[RFC,v2,01/35] ACPI: Move ACPI_HOTPLUG_CPU to be disabled on arm64 and riscv

Neither arm64 nor riscv support physical hotadd of CPUs that were not
present at boot. For arm64 much of the platform description is in static
tables which do not have update methods. arm64 does support HOTPLUG_CPU,
which is backed by a firmware interface to turn CPUs on and off.

acpi_processor_hotadd_init() and acpi_processor_remove() are for adding
and removing CPUs that were not present at boot. arm64 systems that do this
are not supported as there is currently insufficient information in the
platform description. (e.g. did the GICR get removed too?)

arm64 currently relies on the MADT enabled flag check in map_gicc_mpidr()
to prevent CPUs that were not described as present at boot from being
added to the system. Similarly, riscv relies on the same check in
map_rintc_hartid(). Both architectures also rely on the weak 'always fails'
definitions of acpi_map_cpu() and arch_register_cpu().

Subsequent changes will redefine ACPI_HOTPLUG_CPU as making possible
CPUs present. Neither arm64 nor riscv support this.

Disable ACPI_HOTPLUG_CPU for arm64 and riscv by removing 'default y' and
selecting it on the other three ACPI architectures. This allows the weak
definitions of some symbols to be removed.

Signed-off-by: James Morse <james.morse@arm.com>
---
Changes since RFC:
 * Expanded conditions to avoid ACPI_HOTPLUG_CPU being enabled when
   HOTPLUG_CPU isn't.
---
 arch/ia64/Kconfig                |  1 +
 arch/loongarch/Kconfig           |  1 +
 arch/loongarch/include/asm/cpu.h |  7 +++++++
 arch/x86/Kconfig                 |  1 +
 drivers/acpi/Kconfig             |  1 -
 drivers/acpi/acpi_processor.c    | 18 ------------------
 6 files changed, 10 insertions(+), 19 deletions(-)

On Wed, Sep 13, 2023 at 04:37:49PM +0000, James Morse wrote:
> diff --git a/arch/loongarch/include/asm/cpu.h b/arch/loongarch/include/asm/cpu.h
> index 48b9f7168bcc..7afe8cbb844e 100644
> --- a/arch/loongarch/include/asm/cpu.h
> +++ b/arch/loongarch/include/asm/cpu.h
> @@ -128,4 +128,11 @@ enum cpu_type_enum {
>  #define LOONGARCH_CPU_HYPERVISOR	BIT_ULL(CPU_FEATURE_HYPERVISOR)
>  #define LOONGARCH_CPU_PTW		BIT_ULL(CPU_FEATURE_PTW)
>  
> +#if !defined(__ASSEMBLY__)
> +#ifdef CONFIG_HOTPLUG_CPU
> +int arch_register_cpu(int num);
> +void arch_unregister_cpu(int cpu);
> +#endif
> +#endif /* ! __ASSEMBLY__ */

So, for loongarch:

grep arch_.*register_cpu arch/loongarch/ -r
arch/loongarch/kernel/topology.c:int arch_register_cpu(int cpu)
arch/loongarch/kernel/topology.c:EXPORT_SYMBOL(arch_register_cpu);
arch/loongarch/kernel/topology.c:void arch_unregister_cpu(int cpu)
arch/loongarch/kernel/topology.c:EXPORT_SYMBOL(arch_unregister_cpu);

So really this is a fix (since these functions should have prototypes)
and thus should probably be a separate patch.

However, I also wonder whether these prototypes should be added to
linux/cpu.h and be done with it (rather than have every arch prototype
these - it's not like the prototype can be different from this because
of the generic code.

I know in subsequent patches you do that, but it's rather piecemeal,
and I think this is a change that could be submitted now as both a
fix and clean up.