enabled.
+	 * PREEMPT_RT is not yet ready to enter domain idle states.
+	 */
+	if (use_osi && !IS_ENABLED(CONFIG_PREEMPT_RT))
 		pd->power_off = psci_pd_power_off;
 	else
 		pd->flags |= GENPD_FLAG_ALWAYS_ON;

drivers/cpuidle/cpuidle-psci.c

@@ -231,6 +231,9 @@ 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 = psci_dt_attach_cpu(cpu);
 	if (IS_ERR_OR_NULL(data->dev))
 		return PTR_ERR_OR_ZERO(data->dev);