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From: "Rafael J. Wysocki" <rafael@kernel.org>
Date: Wed, 10 Apr 2024 15:28:18 +0200
Message-ID: <CAJZ5v0ggD042sfz3jDXQVDUxQZu_AWaF2ox-Me8CvFeRB8nczw@mail.gmail.com>
Subject: Re: [PATCH RFC v4 02/15] ACPI: processor: Register all CPUs from acpi_processor_get_info()
To: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>, Russell King <rmk+kernel@armlinux.org.uk>, 
	linux-pm@vger.kernel.org, loongarch@lists.linux.dev, 
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	linux-doc@vger.kernel.org, linux-ia64@vger.kernel.org, 
	linux-parisc@vger.kernel.org, Salil Mehta <salil.mehta@huawei.com>, 
	Jean-Philippe Brucker <jean-philippe@linaro.org>, jianyong.wu@arm.com, justin.he@arm.com, 
	James Morse <james.morse@arm.com>
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On Wed, Apr 10, 2024 at 2:43=E2=80=AFPM Jonathan Cameron
<Jonathan.Cameron@huawei.com> wrote:
>
> > >
> > > > diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
> > > > index 47de0f140ba6..13d052bf13f4 100644
> > > > --- a/drivers/base/cpu.c
> > > > +++ b/drivers/base/cpu.c
> > > > @@ -553,7 +553,11 @@ static void __init cpu_dev_register_generic(vo=
id)
> > > >  {
> > > >         int i, ret;
> > > >
> > > > -       if (!IS_ENABLED(CONFIG_GENERIC_CPU_DEVICES))
> > > > +       /*
> > > > +        * When ACPI is enabled, CPUs are registered via
> > > > +        * acpi_processor_get_info().
> > > > +        */
> > > > +       if (!IS_ENABLED(CONFIG_GENERIC_CPU_DEVICES) || !acpi_disabl=
ed)
> > > >                 return;
> > >
> > > Honestly, this looks like a quick hack to me and it absolutely
> > > requires an ACK from the x86 maintainers to go anywhere.
> > Will address this separately.
> >
>
> So do people prefer this hack, or something along lines of the following?
>
> static int __init cpu_dev_register_generic(void)
> {
>         int i, ret =3D 0;
>
>         for_each_online_cpu(i) {
>                 if (!get_cpu_device(i)) {
>                         ret =3D arch_register_cpu(i);
>                         if (ret)
>                                 pr_warn("register_cpu %d failed (%d)\n", =
i, ret);
>                 }
>         }
>         //Probably just eat the error.
>         return 0;
> }
> subsys_initcall_sync(cpu_dev_register_generic);

I would prefer something like the above.

I actually thought that arch_register_cpu() might return something
like -EPROBE_DEFER when it cannot determine whether or not the CPU is
really available.

Then, the ACPI processor enumeration path may take care of registering
CPU that have not been registered so far and in the more-or-less the
same way regardless of the architecture (modulo some arch-specific
stuff).

In the end, it should be possible to avoid changing the behavior of
x86 and loongarch in this series.

> Which may look familiar at it's effectively patch 3 from v3 which was dea=
ling
> with CPUs missing from DSDT (something we think doesn't happen).
>
> It might be possible to elide the arch_register_cpu() in
> make_present() but that will mean we use different flows in this patch se=
t
> for the hotplug and initially present cases which is a bit messy.
>
> I've tested this lightly on arm64 and x86 ACPI + DT booting and it "seems=
" fine.

Sounds promising.

Thanks!