[PATCH v4] cpuidle: psci: Do not suspend topology CPUs on PREEMPT_RT

[Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>]

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.

Cc: Adrien Thierry <athierry@redhat.com>
Cc: Brian Masney <bmasney@redhat.com>
Cc: linux-rt-users@vger.kernel.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>

---

Changes since v3:
1. Rework - disable idle states, mark as always on (Ulf).
2. Extend Kconfig warning (Ulf).

Changes since v1:
1. Re-work commit msg.
2. Add note to Kconfig.

Several other patches were dropped, as this is the only one actually
needed.  It effectively stops PSCI cpuidle power domains from suspending
thus solving all other issues I experienced.
---
 drivers/cpuidle/Kconfig.arm           | 8 ++++++++
 drivers/cpuidle/cpuidle-psci-domain.c | 7 +++++--
 drivers/cpuidle/cpuidle-psci.c        | 3 +++
 3 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/drivers/cpuidle/Kconfig.arm b/drivers/cpuidle/Kconfig.arm
index 747aa537389b..8deaa2e05206 100644
--- a/drivers/cpuidle/Kconfig.arm
+++ b/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
diff --git a/drivers/cpuidle/cpuidle-psci-domain.c b/drivers/cpuidle/cpuidle-psci-domain.c
index c80cf9ddabd8..6ad2954948a5 100644
--- a/drivers/cpuidle/cpuidle-psci-domain.c
+++ b/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;
diff --git a/drivers/cpuidle/cpuidle-psci.c b/drivers/cpuidle/cpuidle-psci.c
index 312a34ef28dc..6de027f9f6f5 100644
--- a/drivers/cpuidle/cpuidle-psci.c
+++ b/drivers/cpuidle/cpuidle-psci.c
@@ -222,6 +222,9 @@ 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);
-- 
2.34.1