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

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Message-ID: <ZTY07FlviAL1DlNz@shell.armlinux.org.uk>
Date:   Mon, 23 Oct 2023 09:55:08 +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 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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