Re: [PATCH v8 13/16] arm64: arch_register_cpu() variant to check if an ACPI handle is now available.

[Gavin Shan <gshan@redhat.com>]

On 4/26/24 23:51, Jonathan Cameron wrote:
> The ARM64 architecture does not support physical CPU HP today.
> To avoid any possibility of a bug against such an architecture if defined
> in future, check for the physical CPU HP case (not present) and
> return an error on any such attempt.
> 
> On ARM64 virtual CPU Hotplug relies on the status value that can be
> queried via the AML method _STA for the CPU object.
> 
> There are two conditions in which the CPU can be registered.
> 1) ACPI disabled.
> 2) ACPI enabled and the acpi_handle is available.
>     _STA evaluates to the CPU is both enabled and present.
>     (Note that in absence of the _STA method they are always in this
>      state).
> 
> If neither of these conditions is met the CPU is not 'yet' ready
> to be used and -EPROBE_DEFER is returned.
> 
> Success occurs in the early attempt to register the CPUs if we
> are booting with DT (no concept yet of vCPU HP) if not it succeeds
> for already enabled CPUs when the ACPI Processor driver attaches to
> them.  Finally it may succeed via the CPU Hotplug code indicating that
> the CPU is now enabled.
> 
> For ACPI if CONFIG_ACPI_PROCESSOR the only path to get to
> arch_register_cpu() with that handle set is via
> acpi_processor_hot_add_init() which is only called from an ACPI bus
> scan in which _STA has already been queried there is no need to
> repeat it here. Add a comment to remind us of this in the future.
> 
> Suggested-by: Rafael J. Wysocki <rafael@kernel.org>
> Tested-by: Miguel Luis <miguel.luis@oracle.com>
> Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> ---
> V8: No change. (tags collected only)
> ---
>   arch/arm64/kernel/smp.c | 53 +++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 53 insertions(+)
> 

Reviewed-by: Gavin Shan <gshan@redhat.com>