linux-kernel - Re: [RFC PATCH v2 24/35] drivers: base: Implement weak arch_unregister

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Message-ID: <ZTYygtKt4w9C0Oxw@shell.armlinux.org.uk>
Date:   Mon, 23 Oct 2023 09:44:50 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Gavin Shan <gshan@...hat.com>
Cc:     James Morse <james.morse@....com>, linux-pm@...r.kernel.org,
        loongarch@...ts.linux.dev, linux-acpi@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, kvmarm@...ts.linux.dev,
        x86@...nel.org, Salil Mehta <salil.mehta@...wei.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        jianyong.wu@....com, justin.he@....com
Subject: Re: [RFC PATCH v2 24/35] drivers: base: Implement weak
 arch_unregister_cpu()

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?

Thanks.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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