Re: [PATCH] kernel/cpu: Disallow offlining boot CPU based on config

[Pankaj Patil]

On 6/5/2026 4:00 PM, Sneh Mankad wrote:
> The Qualcomm SoCs like LeMans, Monaco 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.
> 
> In order to prevent such an issue introduce PM_SLEEP_SMP_CPU_ZERO_STRICT
> config and when enabled prohibit the CPU0 to be offline.
> 
> Signed-off-by: Sneh Mankad <sneh.mankad@xxxxxxxxxxxxxxxx>
> ---
>  kernel/cpu.c         | 4 ++++
>  kernel/power/Kconfig | 8 ++++++++
>  2 files changed, 12 insertions(+)
> 
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index bc4f7a9ba64e62d76fbb2b362f243c918defe5bc..1c878db1d9cb6df0e31cb55343851aab4e38a3a0 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -1478,6 +1478,10 @@ static int cpu_down_maps_locked(unsigned int cpu, enum cpuhp_state target)
>  	 */
>  	if (cpu_hotplug_offline_disabled)
>  		return -EOPNOTSUPP;
> +#ifdef CONFIG_PM_SLEEP_SMP_CPU_ZERO_STRICT
> +	if (cpu == get_boot_cpu_id())
> +		return -EPERM;

Should return -ENOTSUPP ?

> +#endif
>  	if (cpu_hotplug_disabled)
>  		return -EBUSY;
>  	return _cpu_down(cpu, 0, target);
> diff --git a/kernel/power/Kconfig b/kernel/power/Kconfig
> index 05337f437cca29da57864583166d7f9abd526507..0b342b3a4a5650a1927dc4805d1515e1d9e067c6 100644
> --- a/kernel/power/Kconfig
> +++ b/kernel/power/Kconfig
> @@ -401,3 +401,11 @@ config ENERGY_MODEL
>  	  The exact usage of the energy model is subsystem-dependent.
>  
>  	  If in doubt, say N.
> +
> +config PM_SLEEP_SMP_CPU_ZERO_STRICT
> +	bool "Disallow boot CPU (CPU0) offline"
> +	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.
> 
> ---
> base-commit: ba3e43a9e601636f5edb54e259a74f96ca3b8fd8
> change-id: 20260603-disable_boot_cpu_offline-eb4f55ac96f2
> 
> Best regards,