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