linux-kernel - Re: [PATCH RFC v3 02/21] ACPI: processor: Add support for processors described as container packages

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Date: Mon, 18 Dec 2023 21:17:34 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Russell King <rmk+kernel@...linux.org.uk>
Cc: 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, 
	acpica-devel@...ts.linuxfoundation.org, linux-csky@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-ia64@...r.kernel.org, 
	linux-parisc@...r.kernel.org, Salil Mehta <salil.mehta@...wei.com>, 
	Jean-Philippe Brucker <jean-philippe@...aro.org>, jianyong.wu@....com, justin.he@....com, 
	James Morse <james.morse@....com>
Subject: Re: [PATCH RFC v3 02/21] ACPI: processor: Add support for processors
 described as container packages

On Wed, Dec 13, 2023 at 1:49 PM Russell King <rmk+kernel@...linux.org.uk> wrote:
>
> From: James Morse <james.morse@....com>
>
> ACPI has two ways of describing processors in the DSDT. From ACPI v6.5,
> 5.2.12:
>
> "Starting with ACPI Specification 6.3, the use of the Processor() object
> was deprecated. Only legacy systems should continue with this usage. On
> the Itanium architecture only, a _UID is provided for the Processor()
> that is a string object. This usage of _UID is also deprecated since it
> can preclude an OSPM from being able to match a processor to a
> non-enumerable device, such as those defined in the MADT. From ACPI
> Specification 6.3 onward, all processor objects for all architectures
> except Itanium must now use Device() objects with an _HID of ACPI0007,
> and use only integer _UID values."
>
> Also see https://uefi.org/specs/ACPI/6.5/08_Processor_Configuration_and_Control.html#declaring-processors
>
> Duplicate descriptions are not allowed, the ACPI processor driver already
> parses the UID from both devices and containers. acpi_processor_get_info()
> returns an error if the UID exists twice in the DSDT.

I'm not really sure how the above is related to the actual patch.

> The missing probe for CPUs described as packages

It is unclear what exactly is meant by "CPUs described as packages".

>From the patch, it looks like those would be Processor() objects
defined under a processor container device.

> creates a problem for
> moving the cpu_register() calls into the acpi_processor driver, as CPUs
> described like this don't get registered, leading to errors from other
> subsystems when they try to add new sysfs entries to the CPU node.
> (e.g. topology_sysfs_init()'s use of topology_add_dev() via cpuhp)
>
> To fix this, parse the processor container and call acpi_processor_add()
> for each processor that is discovered like this.

Discovered like what?

> The processor container
> handler is added with acpi_scan_add_handler(), so no detach call will
> arrive.

The above requires clarification too.

> Qemu TCG describes CPUs using processor devices in a processor container.
> For more information, see build_cpus_aml() in Qemu hw/acpi/cpu.c and
> https://uefi.org/specs/ACPI/6.5/08_Processor_Configuration_and_Control.html#processor-container-device
>
> Signed-off-by: James Morse <james.morse@....com>
> Tested-by: Miguel Luis <miguel.luis@...cle.com>
> Tested-by: Vishnu Pajjuri <vishnu@...amperecomputing.com>
> Tested-by: Jianyong Wu <jianyong.wu@....com>
> ---
> Outstanding comments:
>  https://lore.kernel.org/r/20230914145353.000072e2@Huawei.com
>  https://lore.kernel.org/r/50571c2f-aa3c-baeb-3add-cd59e0eddc02@redhat.com
> ---
>  drivers/acpi/acpi_processor.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
>
> diff --git a/drivers/acpi/acpi_processor.c b/drivers/acpi/acpi_processor.c
> index 4fe2ef54088c..6a542e0ce396 100644
> --- a/drivers/acpi/acpi_processor.c
> +++ b/drivers/acpi/acpi_processor.c
> @@ -626,9 +626,31 @@ static struct acpi_scan_handler processor_handler = {
>         },
>  };
>
> +static acpi_status acpi_processor_container_walk(acpi_handle handle,
> +                                                u32 lvl,
> +                                                void *context,
> +                                                void **rv)
> +{
> +       struct acpi_device *adev;
> +       acpi_status status;
> +
> +       adev = acpi_get_acpi_dev(handle);
> +       if (!adev)
> +               return AE_ERROR;

Why is the reference counting needed here?

Wouldn't acpi_fetch_acpi_dev() suffice?

Also, should the walk really be terminated on the first error?

> +
> +       status = acpi_processor_add(adev, &processor_device_ids[0]);
> +       acpi_put_acpi_dev(adev);
> +
> +       return status;
> +}
> +
>  static int acpi_processor_container_attach(struct acpi_device *dev,
>                                            const struct acpi_device_id *id)
>  {
> +       acpi_walk_namespace(ACPI_TYPE_PROCESSOR, dev->handle,
> +                           ACPI_UINT32_MAX, acpi_processor_container_walk,
> +                           NULL, NULL, NULL);

This covers processor objects only, so why is this not needed for
processor devices defined under a processor container object?

It is not obvious, so it would be nice to add a comment explaining the
difference.

> +
>         return 1;
>  }
>
> --