[PATCH v2] arm64: Disallow disabling boot CPU based on config

[Mark Rutland]

On Fri, Jul 03, 2026 at 04:50:02PM +0530, Sneh Mankad wrote:
> The Qualcomm SoCs like LeMans, Monaco 

Are those released products?

Are those mobile phone parts, or somthing else?

> support suspend to ram which leads the SoC to ACPI S3 similar state
> where SoC is turned off and DDR is retained. The hardware design on
> these SoCs forces a constraint to suspend and resume the system on
> boot CPU / CPU0.
> 
> If CPU0 is already offline before starting suspend to ram the
> freeze_secondary_cpus() picks alternate CPU as primary / last CPU and
> proceed further to invoke PSCI SYSTEM_SUSPEND.
> This leads to a system crash.

Ok, so that's a firmware bug.

Why does the FW permit CPU0 to be offlined in the first place if it
can't handle this?

What does PSCI_MIGRATE_INFO_TYPE report?

Ideally it'd report Uniprocessor (UP) not migrate capable (1), which
would prevent CPU_OFF on that CPU, and would force suspend to happen
there...

> In order to prevent such an issue introduce PM_SLEEP_SMP_CPU_ZERO_STRICT
> config and when enabled prohibit the CPU0 from getting disabled.

I don't think it makes sense for this to be a config option.

This is a platform-specific property, and it's possible to build a
kernel that boots on this platform and/or other platforms.

> Signed-off-by: Sneh Mankad <sneh.mankad at oss.qualcomm.com>
> ---
> Changes in v2:
> - Moved the check to arm64 specific code.
> - Link to v1: https://lore.kernel.org/r/20260605-disable_boot_cpu_offline-v1-1-4c68fe1a6cf8@oss.qualcomm.com
> ---
>  arch/arm64/Kconfig       | 9 +++++++++
>  arch/arm64/kernel/psci.c | 6 ++++++
>  2 files changed, 15 insertions(+)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index fe60738e5943ba279e5571862423df4fed3db661..21697a535a25d286a2f8afe4921a41b13cc32c0a 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -263,6 +263,15 @@ config ARM64
>  	help
>  	  ARM 64-bit (AArch64) Linux support.
>  
> +config PM_SLEEP_SMP_CPU_ZERO_STRICT
> +       bool "Disallow boot CPU (CPU0) offline"
> +       depends on ARCH_QCOM

Why can't others select this?

> +       depends on HOTPLUG_CPU
> +       depends on SUSPEND
> +       help
> +         Disallow boot CPU (CPU0) offline when the suspend_ops->enter()
> +         has to be executed by boot CPU.

As above, I don't think this makse sense as a config option.

Either we handle the FW bug, or we do not.

Mark.

> +
>  config RUSTC_SUPPORTS_ARM64
>  	def_bool y
>  	depends on CPU_LITTLE_ENDIAN
> diff --git a/arch/arm64/kernel/psci.c b/arch/arm64/kernel/psci.c
> index fabd732d0a2dfee37074ef4ebb6ce5894871c8bd..4ad90ae6f8bacf0cbd3203d66580107d467ea232 100644
> --- a/arch/arm64/kernel/psci.c
> +++ b/arch/arm64/kernel/psci.c
> @@ -49,6 +49,12 @@ static int cpu_psci_cpu_boot(unsigned int cpu)
>  #ifdef CONFIG_HOTPLUG_CPU
>  static bool cpu_psci_cpu_can_disable(unsigned int cpu)
>  {
> +#ifdef CONFIG_PM_SLEEP_SMP_CPU_ZERO_STRICT
> +	if (cpu == get_boot_cpu_id()) {
> +		pr_info("Disabling boot CPU is not supported\n");
> +		return false;
> +	}
> +#endif
>  	return !psci_tos_resident_on(cpu);
>  }
>  
> 
> ---
> base-commit: ba3e43a9e601636f5edb54e259a74f96ca3b8fd8
> change-id: 20260603-disable_boot_cpu_offline-eb4f55ac96f2
> 
> Best regards,
> -- 
> Sneh Mankad <sneh.mankad at oss.qualcomm.com>
> 
>