[PATCH v3] cpuidle: psci: Do not suspend topology CPUs on PREEMPT_RT

[Krzysztof Kozlowski]

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 runtime PM calls from cpuidle-psci, which effectively stops
suspending the cpuidle PSCI domain.  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 at redhat.com>
Cc: Brian Masney <bmasney at redhat.com>
Cc: linux-rt-users at vger.kernel.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski at linaro.org>

---

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    | 3 +++
 drivers/cpuidle/cpuidle-psci.c | 4 ++--
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/cpuidle/Kconfig.arm b/drivers/cpuidle/Kconfig.arm
index 747aa537389b..24429b5bfd1c 100644
--- a/drivers/cpuidle/Kconfig.arm
+++ b/drivers/cpuidle/Kconfig.arm
@@ -24,6 +24,9 @@ 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 is not yet compatible with PREEMPT_RT: no idle states will
+	  be entered by CPUs on such kernel.
+
 config ARM_PSCI_CPUIDLE_DOMAIN
 	bool "PSCI CPU idle Domain"
 	depends on ARM_PSCI_CPUIDLE
diff --git a/drivers/cpuidle/cpuidle-psci.c b/drivers/cpuidle/cpuidle-psci.c
index 312a34ef28dc..c25592718984 100644
--- a/drivers/cpuidle/cpuidle-psci.c
+++ b/drivers/cpuidle/cpuidle-psci.c
@@ -66,7 +66,7 @@ static __cpuidle int __psci_enter_domain_idle_state(struct cpuidle_device *dev,
 	/* Do runtime PM to manage a hierarchical CPU toplogy. */
 	if (s2idle)
 		dev_pm_genpd_suspend(pd_dev);
-	else
+	else if (!IS_ENABLED(CONFIG_PREEMPT_RT))
 		pm_runtime_put_sync_suspend(pd_dev);
 
 	state = psci_get_domain_state();
@@ -77,7 +77,7 @@ static __cpuidle int __psci_enter_domain_idle_state(struct cpuidle_device *dev,
 
 	if (s2idle)
 		dev_pm_genpd_resume(pd_dev);
-	else
+	else if (!IS_ENABLED(CONFIG_PREEMPT_RT))
 		pm_runtime_get_sync(pd_dev);
 
 	cpu_pm_exit();
-- 
2.34.1