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    SubjectRe: [RFC PATCH v2 24/35] drivers: base: Implement weak arch_unregister_cpu()
    On Mon, Oct 23, 2023 at 09:44:50AM +0100, Russell King (Oracle) wrote:
    > On Tue, Sep 19, 2023 at 10:59:23AM +1000, Gavin Shan wrote:
    > > On 9/14/23 02:38, James Morse wrote:
    > > > diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
    > > > index 677f963e02ce..c709747c4a18 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 */
    > > > +#endif /* CONFIG_GENERIC_CPU_DEVICES */
    > >
    > > It seems conflicting with its declaration in include/linux/cpu.h.
    >
    > How so? The declaration is:
    >
    > extern void arch_unregister_cpu(int cpu);
    >
    > So:
    >
    > void __weak arch_unregister_cpu(int num)
    >
    > is compatible.
    >
    > > Besides, the function is still needed by
    > > drivers/acpi/acpi_processor.c::acpi_processor_make_not_present()
    > > even both CONFIG_HOTPLUG_CPU and CONFIG_GENERIC_CPU_DEVICES are disabled?
    >
    > Yes, I agree - it needs to be present when ACPI is built, so I'm
    > thinking the right solution is to move it out from under at least
    > CONFIG_HOTPLUG_CPU.
    >
    > It can't be moved out from under CONFIG_GENERIC_CPU_DEVICES because
    > then we end up referencing the per-cpu variable cpu_devices which only
    > exists when CONFIG_GENERIC_CPU_DEVICES is enabled. Is that a problem
    > though, because in the case of !CONFIG_GENERIC_CPU_DEVICES, aren't
    > architectures required to provide both arch_.*register_cpu() functions?

    I'm also wondering why this patch isn't part of:

    "drivers: base: Allow parts of GENERIC_CPU_DEVICES to be overridden"
    because it seems to be doing something very similar.

    The commit I refer to introduces a weak version of arch_register_cpu(),
    and it seems it would also be appropriate to introduce a weak version
    of its unregister paired function at the same time.

    Any existing definitions of non-weak arch_unregister_cpu() would
    override it so it shouldn't cause any issues.

    Thanks.

    --
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