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From: "Rafael J. Wysocki" <rafael@kernel.org>
Date: Thu, 15 Feb 2024 20:22:29 +0100
Message-ID: <CAJZ5v0iiJpUWq5GMSnKFWQTzn_bdwoQz9m=hDaXNg4Lj_ePF4g@mail.gmail.com>
Subject: Re: [PATCH RFC v4 02/15] ACPI: processor: Register all CPUs from acpi_processor_get_info()
To: Russell King <rmk+kernel@armlinux.org.uk>
Cc: linux-pm@vger.kernel.org, loongarch@lists.linux.dev, 
	linux-acpi@vger.kernel.org, linux-arch@vger.kernel.org, 
	linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, 
	linux-riscv@lists.infradead.org, kvmarm@lists.linux.dev, x86@kernel.org, 
	acpica-devel@lists.linuxfoundation.org, linux-csky@vger.kernel.org, 
	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>, Jonathan Cameron <Jonathan.Cameron@huawei.com>, 
	"Rafael J. Wysocki" <rafael@kernel.org>
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On Wed, Jan 31, 2024 at 5:50=E2=80=AFPM Russell King <rmk+kernel@armlinux.o=
rg.uk> wrote:
>
> From: James Morse <james.morse@arm.com>
>
> To allow ACPI to skip the call to arch_register_cpu() when the _STA
> value indicates the CPU can't be brought online right now, move the
> arch_register_cpu() call into acpi_processor_get_info().
>
> Systems can still be booted with 'acpi=3Doff', or not include an
> ACPI description at all. For these, the CPUs continue to be
> registered by cpu_dev_register_generic().
>
> This moves the CPU register logic back to a subsys_initcall(),
> while the memory nodes will have been registered earlier.
>
> Signed-off-by: James Morse <james.morse@arm.com>
> Reviewed-by: Gavin Shan <gshan@redhat.com>
> Tested-by: Miguel Luis <miguel.luis@oracle.com>
> Tested-by: Vishnu Pajjuri <vishnu@os.amperecomputing.com>
> Tested-by: Jianyong Wu <jianyong.wu@arm.com>
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
> ---
> Changes since RFC v2:
>  * Fixup comment in acpi_processor_get_info() (Gavin Shan)
>  * Add comment in cpu_dev_register_generic() (Gavin Shan)
> ---
>  drivers/acpi/acpi_processor.c | 12 ++++++++++++
>  drivers/base/cpu.c            |  6 +++++-
>  2 files changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/acpi/acpi_processor.c b/drivers/acpi/acpi_processor.=
c
> index cf7c1cca69dd..a68c475cdea5 100644
> --- a/drivers/acpi/acpi_processor.c
> +++ b/drivers/acpi/acpi_processor.c
> @@ -314,6 +314,18 @@ static int acpi_processor_get_info(struct acpi_devic=
e *device)
>                         cpufreq_add_device("acpi-cpufreq");
>         }
>
> +       /*
> +        * Register CPUs that are present. get_cpu_device() is used to sk=
ip
> +        * duplicate CPU descriptions from firmware.
> +        */
> +       if (!invalid_logical_cpuid(pr->id) && cpu_present(pr->id) &&
> +           !get_cpu_device(pr->id)) {
> +               int ret =3D arch_register_cpu(pr->id);
> +
> +               if (ret)
> +                       return ret;
> +       }
> +
>         /*
>          *  Extra Processor objects may be enumerated on MP systems with
>          *  less than the max # of CPUs. They should be ignored _iff

This is interesting, because right below there is the following code:

    if (invalid_logical_cpuid(pr->id) || !cpu_present(pr->id)) {
        int ret =3D acpi_processor_hotadd_init(pr);

        if (ret)
            return ret;
    }

and acpi_processor_hotadd_init() essentially calls arch_register_cpu()
with some extra things around it (more about that below).

I do realize that acpi_processor_hotadd_init() is defined under
CONFIG_ACPI_HOTPLUG_CPU, so for the sake of the argument let's
consider an architecture where CONFIG_ACPI_HOTPLUG_CPU is set.

So why are the two conditionals that almost contradict each other both
needed?  It looks like the new code could be combined with
acpi_processor_hotadd_init() to do the right thing in all cases.

Now, acpi_processor_hotadd_init() does some extra things that look
like they should be done by the new code too.

1. It checks invalid_phys_cpuid() which appears to be a good idea to me.

2. It uses locking around arch_register_cpu() which doesn't seem
unreasonable either.

3. It calls acpi_map_cpu() and I'm not sure why this is not done by
the new code.

The only thing that can be dropped from it is the _STA check AFAICS,
because acpi_processor_add() won't even be called if the CPU is not
present (and not enabled after the first patch).

So why does the code not do 1 - 3 above?

> 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(void)
>  {
>         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_disabled)
>                 return;

Honestly, this looks like a quick hack to me and it absolutely
requires an ACK from the x86 maintainers to go anywhere.

>
>         for_each_present_cpu(i) {
> --