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

[Sneh Mankad <sneh.mankad@oss.qualcomm.com>]

On 03-Jul-26 7:58 PM, Mark Rutland wrote:
> On Fri, Jul 03, 2026 at 04:50:02PM +0530, Sneh Mankad wrote:
>> The Qualcomm SoCs like LeMans, Monaco 
> 
> Are those released products?

Yes, the SoCs are present in upstream:

arch/arm64/boot/dts/qcom/lemans.dtsi
arch/arm64/boot/dts/qcom/monaco.dtsi

> 
> Are those mobile phone parts, or somthing else?

These are ride (automotive) chipsets and IoT boards.

> 
>> 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?

FW permits CPU0 disablement because it functions properly in regular run time.
But there is an additional hardware constraint in above mentioned platforms, that
suspend to ram has to be performed via boot CPU / CPU0. Due to this constraint,
CPU0 is required to be online when suspend to ram is triggered.

> What does PSCI_MIGRATE_INFO_TYPE report?

It is a optional function call, which is not implemented in firmware.

> 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...

Also, above platforms are multicore (8 CPUs each).

>> 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.

Yes, but for that reason the config will only be enabled for SoCs that have this
constraint, it will remain disabled for other SoCs.

>> Signed-off-by: Sneh Mankad <sneh.mankad@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?

I can remove ARCH_QCOM dependency if any other want to use 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.
> 

As mentioned above, its not a FW bug. CPU0 is allowed to be disabled from FW and HW constraint is for
suspend to ram to be performed on boot CPU.

Thanks,
Sneh