[PATCH RFC 08/22] drivers: base: Implement weak arch_unregister_cpu()

[Russell King (Oracle)]

On Tue, Nov 07, 2023 at 10:29:59AM +0000, Russell King wrote:
> From: James Morse <james.morse at arm.com>
> 
> Add arch_unregister_cpu() to allow the ACPI machinery to call
> unregister_cpu(). This is enough for arm64, riscv and loongarch, but
> needs to be overridden by x86 and ia64 who need to do more work.
> 
> CC: Jean-Philippe Brucker <jean-philippe at linaro.org>
> Signed-off-by: James Morse <james.morse at arm.com>
> ---
> Changes since v1:
>  * Added CONFIG_HOTPLUG_CPU ifdeffery around unregister_cpu
> Changes since RFC v2:
>  * Move earlier in the series
> ---
>  drivers/base/cpu.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
> index 579064fda97b..58bb86091b34 100644
> --- a/drivers/base/cpu.c
> +++ b/drivers/base/cpu.c
> @@ -531,7 +531,14 @@ int __weak arch_register_cpu(int cpu)
>  {
>  	return register_cpu(&per_cpu(cpu_devices, cpu), cpu);
>  }
> -#endif
> +
> +#ifdef CONFIG_HOTPLUG_CPU
> +void __weak arch_unregister_cpu(int num)
> +{
> +	unregister_cpu(&per_cpu(cpu_devices, num));
> +}
> +#endif /* CONFIG_HOTPLUG_CPU */

I have previously asked the question whether we should provide a
stub weak function for the !HOTPLUG_CPU case for this, which would
alleviate the concerns around if (IS_ENABLED()) in some of the later
hotplug vCPU patches... which failed to get _any_ responses.

So, I'm now going to deem the comment I received about if (IS_ENABLED())
potentially causing issues to be unimportant, and thus there's no
need for a stub weak function. If we start getting compile errors,
then we can address the issue at that point. So far, however, the
kernel build bot has not identified that this as an issue... and it's
been chewing on this entire patch set for well over a month now.

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