Re: [PATCH v3 01/29] ACPI / PPTT: Add a helper to fill a cpumask from a processor container

[Jonathan Cameron <jonathan.cameron@huawei.com>]

On Fri, 17 Oct 2025 18:56:17 +0000
James Morse <james.morse@arm.com> wrote:

> The ACPI MPAM table uses the UID of a processor container specified in
> the PPTT to indicate the subset of CPUs and cache topology that can
> access each MPAM System Component (MSC).
> 
> This information is not directly useful to the kernel. The equivalent
> cpumask is needed instead.
> 
> Add a helper to find the processor container by its id, then walk
> the possible CPUs to fill a cpumask with the CPUs that have this
> processor container as a parent.
> 
> CC: Dave Martin <dave.martin@arm.com>
> Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
> Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
> Reviewed-by: Fenghua Yu <fenghuay@nvidia.com>
> Reviewed-by: Gavin Shan <gshan@redhat.com>
> Tested-by: Fenghua Yu <fenghuay@nvidia.com>
> Signed-off-by: James Morse <james.morse@arm.com>
Hi James,

V2 has dropped out of my memory more or less so to get this back in
I'll do a fresh review (even of ones I've already given an RB on).
Nothing here to change that tag. Just some naming / comment suggestions
that I think would help a little with readability.

Some of these comments come from me forgetting how the spec named
things and so going to take a look.  The spec isn't consistent with the
naming (e.g. ACPI PROCESSOR ID is not necessarily a processor
ID) but I think keeping closer to spec names will help readers. 

Note this may all be in the category of perfect being the enemy of
good + upstream so I don't mind if you ignore.

> ---
> Changes since v2:
>  * Grouped two nested if clauses differently to reduce scope of cpu_node.
>  * Removed stale comment refering to the return value.
> 
> Changes since v1:
>  * Replaced commit message with wording from Dave.
>  * Fixed a stray plural.
>  * Moved further down in the file to make use of get_pptt() helper.
>  * Added a break to exit the loop early.
> 
> Changes since RFC:
>  * Removed leaf_flag local variable from acpi_pptt_get_cpus_from_container()
> 
> Changes since RFC:
>  * Dropped has_leaf_flag dodging of acpi_pptt_leaf_node()
>  * Added missing : in kernel-doc
>  * Made helper return void as this never actually returns an error.
> ---
>  drivers/acpi/pptt.c  | 82 ++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/acpi.h |  3 ++
>  2 files changed, 85 insertions(+)
> 
> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c
> index 54676e3d82dd..58cfa3916a13 100644
> --- a/drivers/acpi/pptt.c
> +++ b/drivers/acpi/pptt.c
> @@ -817,3 +817,85 @@ int find_acpi_cpu_topology_hetero_id(unsigned int cpu)
>  	return find_acpi_cpu_topology_tag(cpu, PPTT_ABORT_PACKAGE,
>  					  ACPI_PPTT_ACPI_IDENTICAL);
>  }
> +
> +/**
> + * acpi_pptt_get_child_cpus() - Find all the CPUs below a PPTT processor node

The spec calls these Processor Hierarchy Node Structures. I think the
addition of the Hierarchy word will help people understand this isn't
finding things below a node specific to a processor but to some higher
level hierarchy structure.

> + * @table_hdr:		A reference to the PPTT table.
> + * @parent_node:	A pointer to the processor node in the @table_hdr.

Likewise, calling this a "processor hierarchy node" would make things
clearer.

> + * @cpus:		A cpumask to fill with the CPUs below @parent_node.
> + *
> + * Walks up the PPTT from every possible CPU to find if the provided
> + * @parent_node is a parent of this CPU.
> + */
> +static void acpi_pptt_get_child_cpus(struct acpi_table_header *table_hdr,
> +				     struct acpi_pptt_processor *parent_node,
> +				     cpumask_t *cpus)
> +{
> +	struct acpi_pptt_processor *cpu_node;

This is definitely a processor hierarchy node.  To my mind
cpu_node doesn't convey this.  See below for more, but perhaps just renaming
it to include hierarchy in the name would help.

> +	u32 acpi_id;
> +	int cpu;
> +
> +	cpumask_clear(cpus);
> +
> +	for_each_possible_cpu(cpu) {
> +		acpi_id = get_acpi_id_for_cpu(cpu);
> +		cpu_node = acpi_find_processor_node(table_hdr, acpi_id);

Here it is indeed a CPU.

> +
> +		while (cpu_node) {
> +			if (cpu_node == parent_node) {
> +				cpumask_set_cpu(cpu, cpus);
> +				break;
> +			}
> +			cpu_node = fetch_pptt_node(table_hdr, cpu_node->parent);

But here it is a processor container or a private node.  So
cpu_hieriarchy_node or something along those lines would be more appropriate.

> +		}
> +	}
> +}
> +
> +/**
> + * acpi_pptt_get_cpus_from_container() - Populate a cpumask with all CPUs in a
> + *                                       processor container
> + * @acpi_cpu_id:	The UID of the processor container.
> + * @cpus:		The resulting CPU mask.
> + *
> + * Find the specified Processor Container, and fill @cpus with all the cpus
> + * below it.
> + *
> + * Not all 'Processor' entries in the PPTT are either a CPU or a Processor

I'd go with 'Processor Hierarchy' here
 
> + * Container, they may exist purely to describe a Private resource. CPUs
> + * have to be leaves, so a Processor Container is a non-leaf that has the
> + * 'ACPI Processor ID valid' flag set.
> + */
> +void acpi_pptt_get_cpus_from_container(u32 acpi_cpu_id, cpumask_t *cpus)
> +{
> +	struct acpi_table_header *table_hdr;
> +	struct acpi_subtable_header *entry;
> +	unsigned long table_end;
> +	u32 proc_sz;
> +
> +	cpumask_clear(cpus);
> +
> +	table_hdr = acpi_get_pptt();
> +	if (!table_hdr)
> +		return;
> +
> +	table_end = (unsigned long)table_hdr + table_hdr->length;
> +	entry = ACPI_ADD_PTR(struct acpi_subtable_header, table_hdr,
> +			     sizeof(struct acpi_table_pptt));
> +	proc_sz = sizeof(struct acpi_pptt_processor);
> +	while ((unsigned long)entry + proc_sz <= table_end) {
> +
> +		if (entry->type == ACPI_PPTT_TYPE_PROCESSOR) {
> +			struct acpi_pptt_processor *cpu_node;
similar naming thing here. 
> +
> +			cpu_node = (struct acpi_pptt_processor *)entry;
> +			if (cpu_node->flags & ACPI_PPTT_ACPI_PROCESSOR_ID_VALID &&
> +			    !acpi_pptt_leaf_node(table_hdr, cpu_node) &&
> +			    cpu_node->acpi_processor_id == acpi_cpu_id) {
> +					acpi_pptt_get_child_cpus(table_hdr, cpu_node, cpus);
The double tab indent here is odd.
			if (cpu_node->flags & ACPI_PPTT_ACPI_PROCESSOR_ID_VALID &&
			    !acpi_pptt_leaf_node(table_hdr, cpu_node) &&
			    cpu_node->acpi_processor_id == acpi_cpu_id) {
				acpi_pptt_get_child_cpus(table_hdr, cpu_node, cpus);

Isn't find I think for readability.  I could understand the bonus tab if it was
close to aligning with the line above but it isn't.

> +					break;
> +			}
> +		}
> +		entry = ACPI_ADD_PTR(struct acpi_subtable_header, entry,
> +				     entry->length);
> +	}
> +}