Re: [PATCH v2 03/29] ACPI / PPTT: Find cache level by cache-id

Jonathan Cameron
2025-09-11T10:59:51Z

On Wed, 10 Sep 2025 20:42:43 +0000
James Morse <james.morse@arm.com> wrote:

> The MPAM table identifies caches by id. The MPAM driver also wants to know
> the cache level to determine if the platform is of the shape that can be
> managed via resctrl. Cacheinfo has this information, but only for CPUs that
> are online.
>
> Waiting for all CPUs to come online is a problem for platforms where
> CPUs are brought online late by user-space.
>
> Add a helper that walks every possible cache, until it finds the one
> identified by cache-id, then return the level.
>
> Signed-off-by: James Morse <james.morse@arm.com>
> ---
> Changes since v1:
> * Droppeed the cleanup based table freeing, use acpi_get_pptt() instead.
> * Removed a confusing comment.
> * Clarified the kernel doc.
>
> Changes since RFC:
> * acpi_count_levels() now returns a value.
> * Converted the table-get stuff to use Jonathan's cleanup helper.
> * Dropped Sudeep's Review tag due to the cleanup change.
> ---
> drivers/acpi/pptt.c | 62 ++++++++++++++++++++++++++++++++++++++++++++
> include/linux/acpi.h | 5 ++++
> 2 files changed, 67 insertions(+)
>
> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c
> index 7af7d62597df..c5f2a51d280b 100644
> --- a/drivers/acpi/pptt.c
> +++ b/drivers/acpi/pptt.c
> @@ -904,3 +904,65 @@ void acpi_pptt_get_cpus_from_container(u32 acpi_cpu_id, cpumask_t *cpus)
> entry->length);
> }
> }
> +
> +/*
/**

It's an exposed interface so nice to have formal kernel-doc and automatic
checks that brings.

> + * find_acpi_cache_level_from_id() - Get the level of the specified cache
> + * @cache_id: The id field of the unified cache
> + *
> + * Determine the level relative to any CPU for the unified cache identified by
> + * cache_id. This allows the property to be found even if the CPUs are offline.
> + *
> + * The returned level can be used to group unified caches that are peers.

Silly question but why do we care if this a unified cache?
It's a bit odd to have a general sounding function fail for split caches.
The handling would have to be more complex but if we really don't want
to do it maybe rename the function to find_acpi_unifiedcache_level_from_id()
and if the general version gets added later we can switch to that.

> + *
> + * The PPTT table must be rev 3 or later,
> + *
> + * If one CPUs L2 is shared with another as L3, this function will return
> + * an unpredictable value.
> + *
> + * Return: -ENOENT if the PPTT doesn't exist, the revision isn't supported or
> + * the cache cannot be found.
> + * Otherwise returns a value which represents the level of the specified cache.
> + */
> +int find_acpi_cache_level_from_id(u32 cache_id)
> +{
> + u32 acpi_cpu_id;
> + int level, cpu, num_levels;
> + struct acpi_pptt_cache *cache;
> + struct acpi_table_header *table;
> + struct acpi_pptt_cache_v1 *cache_v1;
> + struct acpi_pptt_processor *cpu_node;
> +
> + table = acpi_get_pptt();
> + if (!table)
> + return -ENOENT;
> +
> + if (table->revision < 3)
> + return -ENOENT;
> +
> + for_each_possible_cpu(cpu) {
> + acpi_cpu_id = get_acpi_id_for_cpu(cpu);
> + cpu_node = acpi_find_processor_node(table, acpi_cpu_id);
> + if (!cpu_node)
> + return -ENOENT;
> + num_levels = acpi_count_levels(table, cpu_node, NULL);
> +
> + /* Start at 1 for L1 */
> + for (level = 1; level <= num_levels; level++) {
> + cache = acpi_find_cache_node(table, acpi_cpu_id,
> + ACPI_PPTT_CACHE_TYPE_UNIFIED,
> + level, &cpu_node);
> + if (!cache)
> + continue;
> +
> + cache_v1 = ACPI_ADD_PTR(struct acpi_pptt_cache_v1,
> + cache,
> + sizeof(struct acpi_pptt_cache));

sizeof(*cache) to me makes this more obvious.

> +
> + if (cache->flags & ACPI_PPTT_CACHE_ID_VALID &&
> + cache_v1->cache_id == cache_id)
> + return level;
> + }
> + }
> +
> + return -ENOENT;
> +}