Re: [PATCH v2 01/29] ACPI / PPTT: Add a helper to fill a cpumask from a processor container

Jonathan Cameron
2025-09-11T10:43:42Z

On Wed, 10 Sep 2025 20:42:41 +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>
> Signed-off-by: James Morse <james.morse@arm.com>

Hi James,

Sorry I missed v1. Busy few weeks.

I think one resource leak plus a few suggested changes that
I'm not that bothered about.

Jonathan

> ---
> 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 | 83 ++++++++++++++++++++++++++++++++++++++++++++
> include/linux/acpi.h | 3 ++
> 2 files changed, 86 insertions(+)
>
> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c
> index 54676e3d82dd..1728545d90b2 100644
> --- a/drivers/acpi/pptt.c
> +++ b/drivers/acpi/pptt.c
> @@ -817,3 +817,86 @@ 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_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
> + * 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.
> + *
> + * Return: 0 for a complete walk, or an error if the mask is incomplete.
> + */
> +void acpi_pptt_get_cpus_from_container(u32 acpi_cpu_id, cpumask_t *cpus)
> +{
> + struct acpi_pptt_processor *cpu_node;
> + 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();

This calls acpi_get_table() so you need to put it again or every call
to this leaks a reference count. I messed around with DEFINE_FREE() for this
but it doesn't fit that well as the underlying call doesn't return the table.
This one does though so you could do a pptt specific one.

Or just acpi_put_table(table_hdr); at exit path from this function.

> + 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));
Hmm. Not related to this patch but I have no idea why acpi_get_pptt()
doesn't return a struct acpi_table_pptt as if it did this would be a simple
+ 1 and not require those who only sometimes deal with ACPI code to go
check what that macro actually does!

> + proc_sz = sizeof(struct acpi_pptt_processor);
Maybe sizeof (*cpu_node) is more helpful to reader.
Also shorter so you could do
while ((unsigned long)entry + sizeof(*cpu_node) <= table_end)

> + while ((unsigned long)entry + proc_sz <= table_end) {
> + cpu_node = (struct acpi_pptt_processor *)entry;

For me, assigning this before checking the type is inelegant.
but the nesting does get deep without it so I guess this is ok maybe, though
I wonder if better reorganized to combine a different bunch of conditions.
I think this is functionally identival.

if (entry->type == ACPI_PTT_TYPE_PROCESSOR) {
struct acpi_pptt_processor *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);
break;

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

More generally I wonder if it is worth adding a for_each_acpi_pptt_entry() macro.
There is some precedence in drivers acpi such as for_each_nhlt_endpoint()

That's probably material for another day though unless you think it brings
enough benefits to do it here.

> + if (entry->type == ACPI_PPTT_TYPE_PROCESSOR &&
> + cpu_node->flags & ACPI_PPTT_ACPI_PROCESSOR_ID_VALID) {
> + if (!acpi_pptt_leaf_node(table_hdr, cpu_node)) {
> + if (cpu_node->acpi_processor_id == acpi_cpu_id) {
> + acpi_pptt_get_child_cpus(table_hdr, cpu_node, cpus);
> + break;
> + }
> + }
> + }
> + entry = ACPI_ADD_PTR(struct acpi_subtable_header, entry,
> + entry->length);
> + }
> +}
> diff --git a/include/linux/acpi.h b/include/linux/acpi.h
> index 1c5bb1e887cd..f97a9ff678cc 100644
> --- a/include/linux/acpi.h
> +++ b/include/linux/acpi.h
> @@ -1541,6 +1541,7 @@ int find_acpi_cpu_topology(unsigned int cpu, int level);
> int find_acpi_cpu_topology_cluster(unsigned int cpu);
> int find_acpi_cpu_topology_package(unsigned int cpu);
> int find_acpi_cpu_topology_hetero_id(unsigned int cpu);
> +void acpi_pptt_get_cpus_from_container(u32 acpi_cpu_id, cpumask_t *cpus);
> #else
> static inline int acpi_pptt_cpu_is_thread(unsigned int cpu)
> {
> @@ -1562,6 +1563,8 @@ static inline int find_acpi_cpu_topology_hetero_id(unsigned int cpu)
> {
> return -EINVAL;
> }
> +static inline void acpi_pptt_get_cpus_from_container(u32 acpi_cpu_id,
> + cpumask_t *cpus) { }
> #endif
>
> void acpi_arch_init(void);