Re: [PATCH RFC] arch_topology: Introduce nr_possible_packages

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, May 12, 2026 at 11:05:05PM +0800, Feng Tang wrote:
> In multi-sockets platform, kernel or driver code may need the number
> of packages for chosing different code directions. Some architecture
> already provide such kind of interface like x86, which is being used
> in some architecture code and drivers.
> 
> Add similar interface 'nr_possible_packages' for platforms which can
> get package topology information by parsing ACPI tables in boot phase.
> 
> Signed-off-by: Feng Tang <feng.tang@linux.alibaba.com>
> ---
>  drivers/base/arch_topology.c  | 21 +++++++++++++++++++++
>  include/linux/arch_topology.h |  5 +++++
>  2 files changed, 26 insertions(+)
> 
> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
> index 8c5e47c28d9a..796d8a7aceea 100644
> --- a/drivers/base/arch_topology.c
> +++ b/drivers/base/arch_topology.c
> @@ -850,6 +850,16 @@ static bool __init acpi_cpu_is_threaded(int cpu)
>  	return !!is_threaded;
>  }
>  
> +unsigned int nr_possible_packages __ro_after_init = 1;
> +EXPORT_SYMBOL(nr_possible_packages);

EXPORT_SYMBOL_GPL() please?

And you don't have a user for this, so we can't verify how it actually
works :(

thanks,

greg k-h

> +/*
> + * Assuming silicon has a sane package ID decoding method to not have
> + * an ID bigger than 255 (1 byte).

Is that a valid assumption?  What about packages bigger than 255?  Don't
we have those today?

> + */
> +#define MAX_PACKAGE_ID	255
> +DECLARE_BITMAP(package_id_mask, MAX_PACKAGE_ID + 1);
> +
>  /*
>   * Propagate the topology information of the processor_topology_node tree to the
>   * cpu_topology array.
> @@ -912,6 +922,13 @@ __weak int __init parse_acpi_topology(void)
>  		cpu_topology[cpu].cluster_id = topology_id;
>  		topology_id = find_acpi_cpu_topology_package(cpu);
>  		cpu_topology[cpu].package_id = topology_id;
> +
> +
> +		if (topology_id >= 0 && topology_id <= MAX_PACKAGE_ID)
> +			bitmap_set(package_id_mask, topology_id, 1);
> +		else
> +			pr_warn("ACPI: abnormal package ID: %d !\n",
> +				topology_id);
>  	}
>  
>  	/*
> @@ -927,6 +944,10 @@ __weak int __init parse_acpi_topology(void)
>  
>  	cpu_smt_set_num_threads(max_smt_thread_num, max_smt_thread_num);
>  	xa_destroy(&hetero_cpu);
> +
> +	/* Count the number of possible packages in system */
> +	nr_possible_packages = bitmap_weight(package_id_mask, MAX_PACKAGE_ID + 1);
> +	pr_info("ACPI: System has %u Package(s) detected\n", nr_possible_packages);

Why "P" and not "p"?


>  	return 0;
>  }
>  
> diff --git a/include/linux/arch_topology.h b/include/linux/arch_topology.h
> index ebd7f8935f96..dee076cc9c7a 100644
> --- a/include/linux/arch_topology.h
> +++ b/include/linux/arch_topology.h
> @@ -111,4 +111,9 @@ static inline bool topology_core_has_smt(int cpu) { return false; }
>  
>  #endif /* CONFIG_GENERIC_ARCH_TOPOLOGY */
>  
> +
> +#if defined(CONFIG_ARM64) || defined(CONFIG_RISCV)
> +extern unsigned int nr_possible_packages;
> +#endif

Why the ifdef?

thanks,

greg k-h